**Table of contents**

[4 Designing equipment for maintenance
4-1](#designing-equipment-for-maintenance)

[4.1 General 4-1](#general)

[4.1.1 General design guidance 4-1](#general-design-guidance)

[4.1.2 Emphasizing maintenance during design
4-3](#emphasizing-maintenance-during-design)

[4.1.3 Optimize skills and training 4-4](#optimize-skills-and-training)

[4.1.4 Minimizing need for maintenance
4-5](#minimizing-need-for-maintenance)

[4.2 Designing equipment for handling
4-5](#designing-equipment-for-handling)

[4.2.1 General 4-5](#general-1)

[4.2.2 Weight 4-6](#weight)

[4.2.3 Size 4-8](#size)

[4.2.4 Shape 4-8](#shape)

[4.2.5 Handles 4-8](#handles)

[4.2.5.1 When handles are needed 4-8](#when-handles-are-needed)

[4.2.5.2 Handle characteristics 4-9](#handle-characteristics)

[4.2.5.3 Dimensions 4-10](#dimensions)

[4.2.5.4 Location 4-12](#location)

[4.2.6 Grasp areas 4-13](#grasp-areas)

[4.2.7 Stands and rests 4-13](#stands-and-rests)

[4.2.8 Alignment aids 4-14](#alignment-aids)

[4.2.9 Designing for remote handling
4-14](#designing-for-remote-handling)

[4.2.10 Designing for use of hoists, jacks, and cranes
4-15](#designing-for-use-of-hoists-jacks-and-cranes)

[4.3 Packaging, arrangement, and mounting of equipment
4-15](#packaging-arrangement-and-mounting-of-equipment)

[4.3.1 Unitization of equipment 4-16](#unitization-of-equipment)

[4.3.2 Interchangeability, non-interchangeability
4-17](#interchangeability-non-interchangeability)

[4.3.3 Mounting in drawers, on racks, and on hinges
4-18](#mounting-in-drawers-on-racks-and-on-hinges)

[4.3.3.1 General 4-18](#general-2)

[4.3.3.2 Restraints and supports 4-19](#restraints-and-supports)

[4.3.3.3 External connectors and interlocks
4-19](#external-connectors-and-interlocks)

[4.3.4 Positioning equipment 4-20](#positioning-equipment)

[4.3.4.1 Physical accessibility 4-20](#physical-accessibility)

[4.3.4.2 Relative accessibility 4-21](#relative-accessibility)

[4.3.5 Labeling and marking 4-22](#labeling-and-marking)

[4.3.5.1 Types of labels 4-22](#types-of-labels)

[4.3.5.2 Location and orientation 4-23](#location-and-orientation)

[4.3.5.3 Typographic matters 4-24](#typographic-matters)

[4.3.5.4 Wording 4-25](#wording)

[4.3.5.5 Markings 4-26](#markings)

[4.4 Access openings 4-27](#access-openings)

[4.4.1 General 4-27](#general-3)

[4.4.2 Access 4-27](#access)

[4.4.3 Size 4-28](#size-1)

[4.4.4 Shape 4-31](#shape-1)

[4.4.5 Location 4-31](#location-1)

[4.4.6 Labeling and marking 4-32](#labeling-and-marking-1)

[4.5 Cases, covers, guards, and shields
4-33](#cases-covers-guards-and-shields)

[4.5.1 General 4-33](#general-4)

[4.5.2 Size 4-36](#size-2)

[4.5.3 Shape 4-37](#shape-2)

[4.5.4 Hinged covers 4-37](#hinged-covers)

[4.5.5 Sliding doors and caps 4-37](#sliding-doors-and-caps)

[4.5.6 Interlocks 4-38](#interlocks)

[4.5.7 Labeling and marking 4-39](#labeling-and-marking-2)

[4.6 Fasteners 4-40](#fasteners)

[4.6.1 General 4-40](#general-5)

[4.6.2 Number 4-43](#number)

[4.6.3 Types 4-43](#types)

[4.6.3.1 Nuts and bolts 4-43](#nuts-and-bolts)

[4.6.3.2 Screws 4-44](#screws)

[4.6.3.3 Screw and bolt heads 4-45](#screw-and-bolt-heads)

[4.6.3.4 Latches and catches 4-46](#latches-and-catches)

[4.6.3.5 Other fastening devices 4-47](#other-fastening-devices)

[4.6.3.6 Quick fastening and releasing devices
4-49](#quick-fastening-and-releasing-devices)

[4.6.3.7 Captive versus removable 4-50](#captive-versus-removable)

[4.6.4 Labeling, marking, and coding 4-50](#labeling-marking-and-coding)

[4.7 Connectors 4-51](#connectors)

[4.7.1 General 4-51](#general-6)

[4.7.2 Types 4-52](#types-1)

[4.7.2.1 Distinctive 4-52](#distinctive)

[4.7.2.2 Plug-in 4-52](#plug-in)

[4.7.2.3 Threaded 4-52](#threaded)

[4.7.2.4 Quick-action 4-53](#quick-action)

[4.7.3 Location and accessibility 4-53](#location-and-accessibility)

[4.7.4 Alignment aids 4-54](#alignment-aids-1)

[4.7.5 Electrical connections 4-55](#electrical-connections)

[4.7.5.1 Plugs and receptacles 4-55](#plugs-and-receptacles)

[4.7.5.2 Wire connections 4-56](#wire-connections)

[4.7.6 Labeling, marking, and coding
4-58](#labeling-marking-and-coding-1)

[4.8 Lines and cables 4-59](#lines-and-cables)

[4.8.1 Electrical 4-59](#electrical)

[4.8.1.1 General 4-59](#general-7)

[4.8.1.2 Length of cables and leads 4-61](#length-of-cables-and-leads)

[4.8.1.3 Routing and mounting 4-63](#routing-and-mounting)

[4.8.1.4 Leads 4-65](#leads)

[4.8.1.5 Clamps and mounting plates 4-65](#clamps-and-mounting-plates)

[4.8.1.6 Test and extension cables 4-67](#test-and-extension-cables)

[4.8.1.7 Bench mockup cables 4-68](#bench-mockup-cables)

[4.8.1.8 Labeling, marking, and coding
4-69](#labeling-marking-and-coding-2)

[4.8.2 Fluid and gas lines 4-71](#fluid-and-gas-lines)

[4.8.2.1 General 4-71](#general-8)

[4.8.2.2 Routing and mounting 4-72](#routing-and-mounting-1)

[4.8.2.3 Clamps and supports 4-73](#clamps-and-supports)

[4.8.2.4 Gaskets and seals 4-73](#gaskets-and-seals)

[4.8.2.5 Labeling, marking, and coding
4-75](#labeling-marking-and-coding-3)

[4.9 Packaging, layout, and mounting of internal components
4-79](#packaging-layout-and-mounting-of-internal-components)

[4.9.1 General 4-79](#general-9)

[4.9.2 Packaging 4-80](#packaging)

[4.9.2.1 Modularization 4-80](#modularization)

[4.9.2.2 Modularization methods 4-81](#modularization-methods)

[4.9.3 Layout 4-82](#layout)

[4.9.3.1 Accessibility 4-82](#accessibility)

[4.9.3.2 Grouping of parts 4-83](#grouping-of-parts)

[4.9.3.3 Hazard protection 4-84](#hazard-protection)

[4.9.4 Mounting 4-85](#mounting)

[4.9.5 Labeling and marking 4-87](#labeling-and-marking-3)

[4.10 Adjustment controls 4-89](#adjustment-controls)

[4.11 Fuses and circuit breakers 4-91](#fuses-and-circuit-breakers)

[4.11.1 General 4-91](#general-10)

[4.11.2 Fuses 4-92](#fuses)

[4.11.3 Push-pull circuit breakers 4-93](#push-pull-circuit-breakers)

[4.11.4 Toggle bat and legend switch circuit breakers
4-95](#toggle-bat-and-legend-switch-circuit-breakers)

[4.11.5 Labeling and marking 4-97](#labeling-and-marking-4)

[4.12 Test points and service points
4-97](#test-points-and-service-points)

[4.12.1 Adjustment controls 4-97](#adjustment-controls-1)

[4.12.2 Location and arrangement 4-98](#location-and-arrangement)

[4.12.3 Drain points 4-99](#drain-points)

[4.12.4 Accessibility 4-100](#accessibility-1)

[4.12.5 Safety 4-101](#safety)

[4.12.6 Labeling, marking, and coding
4-102](#labeling-marking-and-coding-4)

[4.13 Test equipment 4-103](#test-equipment)

[4.13.1 General 4-103](#general-11)

[4.13.1.1 General characteristics 4-103](#general-characteristics)

[4.13.1.2 Safety 4-104](#safety-1)

[4.13.1.3 Ease of use 4-104](#ease-of-use)

[4.13.1.4 Controls and displays 4-105](#controls-and-displays)

[4.13.2 Built-in test equipment (and partially built-in, portable, and
test panels)
4-106](#built-in-test-equipment-and-partially-built-in-portable-and-test-panels)

[4.13.2.1 Completely built-in test equipment
4-106](#completely-built-in-test-equipment)

[4.13.2.2 Partially built-in test equipment
4-106](#partially-built-in-test-equipment)

[4.13.2.3 Portable test equipment 4-107](#portable-test-equipment)

[4.13.2.4 Built-in test panel 4-107](#built-in-test-panel)

[4.13.3 Test equipment 4-108](#test-equipment-1)

[4.13.4 Bench mockups 4-109](#bench-mockups)

[4.13.5 Storage space 4-110](#storage-space)

[4.14 Tools 4-110](#tools)

[4.14.1 General 4-110](#general-12)

[4.14.2 Common hand tools 4-111](#common-hand-tools)

[4.14.3 Special tools 4-111](#special-tools)

[Glossary 4-112](#__RefHeading___Toc34717537)

[References 4-114](#__RefHeading___Toc34717538)

[Index 4-115](#__RefHeading___Toc34717539)

**List of exhibits**

[Exhibit 4.2.2.1 Maximum weight limits for objects lifted by one person
using both hands 4-6](#__RefHeading___Toc27272136)

[Exhibit 4.2.5.1.5 Maximum force limits for pulling and pushing units of
equipment using handles or grasp areas.
4-9](#__RefHeading___Toc27272137)

[Exhibit 4.2.5.3.1 Minimum handle dimensions
4-11](#__RefHeading___Toc27272138)

[Exhibit 4.2.5.3.2 Minimum handle diameter required by weight of unit of
equipment 4-12](#__RefHeading___Toc27272139)

[Exhibit 4.3.5.3.1 Minimum character height for various viewing
distances 4-24](#__RefHeading___Toc27272140)

[Exhibit 4.3.5.5.2 Recommended colors 4-26](#__RefHeading___Toc27272141)

[Exhibit 4.3.5.5.3 Good and bad arrows
4-26](#__RefHeading___Toc27272142)

[Exhibit 4.4.3.2 Minimum dimensions of openings designed for access by
one or two fingers without visual access
4-28](#__RefHeading___Toc27272143)

[Exhibit 4.4.3.3 Minimum dimensions of openings designed for access by
one hand or arm without visual access.
4-29](#__RefHeading___Toc27272144)

[Exhibit 4.4.3.4 Minimum dimensions of openings designed for access by
two hands without visual access 4-30](#__RefHeading___Toc27272145)

[Exhibit 4.5.1.2 Type of covering for type of access and environmental
conditions 4-34](#__RefHeading___Toc27272146)

[Exhibit 4.6.3.3.2 Examples of combination-head bolts and screws
4-45](#__RefHeading___Toc27272147)

[Exhibit 4.6.3.3.4 Example of an internal-wrenching bolt and nut
4-46](#__RefHeading___Toc27272148)

[Exhibit 4.7.2.4 Example of a quick-action connector
4-53](#__RefHeading___Toc27272149)

[Exhibit 4.7.5.2.2 Example of a crimp-on splice.
4-57](#refheading___toc27272150)

[Exhibit 4.7.5.2.5 Examples of a U-lug and an O-lug
4-57](#__RefHeading___Toc27272151)

[Exhibit 4.8.1.1.5 Fanning out cables 4-60](#__RefHeading___Toc27272152)

[Exhibit 4.8.1.1.13 Cable winder 4-61](#__RefHeading___Toc27272153)

[Exhibit 4.8.1.2.6 Recoiling slack cable
4-63](#__RefHeading___Toc27272154)

[Exhibit 4.8.1.5.3 Quick-release clamps, hinged and spring
4-66](#__RefHeading___Toc27272155)

[Exhibit 4.8.1.6.6 An example of a line and cable reel cart.
4-67](#__RefHeading___Toc27272156)

[Exhibit 4.8.1.6.8 Line and cable mobile support
4-68](#__RefHeading___Toc27272157)

[Exhibit 4.8.1.8.4 Electrical cable coding
4-70](#__RefHeading___Toc27272158)

[Exhibit 4.8.2.4.7 Externally visible seals
4-74](#__RefHeading___Toc27272159)

[Exhibit 4.8.2.5.1 Color-coding of fluid conductors
4-75](#__RefHeading___Toc27272160)

[Exhibit 4.8.2.5.2 Valve color-coding scheme
4-76](#__RefHeading___Toc27272161)

[Exhibit 4.8.2.5.3 Hydraulic and pneumatic line color-coding scheme
4-77](#__RefHeading___Toc27272162)

[Exhibit 4.8.2.5.5 Value position labeling
4-78](#__RefHeading___Toc27272163)

[Exhibit 4.9.4.1 Example of foldout mounting construction
4-85](#__RefHeading___Toc27272164)

[Exhibit 4.9.4.7 Error-free mounting provisions
4-86](#__RefHeading___Toc27272165)

[Exhibit 4.11.1.1 General comparison of fuses and circuit breakers
4-92](#__RefHeading___Toc27272166)

[Exhibit 4.11.3.1 Push-pull circuit breaker specifications
4-94](#__RefHeading___Toc27272167)

[Exhibit 4.11.4.1 Toggle bat specifications
4-95](#__RefHeading___Toc27272168)

[Exhibit 4.11.4.2 Legend switch specifications.
4-96](#__RefHeading___Toc27272169)

[Exhibit 4.12.2.13 Terminal strips 4-99](#__RefHeading___Toc27272170)

[Exhibit 4.12.4.4 (a) Test plug with sliding cover
4-101](#__RefHeading___Toc27272171)

[Exhibit 4.12.4.4 (b) Test point adapter
4-101](#__RefHeading___Toc27272172)

#  Designing equipment for maintenance

This section contains human factors rules intended to make equipment
maintenance easy, fast, and safe. Equipment maintenance does not, of
course, occur in isolation. The overall system of which the equipment is
a part affects the design of the equipment; for example, if the system
must run continuously, the equipment might be designed to allow
maintenance while it is in operation, or some sort of redundancy might
be provided. The system maintenance concept also affects equipment
design; for example, is a particular unit of equipment intended to be
repaired on site? Is it intended to be removed and repaired at another
location? Or is it intended to be discarded and replaced with another
unit? A third factor affecting equipment design is the physical
environment in which it will be located; will it be exposed to weather
or to temperature extremes? Will the user be wearing gloves or other
protective clothing? Finally, equipment must accommodate characteristics
of the users themselves, their sizes and shapes, their skills, and their
training.

## 4.1 General

> This section contains rules for: (a) designing equipment in general,
> (b) emphasizing maintenance during the design of equipment, modules,
> and components, (c) maximizing the use of existing equipment and
> tools, (d) minimizing the skill and training requirements for users,
> and (e) minimizing the need for maintenance.

### 4.1.1 General design guidance

One of the most important aspects of designing equipment for maintenance
is the breaking up of a unit of equipment into modules that are
independent, interchangeable, and easily replaced. If warranted, these
modules can also be disposable. Other important aspects are: ease of
access to test and service points, ease of access to internal parts and
components, and, if warranted, built-in testing, diagnostic and fault
localization capability.

**Definitions.** A **unit of equipment** is an assemblage of items that
may include modules, components, and parts that are packaged together
into a single hardware package. For example, a computer, its keyboard,
and its visual display are all units of equipment, as are radio
transmitters and receivers. A **module** is an assemblage of two or more
interconnected parts or components that comprise a single, physical and
functional entity. It is this singular functionality that defines a
module. A **component** is a subdivision of a unit of equipment that can
be treated as an object by the user, but which can be further broken
down into parts. A mounting board together with its mounted parts is an
example of a component. A **part** is an object that cannot normally be
broken down further without destroying its designated use. Fuses,
resistors, and capacitors are examples of parts.

-   **4.1.1.1 General design guidance.** The following features should
    be incorporated into the design of all equipment, modules, and
    components, as appropriate:

a.  simplification of maintenance functions,

b.  modularization of equipment and components,

c.  minimization of the number and complexity of maintenance tasks,

d.  use of built-in testing, diagnostic, and fault localization
    capabilities,

e.  use of disposable modules, components, and parts, where cost
    effective and appropriate,

f.  simplification of design,

g.  quick and easy access to all units of equipment, modules, components
    and parts that require maintenance, inspection, removal, or
    replacement,

h.  compliance with lifting, carrying, and force criteria,

i.  minimization of the numbers and types of tools and test equipment
    required for maintenance,

j.  design alternatives dependent on the skills and training needed by
    maintainers and operators,

k.  maximization of the safety and protection of users and equipment,

l.  ease of assembly, disassembly, installation, and removal,

m.  elimination of precise torque requirements,

n.  ability to perform maintenance from above and outside rather than
    from underneath and inside,

o.  use of self lubrication, and

p.  use of sealed and lubricated modules. \[Source: Department of
    Defense (MIL-HDBK-759B), 1992\]

-   **4.1.1.2** **Use of existing items.** If an existing unit of
    equipment, module, component, or part meets the relevant
    requirements and the applicable human engineering criteria in this
    document, designers shall use that existing item rather than design
    a new one. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999\]

    **Discussion.** Relevant requirements include performance,
    maintainability, and reliability criteria designated by the
    acquisition program office. \[Source: AFSC DH 1-3, 1980;
    MIL-STD-1472F, 1999\]

    **Definition.** **Item** is a nonspecific term used to denote any
    product, available or in design or development, including parts,
    components, modules, and units of equipment. \[Source: AFSC DH 1-3,
    1980; MIL-STD-1472F, 1999\]

### 4.1.2 Emphasizing maintenance during design

-   **4.1.2.1** **Non-interruption of continuous operation.** Equipment
    that is part of a system that must operate continuously shall be
    capable of undergoing maintenance without interrupting the
    operation. \[Source: NASA-STD-3000A, 1989\]

-   **4.1.2.2** **Redundancy to prevent interruption.** If continuous
    operation is required and required maintenance on a unit of
    equipment would interrupt the operation, redundant equipment shall
    be provided. \[Source: NASA-STD-3000A, 1989\]

-   **4.1.2.3** **Degraded operation.** When warranted by its importance
    in a system, a unit of equipment that has a partial failure shall be
    designed to operate in a degraded mode while awaiting maintenance.
    This degraded operation shall not cause damage to other equipment or
    components, nor shall it aggravate the original fault. Degraded
    operation and faults shall be sensed and appropriate information
    identified, displayed, or transmitted to users and, if appropriate,
    to operators. \[Source: NASA-STD-3000A, 1989\]

-   **4.1.2.4** **Automation of fault detection and isolation.** When
    warranted, equipment shall have automatic fault detection and
    isolation capability. \[Source: NASA-STD-3000A, 1989\]

-   **4.1.2.5** **Equipment independence for maintenance.** Units of
    equipment shall be as independent (functionally, mechanically,
    electrically, and electronically) as is practical (see Paragraph
    4.3.1.1). \[Source: NASA-STD-3000A, 1989\]

-   **4.1.2.6** **Designing for safety of users.** Equipment shall not
    present hazardous conditions to users as they perform maintenance
    procedures. A positive means (for example, disconnects or lockouts)
    shall be designed into equipment and used to control hazardous
    conditions and facilitate safety as outlined in OSHA 29 CFR
    1910.147. \[Source: 29 CFR 1910.147; NASA-STD-3000A, 1989\]

    **Definitions.** A **hazardous condition** is the presence of energy
    or a substance which is likely to cause death or injury by reason of
    physical force, shock, radiation, explosion, flames, poison,
    corrosion, oxidation, irritation or other debilitation. Biological
    and chemical hazards can have debilitating effects through disease
    or interference with physiological functions. A **hazardous
    location** is a space within a facility, room, or open environment
    where a hazardous condition exists or is accessible or exposed
    within the system or equipment located within the space. \[Source:
    NASA-STD-3000A, 1989\]

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-   **4.1.2.7** **Dividing equipment into modules.** Heavy, large, or
    complex equipment should be divided into modules. \[Source: AFSC DH
    1-3, 1980\]

    **Discussion.** Modularization of equipment can make it easier
    to: (a) locate and isolate malfunctions, (b) properly allocate
    maintenance functions and responsibilities, (c) reach, remove, and
    maintain components, and (d) handle the equipment for installation
    and repair. \[Source: AFSC DH 1-3, 1980\]

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-   **4.1.2.8** **Replacement of failed components.** Equipment shall be
    designed so that components that fail frequently (such as lamps and
    fuses) can be easily replaced. \[Source: NASA-STD-3000A, 1989\]

### 4.1.3 Optimize skills and training

-   **4.1.3.1** **Optimize balance between use, maintenance, and special
    skills.** As practical, the balance between ease of use,
    maintenance, and the need for special skills on the part of the
    users should be optimized. \[Source: NASA-STD-3000A, 1989\]

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-   **4.1.3.2** **Optimize balance between ease of use and training.**
    As practical, optimize the balance between ease of use and training
    on the part of the users. Special training shall only be required
    when equipment or automation is implemented or has undergone major
    modification. \[Source: NASA-STD-3000A, 1989\]

### 4.1.4 Minimizing need for maintenance

-   **4.1.4.1 Ease of servicing.** Equipment shall be designed so that
    it can be serviced in its installed position. \[Source: AFSC DH 1-3,
    1980\]

-   **4.1.4.2** **Minimize maintenance time.** Equipment shall be
    designed to minimize the time required for maintenance. \[Source:
    NASA-STD-3000A, 1989\]

## 4.2 Designing equipment for handling

The purpose in designing equipment for handling is to increase the
efficiency of the user and to reduce the likelihood of injury to the
user or damage to the equipment. The topics covered in this section
include (a) the weight, size, and shape of the equipment, (b) the
provision of handles and grasp areas, (c) the provision of stands,
rests, and alignment aids, (d) designing for remote handling, and (e)
designing for the use of hoists, jacks, and cranes.

### 4.2.1 General

-   **4.2.1.1** **Prevention of damage.** Units of equipment shall be
    designed, located, and protected so that they will not be damaged
    when they are stored, shipped, handled, installed, operated, or
    maintained. Susceptibility to damage shall be clearly identified.
    Procedural guidance and suitable warning labels shall be provided to
    help prevent such damage. \[Source: MIL-STD-1800A, 1990\]

-   **4.2.1.2** **Minimal number of maintainers.** Units of equipment
    shall be designed, placed, and mounted so that they can be installed
    and removed by a minimum number of people wearing clothing
    appropriate to the environment. \[Source: MIL-STD-1472F, 1999\]

### 4.2.2 Weight

The weight limits provided in this section assume that they will be
lifted by able-bodied people.

-   **4.2.2.1** **Maximum weight of units of equipment to be lifted by
    one person.** If a unit of equipment is designed to be lifted by a
    single person, its weight shall not exceed the value in Exhibit
    4.2.2.1 that is appropriate for the height to which it is to be
    lifted and the size of the unit as it affects the distance between
    the body and the grip. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999\]

[]{#__RefHeading___Toc27272136 .anchor}**Exhibit 4.2.2.1** Maximum
weight limits for objects lifted by one person using both hands

-   **4.2.2.2** **Lifting in the presence of obstacles.** The values
    given in Exhibit 4.2.2.1 assume that there are no obstacles between
    the person lifting and the surface onto which the object is to be
    placed. If there is an obstacle, such as a lower shelf, the weight
    limit shall be reduced by 33% for an obstacle protruding 300 mm (12
    in), 50% for an obstacle protruding 460 mm (18 in), and 66% for an
    obstacle protruding 610 mm (24 in). No lift shall be performed at a
    reach distance greater than 635 mm (25 in). If the allowable weight
    must be reduced by both size (distance between body and grip) and
    obstacle considerations, only the more restrictive single value
    shall apply, that is, two reductions shall not be applied. \[Source:
    MIL-STD-1472F, 1999\]

-   **4.2.2.3** **Maximum weight of units of equipment to be lifted by
    two people.** If a unit of equipment is designed to be lifted by two
    people, the weight lifted by either one of them shall not exceed the
    appropriate value given in Exhibit 4.2.2.1; thus, if the weight of
    the unit is distributed uniformly, the maximum weight is twice that
    for a single person. \[Source: MIL-STD-1472F, 1999\]

-   **4.2.2.4** **Maximum weight of units of equipment to be lifted by
    three or more people.** If a unit of equipment is designed to be
    lifted by three or more people, the weight lifted by any one of them
    shall not exceed the appropriate value given in Exhibit 4.2.2.1. The
    maximum weight of the unit may be increased by three-fourths of the
    single person value for each person in addition to the first. Thus,
    the maximum weight shall not exceed

    X + 0.75(N-1) X

    where X is the appropriate value from Exhibit 4.2.2.1, and N is the
    number of people lifting. This increase assumes that the unit is
    large enough that the people lifting do not interfere with each
    other. \[Source: MIL-STD-1472F, 1999\]

-   **4.2.2.5** **Maximum weight of units of equipment to be carried by
    one person.** The weight of a unit of equipment designed to be
    carried by one person shall not exceed 16 kg (35 lb). This limit
    applies to carrying distances up to 10 m (33 ft). \[Source:
    MIL-STD-1472F, 1999\]

-   **4.2.2.6** **Maximum weight of units of equipment to be carried by
    more than one person.** If a unit of equipment is designed to be
    carried by two people, the weight carried by either one of them
    shall not exceed 19 kg (42 lb); thus, if the weight of the unit is
    distributed uniformly, the maximum weight of the unit is 38 kg (84
    lb). This limit applies to carrying distances up to 10 m (33 ft).
    \[Source: MIL-STD-1472F, 1999\]

-   **4.2.2.7** **Maximum weight of units of equipment to be carried by
    more than two people.** If a unit of equipment is designed to be
    carried by more than two people, the total weight shall not exceed
    19 kg (42 lb) plus 14.3 kg (31.5 lb) for each person carrying in
    addition to the first. This increase in weight assumes that the unit
    is large enough that the people carrying do not interfere with each
    other. This limit applies to carrying distances up to 10 m (33 ft).
    \[Source: MIL-STD-1472F, 1999\]

-   **4.2.2.8** **Lifting eyes or jacking points.** Units of equipment
    weighing more than 68 kg (150 lb) shall have lifting eyes or jacking
    points (see Paragraph 4.2.10 Designing for use of hoists, jacks, and
    cranes). \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992;
    MIL-STD-1800A, 1990\]

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-   **4.2.2.9** **Reducing weight by removing parts.** Heavy pieces of
    equipment should be made more manageable by designing them with
    removable parts. \[Source: AFSC DH 1-3, 1980\]

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-   **4.2.2.10** **Labeling heavy units.** Any unit of equipment heavier
    than 13.6 kg (30 lbs) or designed to be lifted or carried by more
    than one person shall be labeled according to Paragraph 4.3.5.1.3.
    \[Source: MIL-STD-1800A, 1990\]

### 4.2.3 Size 

The size of a unit of equipment affects its weight limits; a large unit
intended to be handled by one person cannot weigh as much as a smaller
one, as can be seen in Exhibit 4.2.2.1. Similarly, a unit intended to be
handled by two or more people that is so small that the people interfere
with each other cannot weigh as much as a unit that is large enough to
avoid such interference.

-   **4.2.3.1** **Desirable size.** Each unit of equipment should be
    small enough for one person to lift or carry. \[Source: AFSC DH 1-3,
    1980\]

-   **4.2.3.2** **Reducing size by removing parts.** Units of equipment
    that are too large to be handled by one person should be designed
    with removable parts to reduce their size. \[Source: AFSC DH 1-3,
    1980\]

### 4.2.4 Shape

-   **4.2.4.1** **Avoiding protuberances.** Equipment shall be designed
    with a minimum number of bulges or extensions that might interfere
    with handling. \[Source: MIL-STD-1472F, 1999\]

-   **4.2.4.2** **Removing protuberances.** If a unit of equipment
    includes irregular bulges or extensions that make handling
    difficult, the bulges or extensions shall be easily removable by
    hand or with common hand tools. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

### 4.2.5 Handles 

A handle is a permanent part of a unit of equipment that is designed to
be grasped by the hand. Handles may extend out from the unit so that the
fingers wrap around them, or they may be recessed areas so that the
fingers fit inside an opening. Extended handles may be rigid or folding.

The size, number, and location of handles depend upon: (a) the weight
and center of gravity of the unit, (b) the number of people lifting or
carrying the unit, (c) the type of clothing worn and whether or not
gloves are worn, (d) the position of the unit before handling and its
final position, (e) the frequency with which the unit is handled, and
(f) any additional uses the handles may serve. \[Source: UCRL-15673,
1985\]

#### 4.2.5.1 When handles are needed

-   **4.2.5.1.1** **Units of equipment designed for carrying.** Units of
    equipment intended to be carried shall have handles or grasp areas.
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1800A, 1990\]

-   **4.2.5.1.2 Units of equipment weighing less than 4.5 kg (10 lb).**
    Units of equipment weighing less than 4.5 kg (10 lb) shall have
    handles if they would otherwise be difficult to grasp, remove, or
    carry. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992\]

-   **4.2.5.1.3** **Units of equipment weighing between 4.5 and 18 kg
    (10 to 40 lb).** Units of equipment weighing between 4.5 kg (10 lb)
    and 18 kg (40 lb) shall have one or more handles that permit easy
    handling of the unit by one person. If the unit is bulky or if its
    weight is unevenly distributed, the handles shall permit easy
    handling by two people. \[Source: AFSC DH 1-3, 1980; UCRL-15673,
    1985\]

-   **4.2.5.1.4** **Units of equipment weighing between 18 and 68 kg (40
    to 150 lb).** Units of equipment weighing between 18 kg (40 lb) and
    68 kg (150 lb) shall have handles that provide easy handling of the
    unit by two or more people. If the unit is very large, it shall have
    lifting eyes (see Paragraph 4.2.10.1). \[Source: AFSC DH 1-3, 1980;
    UCRL-15673, 1985\]

-   **4.2.5.1.5** **Force limits.** The force exerted pulling or pushing
    a handle or grasp area shall not exceed the values given in Exhibit
    4.2.5.1.5 for the appropriate elbow angle. The values provided in
    the Exhibit shall be reduced by 30% if the work is performed in
    excess of 30 minutes. The values given are projected to a mixed male
    and female population by taking two-thirds of the values for a male
    population. For a more detailed coverage of pulling and pushing
    limits, see Snook & Ciriello (1991). \[Source: MIL-STD-1472F, 1999\]

> []{#__RefHeading___Toc27272137 .anchor}**Exhibit 4.2.5.1.5** Maximum
> force limits for pulling and pushing units of equipment using handles
> or grasp areas.

+-----------+-------------+-------------+-------------+-------------+
| **Degree  | Pulling     |             | Pushing     |             |
| of elbow  |             |             |             |             |
| flexion** |             |             |             |             |
+-----------+-------------+-------------+-------------+-------------+
|           | Left arm    | Right arm   | Left arm    | Right arm   |
|           |             |             |             |             |
|           | N (lbf)     | N (lbf)     | N (lbf)     | N (lbf)     |
+-----------+-------------+-------------+-------------+-------------+
| 180       | 148 (33)    | 154 (35)    | 125 (28)    | 148 (33)    |
+-----------+-------------+-------------+-------------+-------------+
| 150       | 125 (28)    | 166 (37)    | 89 (20)     | 125 (28)    |
+-----------+-------------+-------------+-------------+-------------+
| 120       | 101 (23)    | 125 (28)    | 77 (17)     | 107 (24)    |
+-----------+-------------+-------------+-------------+-------------+
| 90        | 95 (21)     | 110 (25)    | 65 (15)     | 107 (24)    |
+-----------+-------------+-------------+-------------+-------------+
| 60        | 77 (17)     | 71 (16)     | 65 (15)     | 101 (23)    |
+-----------+-------------+-------------+-------------+-------------+

#### 4.2.5.2 Handle characteristics

-   **4.2.5.2.1** **Handle comfort.** Handles shall be comfortable and
    easy to grasp; they shall not cut into the hand or cause undue
    pressure on the fingers. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

-   **4.2.5.2.2 Handle surface.** The surface of handles shall be
    sufficiently hard that grit and grime do not become embedded during
    normal use. \[Source: MIL-STD-1472F, 1999\]

-   **4.2.5.2.3** **Handle conductivity.** The handle material that
    comes into contact with a user\'s hand shall not conduct heat or
    electricity. \[Source: MIL-STD-1472F, 1999\]

-   **4.2.5.2.4** **Handle attachment.** Handles shall be permanently
    attached to the unit of equipment. \[Source: MIL-HDBK-759B, 1992\]

-   **4.2.5.2.5** **Stops for hinged or folding handles.** Hinged or
    folding handles shall have a stop that holds them perpendicular to
    the surface on which they are mounted when they are moved into
    carrying position. It shall require only one hand to move them into
    this position. \[Source: AFSC DH 1-3, 1980; MIL-STD-1800A, 1990;
    MIL-STD-1472F, 1999\]

#### 4.2.5.3 Dimensions 

The dimensions of handles depend primarily upon the type of handle, the
weight of the unit of equipment, and the type of hand covering the user
wears (none, gloves, or mittens). Other factors affecting handle
dimensions include the normal operating position of the unit, the
frequency and distance it is lifted or carried, and whether or not the
handle has an additional purpose, such as protecting the front of the
equipment or serving as a stand when the equipment is in its maintenance
position. \[Source: MIL-HDBK-759B, 1992\]

-   **4.2.5.3.1** **Minimum handle dimensions by type of handle and hand
    covering.** Handles shall equal or exceed the dimensions in Exhibit
    4.2.5.3.1 for the appropriate type of handle and the user\'s hand
    covering. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999;
    UCRL-15673, 1985\]

[]{#__RefHeading___Toc27272138 .anchor}**Exhibit 4.2.5.3.1** Minimum
handle dimensions

![](media/image1.png){width="6.75in" height="7.5in"}

-   **4.2.5.3.2** **Minimum handle diameter by weight of unit of
    equipment.** Heavier units require handles of greater diameter. The
    diameter of the handle shall equal or exceed the value given in
    Exhibit 4.2.5.3.2 for the appropriate weight range. \[Source: AFSC
    DH 1-3, 1980; MIL-STD-1472F, 1999\]

> []{#__RefHeading___Toc27272139 .anchor}**Exhibit 4.2.5.3.2** Minimum
> handle diameter required by weight of unit of equipment

-   **4.2.5.3.3** **Finger curl.** The size and shape of a handle shall
    allow the user\'s fingers to curl around the handle at least 120°.
    \[Source: MIL-STD-1472F, 1999\]

#### 4.2.5.4 Location

The location of handles with respect to the center of gravity of a unit
of equipment determines the tendency of the unit to tip or sway when it
is lifted or carried. Placing the handles above the center of gravity
and placing pairs of handles on opposite sides of the unit on a line
passing through the center of gravity horizontally ensure the stability
of the unit. However, other considerations may outweigh these
\"balance\" considerations. For example, if a unit is intended to be
pulled out of a rack, its handles will probably be located on the front
of the unit.

-   **4.2.5.4.1** **Single handles.** A single handle should be located
    directly above the center of gravity of a unit of equipment.
    \[Source: MIL-STD-1472F, 1999; UCRL-15673, 1985\]

-   **4.2.5.4.2** **Pairs of handles.** The two handles of a pair of
    handles should be located on opposite sides of the unit of equipment
    on or above a line passing horizontally through the unit\'s center
    of gravity. \[Source: MIL-STD-1472F, 1999; UCRL-15673, 1985\]

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-   **4.2.5.4.3** **Exposure to hazards.** Handles shall be located so
    that their use does not expose a user to thermal or electrical
    hazards. \[Source: MIL-STD-1800A, 1990\]

-   **4.2.5.4.4** **Structural clearance.** Handles shall be located to
    provide a clearance of at least 50 mm (2 in) between the handle and
    any obstruction when the equipment is in its installed or
    maintenance position. \[Source: MIL-STD-1472F, 1999\]

### 4.2.6 Grasp areas

-   **4.2.6.1** **Location of grasp area.** Grasp areas should be
    located above the center of gravity of a unit of equipment so that
    the unit does not tend to tip or sway when it is lifted or carried.
    \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

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-   **4.2.6.2** **Grasp area finish.** Grasp areas shall have a nonslip
    finish. If the bottom of a unit of equipment is designed to serve as
    a grasp area, the bottom surface shall have a nonslip finish.
    \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990\]

-   **4.2.6.3** **Grasp area material.** The material used for the grasp
    area shall be sufficiently hard that grit and grime do not become
    embedded in it during normal use. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

-   **4.2.6.4** **Grasp area conductivity.** Grasp area material shall
    not conduct heat or electricity. \[Source: MIL-STD-1472F, 1999\]

### 4.2.7 Stands and rests

If a unit of equipment contains components that could be damaged easily
during maintenance, stands or rests might be provided to protect the
susceptible parts. If the unit has handles, the handles can be designed
to serve as the stands or rests.

-   **4.2.7.1 Prevention of damage.** If appropriate and practical,
    units of equipment that contain components that are susceptible to
    damage shall have stands or rests that protect the susceptible
    components when the unit is in its maintenance position. \[Source:
    AFSC DH 1-3, 1980; MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

-   **4.2.7.2** **Integral to chassis.** When provided, stands or rests
    shall be part of the basic chassis of the unit of equipment.
    \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990\]

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-   **4.2.7.3** **Handles as stands or rests.** If a unit of equipment
    requiring stands or rests has handles, the handles should be
    designed to serve as the stands or rests. \[Source: AFSC DH 1-3,
    1980\]

### 4.2.8 Alignment aids

Ideally, it would be impossible to install equipment incorrectly.
Alignment aids can help to achieve correct installation. The emphasis in
this section is on physical devices, such as guides and pins, but labels
can also serve as alignment aids.

-   **4.2.8.1** **Guides, tracks, and stops.** Guides, tracks, and stops
    shall be provided wherever appropriate to facilitate handling and to
    prevent damage to equipment and injury to users. \[Source:
    MIL-STD-1472F, 1999\]

-   **4.2.8.2** **Prevention of improper mounting.** Units of equipment
    shall include physical features (such as supports, guides, or
    alignment pins) that prevent improper mounting. \[Source: AFSC DH
    1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990; NASA-STD-3000A, 1989\]

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-   **4.2.8.3** **Alignment of light-weight units of equipment.** Units
    of equipment weighing less than 9 kg (20 lb) should have
    bottom-mounted alignment pins. \[Source: MIL-HDBK-759B, 1992\]

-   **4.2.8.4** **Alignment of heavy units of equipment.** Units of
    equipment weighing more than 9 kg (20 lb) should have side-aligning
    guides so that the unit can be slid into place. \[Source:
    MIL-HDBK-759B, 1992\]

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-   **4.2.8.5** **Labeling units of equipment.** Units of equipment
    shall have labels that identify their proper alignment, unless the
    alignment is immediately obvious. \[Source: MIL-STD-1472F, 1999\]

-   **4.2.8.6** **Labeling insertion holes.** If a unit of equipment has
    holes through which connectors or other objects that require proper
    alignment are inserted, the holes shall have labels showing proper
    alignment of the object to be inserted. \[Source: AFSC DH 1-3, 1980;
    UCRL-15673, 1985\]

### 4.2.9 Designing for remote handling

-   **4.2.9.1** **Alignment aids.** All units of equipment designed for
    remote handling shall have alignment aids. \[Source: MIL-STD-1800A,
    1990\]

-   **4.2.9.2** **Quick-action connectors.** All connectors on units of
    equipment designed for remote handling shall be of the quick-action
    type. \[Source: MIL-STD-1800A, 1990\]

-   **4.2.9.3** **Fasteners.** All fasteners on units of equipment
    designed for remote handling shall be of the captive type and shall
    be operable by remote handling techniques. \[Source: MIL-STD-1800A,
    1990\]

-   **4.2.9.4** **Latches.** All latches on units of equipment designed
    for remote handling shall: (a) be operable from a single point, (b)
    have positive catches, and (c) provide a clear visual indication of
    the latch position. \[Source: MIL-STD-1800A, 1990\]

### 4.2.10 Designing for use of hoists, jacks, and cranes

-   **4.2.10.1** **Location of assistance points.** The lifting eyes or
    jacking points shall be located so that the unit of equipment does
    not tilt or swing uncontrollably while it is being lifted. \[Source:
    MIL-STD-1472F, 1999\]

-   **4.2.10.2** **Labeling.** Lifting eyes and jacking points shall be
    labeled conspicuously. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1800A,
    1990\]

## 4.3 Packaging, arrangement, and mounting of equipment

> Equipment can be packaged, arranged, and mounted in a variety of ways.
> This section gives rules to narrow the choices. Several goals and
> principles underlie the requirements and rules of this section. They
> include:

a.  avoiding irregular, fragile, or awkward extensions to equipment, or,
    if they cannot be avoided, ensuring that such extensions are easy to
    remove,

b.  packaging equipment so that it can be handled by one person or as
    few people as possible,

c.  arranging different units of equipment so that maintenance by one
    specialist does not require moving or handling equipment maintained
    by another specialist,

d.  mounting equipment so that it is easily installed or removed and
    readily accessible for maintenance,

e.  mounting equipment in a way that minimizes the need for the user to
    bend, stretch, crawl, assume awkward positions, or move from place
    to place in performing maintenance tasks, and

f.  minimizing the need for tools, particularly specialized tools.

### 4.3.1 Unitization of equipment

> Unitization is the packaging of equipment in physically and
> functionally distinct units that can be easily removed and replaced.
> This sort of separation can have a number of advantages, such as
> providing easy access to malfunctioning equipment, allowing a high
> degree of standardization, simplifying and speeding equipment design
> by using previously developed standardized designs, and reducing the
> skill and training requirements for users.

**Definitions.** A **unit of equipment** is an assemblage of items that
may include modules, components, and parts that are packaged together
into a single hardware package. For example, a computer, its keyboard,
and its visual display are all units of equipment, as are radio
transmitters and receivers. A **module** is an assemblage of two or more
interconnected parts or components that comprise a single, physical and
functional entity. It is this singular functionality that defines a
module. A **component** is a subdivision of a unit of equipment that can
be treated as an object by the user, but which can be further broken
down into parts. A mounting board together with its mounted parts is an
example of a component. A **part** is an object that cannot normally be
broken down further without destroying its designated use. Fuses,
resistors, and capacitors are examples of parts. The **packaging** of a
unit of equipment is the assembling, mounting, and enclosing of the
items it includes.

-   **4.3.1.1** **Functional independence.** Units of equipment shall
    correspond to the functional design of the equipment and shall
    maximize the functional independence of each unit while minimizing
    the interaction between units (see Paragraph 4.1.2.5). \[Source:
    UCRL-15673, 1985\]

-   **4.3.1.2** **Packaging equipment.** Whenever possible, units of
    equipment shall be independent, interchangeable, and easy to
    replace. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999;
    UCRL-15673, 1985\]

-   **4.3.1.3** **Ease of installation.** All equipment shall be easy to
    mount and easy to connect to other equipment. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.3.1.4** **Independent adjustment.** Units of equipment shall be
    capable of being checked and adjusted separately; when
    interconnected with other units, they shall require little or no
    additional adjustment. \[Source: AFSC DH 1-3, 1980; UCRL-15673,
    1985\]

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-   **4.3.1.5** **Handling by one person.** Units of equipment should be
    installable and removable by one person (see Paragraph 4.2.2 on
    weight limits). \[Source: UCRL-15673, 1985\]

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-   **4.3.1.6** **Interconnectivity.** The number of inputs and outputs
    associated with a unit of equipment shall be minimized. \[Source:
    AFSC DH 1-3, 1980\]

-   **4.3.1.7** **Protrusions.** Any irregular protrusions on a unit of
    equipment, such as cables, waveguides, or hoses shall be easily
    removable to prevent damage during installation and maintenance.
    \[Source: AFSC DH 1-3, 1980\]

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-   **4.3.1.8** **Prevention of incorrect mounting.** Units of equipment
    should be designed so that they cannot be mounted incorrectly.
    \[Source: MIL-HDBK-759B, 1992\]

### 4.3.2 Interchangeability, non-interchangeability

Units of equipment may be interchangeable physically, functionally, or
both. This section contains rules that might be summarized in the
general statements that if two units of equipment are interchangeable
functionally, they will also be interchangeable physically; if they are
not interchangeable functionally, they will not be interchangeable
physically.

-   **4.3.2.1** **Interchangeability of equivalent units of equipment.**
    Units of equipment having the same form and function shall be
    interchangeable throughout a system and related systems. \[Source:
    UCRL-15673, 1985\]

-   **4.3.2.2** **Identifiable interchangeable units of equipment.**
    Interchangeable units of equipment shall be clearly identifiable and
    easily distinguishable from units that are similar, but not
    interchangeable. Identification methods might be physical (such as
    size, shape, and mounting provisions), visual (such as
    color-coding), or verbal (such as labeling). \[Source:
    MIL-STD-1472F, 1999\]

-   **4.3.2.3** **Non-interchangeability of nonequivalent units of
    equipment.** Units of equipment that are not functionally
    interchangeable shall not be physically interchangeable. \[Source:
    MIL-STD-1472F, 1999; UCRL-15673, 1985\]

-   **4.3.2.4** **Identifiable non-interchangeable units of equipment.**
    Non-interchangeable units of equipment shall be clearly identifiable
    and readily distinguishable from units that are interchangeable.
    Identification methods might be physical (such as size, shape, and
    mounting provisions), visual (such as color-coding), or verbal (such
    as labeling). \[Source: UCRL-15673, 1985\]

    []{.mark}

### 4.3.3 Mounting in drawers, on racks, and on hinges 

#### 4.3.3.1 General 

-   **4.3.3.1.1 Mounting frequently-moved units of equipment.** Units of
    equipment that must be moved frequently from their installed
    positions for maintenance shall be mounted in drawers, on sliding
    racks, or on hinges. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F,
    1999\]

-   **4.3.3.1.2** **Mounting of heavy units of equipment.** Heavy,
    relatively inaccessible units of equipment that must be inspected or
    maintained shall be mounted in drawers, on sliding racks, or
    equipment covers on hinges. \[Source: MIL-HDBK-759B, 1992\]

-   **4.3.3.1.3** **Access to rear or bottom of units of equipment.** If
    the maintainer must have access to the rear or bottom of units of
    equipment mounted in drawers, on sliding racks, or equipment covers
    on hinges, the units shall open or rotate fully and remain in that
    position (held by braces, for example) without being supported by
    the maintainer. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992;
    MIL-STD-1800A, 1990\]

-   **4.3.3.1.4** **Avoidance of instability.** If opening or extending
    a unit of equipment that is mounted in a drawer, on a sliding rack,
    or on hinges would shift the center of gravity of the mounting
    structure so that it becomes unstable, the structure shall be
    securely fastened. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990\]

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-   **4.3.3.1.5** **Attachment of equipment.** Units of equipment
    mounted in drawers, on sliding racks, or on hinges should be
    attached only to the drawer, rack, or hinge and to interconnecting
    lines and cables. \[Source: UCRL-15673, 1985\]

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-   **4.3.3.1.6** **Ease of moving mounted units of equipment.** When
    units of equipment are mounted in drawers, on sliding racks, or on
    hinges, the drawers, racks, or hinges shall be easy (require few
    operations) to open or extend. \[Source: UCRL-15673, 1985\]

-   **4.3.3.1.7** **Maximum force to move mounted units of equipment**.
    The force needed to open or rotate a drawer, slide, or hinged mount
    shall not exceed the values given in Paragraph 4.2.5.1.5. \[Source:
    UCRL-15673, 1985\]

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-   **4.3.3.1.8 Guards and shields.** If needed to protect fragile or
    sensitive components, drawers and racks should include guards or
    shields. \[Source: UCRL-15673, 1985\]

#### 4.3.3.2 Restraints and supports

Equipment mounted in drawers, on sliding racks, or on hinges must be
protected with stops or supports that prevent it from falling or tipping
over and that hold it in position both for operation and for
maintenance. These stops and supports must be easily overridden so that
the equipment can be easily removed and replaced.

-   **4.3.3.2.1** **Limit stops.** Limit stops shall be provided on all
    drawer, slide, or hinge mounted equipment that must be moved from
    its operating position to a maintenance position. \[Source: AFSC DH
    1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990; NASA-STD-3000A, 1989\]

    **Definition. Limit stops** are mechanisms that restrict a moving
    object or part by stopping it at predetermined (limit) positions.
    \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999; MIL-STD-1800A, 1990; NASA-STD-3000A, 1989\]

-   **4.3.3.2.2** **Automatic locks.** Drawers and slides shall lock
    automatically in both the operating and maintenance positions.
    \[Source: MIL-STD-1472F, 1999; UCRL-15673, 1985\]

-   **4.3.3.2.3** **Lock release.** The locks holding drawers and slides
    in the operating and maintenance positions shall be easy to release,
    preferably requiring only one hand to operate. \[Source: AFSC DH
    1-3, 1980; MIL-STD-1472F, 1999; MIL-STD-1800A, 1990; NASA-STD-3000A,
    1989\]

-   **4.3.3.2.4** **Supports for hinge-mounted equipment.**
    Hinge-mounted equipment shall have a means of support to hold it in
    both the operating and maintenance positions. \[Source: AFSC DH 1-3,
    1980; MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

#### 4.3.3.3 External connectors and interlocks 

-   **4.3.3.3.1** **Preservation of external connections.** If external
    connections are required for maintenance as well as for normal
    operation, mounting shall be designed so that these connections are
    not broken when the unit of equipment is slid or rotated into its
    maintenance position. \[Source: UCRL-15673, 1985\]

-   **4.3.3.3.2** **Breaking of external connections.** If it is
    required that external connections be broken for maintenance,
    interlocks shall be provided that break the connections when the
    equipment is slid or rotated into its maintenance position.
    \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

-   **4.3.3.3.3** **External connectors as part of supporting
    structure.** If equipment mounted in a drawer or on a sliding rack
    is intended to be removed and replaced by users, and if external
    connections are not required during maintenance, the drawer or rack
    shall be provided with connectors on the rear of the equipment that
    mate with connectors mounted on the structure. \[Source:
    MIL-STD-1472F, 1999\]

### 4.3.4 Positioning equipment

> Some general considerations affecting the positioning of equipment
> are:

a.  Avoid locations where the equipment or the user would be exposed to
    damage or injury.

b.  Avoid locations where the equipment or the user would be exposed to
    oil, dirt, or other contaminants.

c.  Choose the most accessible locations for the most frequently
    serviced equipment.

d.  Choose the most accessible locations for the heaviest or bulkiest
    equipment.

e.  Choose the most protected locations for the most fragile or
    sensitive equipment.

#### 4.3.4.1 Physical accessibility 

-   **4.3.4.1.1** **Complete visual and physical access.** Equipment
    shall be positioned so that the maintainer has complete visual and
    physical access to all parts of the equipment on which maintenance
    is performed; this includes access openings, adjustment points, test
    points, cables, connectors, labels, and mounting fasteners.
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990; NASA-STD-3000A, 1989\]

-   **4.3.4.1.2** **Freedom from structural obstruction.** Units of
    equipment shall be positioned so that neither visual nor physical
    access is obstructed by structural members or permanently installed
    equipment. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990; UCRL-15673, 1985\]

-   **4.3.4.1.3** **Working space.** Units of equipment shall be
    positioned so that there is sufficient space around them for the use
    of any tools and test equipment required for their maintenance
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; NASA-STD-3000A,
    1989\]

-   **4.3.4.1.4** **Room to open covers.** Units of equipment shall be
    positioned with sufficient clearance from other equipment and
    structures to permit unhindered opening of any covers that are
    opened during maintenance tasks. \[Source: AFSC DH 1-3, 1980\]

-   **4.3.4.1.5** **Stacking or blocking equipment.** Units of equipment
    shall not be stacked or placed in front of or behind other units;
    each unit shall be positioned so that it is both visually and
    physically accessible without the removal of another unit. \[Source:
    AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1800A, 1990;
    NASA-STD-3000A, 1989; UCRL-15673, 1985\]

-   **4.3.4.1.6** **Full extension or rotation.** Units of equipment
    mounted in drawers, on sliding racks, or on hinges shall be
    positioned so that the drawer, rack, or hinge can be opened or
    extended without hindrance. \[Source: MIL-STD-1472F, 1999;
    NASA-STD-3000A, 1989\]

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-   **4.3.4.1.7** **Working level.** Units of equipment should be
    positioned so that they are at the most favorable working level;
    this is usually between the maintainer\'s hip and shoulder height,
    from approximately 1 to 1.5 m (3 to 5 ft). \[Source: AFSC DH 1-3,
    1980; MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

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-   **4.3.4.1.8** **Visual access.** Units of equipment that require
    frequent visual inspection shall be positioned so that the
    components to be inspected (such as displays, test points, and
    labels) can be seen easily without the removal of any other
    equipment. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992\]

-   **4.3.4.1.9** **Removal path.** Units of equipment intended to be
    replaceable by maintainers shall be positioned so that they can be
    removed along a straight or moderately curved path, not along a
    sharply bent path. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F,
    1999\]

#### 4.3.4.2 Relative accessibility

In addition to considerations of physical accessibility, the positioning
of equipment is affected by its indispensability, the frequency with
which it is serviced or maintained, the relationship of one unit of
equipment to other units, and the difficulty with which it is serviced
or maintained.

-   **4.3.4.2.1** **Criticality of equipment.** The most critical units
    of equipment shall be the most accessible. Accessibility may be
    compromised for highly reliable critical equipment. \[Source:
    MIL-STD-1472F, 1999; NASA-STD-3000A, 1989\]

-   **4.3.4.2.2** **Frequency of access.** If criticality is not a
    factor, equipment requiring the most frequent servicing or
    maintenance shall be the most accessible. \[Source: AFSC DH 1-3,
    1980; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; NASA-STD-3000A,
    1989; UCRL-15673, 1985\]

-   **4.3.4.2.3** **Grouping to minimize movement.** Units of equipment
    maintained by the same person shall be positioned near each other
    (provided the operational grouping of equipment remains in close
    proximity) to minimize the amount of movement required of the
    maintainer. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992;
    MIL-STD-1472F, 1999\]

-   **4.3.4.2.4** **Non-interaction with equipment maintained by
    others.** Access to units of equipment maintained by one type of
    maintenance specialist shall not require moving equipment maintained
    by another type of specialist. When conflicts with Paragraphs
    4.3.4.2.1 through 4.3.4.2.3 occur, the previous paragraphs shall
    have priority. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

-   **4.3.4.2.5** **Difficulty of moving.** Units of equipment that are
    difficult to move shall not prevent convenient access to other
    units. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

-   **4.3.4.2.6** **Non-interaction with equipment not in need of
    maintenance.** It shall not be necessary to remove or disable an
    operable unit of equipment to obtain access to a unit requiring
    maintenance. \[Source: NASA-STD-3000A, 1989\]

### 4.3.5 Labeling and marking

Labels on equipment can be used to (a) identify the equipment, (b) state
warnings or cautions, (c) supply useful information, such as
instructions, the weight of the equipment, or calibration information,
and (d) record and supply historical data, such as periodic readings or
the date of servicing or replacement.

**Definitions.** A **label** is alphanumeric information that identifies
or describes an object. Labels can be printed directly on or adjacent to
the object, or they can be printed on a card or plate that is attached
to the object or adjacent to the object. **Marking** is nonverbal
information, such as colors or symbols, which identifies or describes an
object. Marking can appear directly on or adjacent to the object, or it
can be printed on a card or plate that is attached to the object or
adjacent to the object.

#### 4.3.5.1 Types of labels

-   **4.3.5.1.1** **Equipment identification.** All units of equipment
    shall have identifying labels. These labels shall be securely
    attached, permanent, nonfading, oil-, gasoline-, and
    corrosion-resistant, and shall include all of the following that are
    applicable: (a) contract order or task number, (b) equipment
    name, (c) specification number, (d) manufacturer\'s part number, (e)
    serial number, (f) manufacturer\'s name and address, and (g) if
    necessary, stock number. \[Source: UCRL-15673, 1985\]

    **Discussion.** Note that OSHA 29 CFR 1910.303 (e) requires that
    electrical equipment itself must have a manufacturers name or
    trademark or other descriptive material which identifies product
    responsibility and includes markings giving voltage, current,
    wattage or other ratings. \[Source: 29 CFR 1910.303; UCRL-15673,
    1985\]

-   **4.3.5.1.2** **Hazard labels.** If any hazard exists in servicing
    or maintaining a unit of equipment, the equipment shall have a
    warning label attached that describes the hazard. \[Source:
    MIL-HDBK-759B, 1992\]

    **Discussion.** Electrical equipment that is to be used in hazardous
    locations must be marked to show the hazardous location class and
    group (from National Fire Protection Association 70) and operating
    temperatures. \[Source: MIL-HDBK-759B, 1992\]

-   **4.3.5.1.3** **Weight labels.** Weight and center of gravity
    caution placards shall be placed on any unit of equipment to be
    moved for maintenance if its weight exceeds 13.6 kg (30 lbs). If it
    is designed to be lifted or carried by more than one person, the
    label shall include the number of people recommended to lift or
    carry it (see Paragraph 4.2.2.10). \[Source: MIL-HDBK-759B, 1992;
    MIL-STD-1800A, 1990\]

-   **4.3.5.1.4** **Instruction labels.** If there are critical
    instructions for the servicing or maintenance of a unit of
    equipment, and if these instructions are not likely to be available
    through other means, they shall be provided in a label on the
    equipment. \[Source: UCRL-15673, 1985\]

-   **4.3.5.1.5** **Data labels.** If there are critical data that must
    be available to or recorded by the maintainer of a unit of
    equipment, and if there is no other provision for them, a label
    shall be provided for these data. \[Source: UCRL-15673, 1985\]

#### 4.3.5.2 Location and orientation 

-   **4.3.5.2.1** **Readability.** Equipment labels shall be located so
    that they are visible and readable with the equipment in its
    installed position. \[Source: UCRL-15673, 1985\]

-   **4.3.5.2.2** **Preserving readability.** Equipment labels shall be
    located so that they will not become obscured by dirt, moisture, or
    other foreign materials. If these materials are likely to
    accumulate, the labels shall be mounted on a vertical surface.
    \[Source: UCRL-15673, 1985\]

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-   **4.3.5.2.3** **Consistent location.** Labels on similar units of
    equipment should be placed in approximately the same location on
    each. \[Source: UCRL-15673, 1985\]

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-   **4.3.5.2.4** **Horizontal orientation.** Labels shall be oriented
    so that alphanumeric characters are read horizontally, not
    vertically. \[Source: UCRL-15673, 1985\]

#### 4.3.5.3 Typographic matters

-   **4.3.5.3.1** **Character height for viewing distance.** Unless
    circumstances require otherwise, labels shall be clearly legible at
    a viewing distance of 710 mm (28 in). The recommended height for
    letters and numerals at this distance is approximately 5 mm (.18
    in). Exhibit 4.3.5.3.1 gives minimum character heights for other
    viewing distances. \[Source: UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272140 .anchor}**Exhibit 4.3.5.3.1** Minimum
> character height for various viewing distances

-   **4.3.5.3.2** **Stroke width in normal illumination.** If labels are
    expected to be read under normal illumination, characters shall be
    black on a white or light background, and stroke width shall be 1/6
    to 1/7 of the height. \[Source: UCRL-15673, 1985\]

-   **4.3.5.3.3** **Stroke width in dim illumination.** If labels are
    expected to be read under dim illumination, characters shall be
    white on a black or dark background, and stroke width shall be from
    1/7 to 1/8 of the height. \[Source: UCRL-15673, 1985\]

-   **4.3.5.3.4** **Width to height ratios.** The width to height ratio
    of letters and numerals shall be 3:5 with the exceptions of \"M\"
    and \"W,\" which shall be 4:5, \"4,\" which shall be one stroke
    width wider, and \"I\" and \"1,\" which shall be one stroke wide.
    \[Source: UCRL-15673, 1985\]

-   **4.3.5.3.5** **Character spacing.** The spacing between characters
    shall be at least one stroke width. \[Source: UCRL-15673, 1985\]

-   **4.3.5.3.6** **Word spacing.** The spacing between words shall be
    approximately the width of one normal-width character. \[Source:
    UCRL-15673, 1985\]

-   **4.3.5.3.7** **Line spacing.** The spacing between lines shall be
    at least one-half the character height. \[Source: UCRL-15673, 1985\]

-   **4.3.5.3.8** **Case of letters.** If the text on a label is
    exclusively single words, such as names, the words shall appear as
    all capital letters; if the text is phrases or sentences, the text
    shall appear as mixed case letters. \[Source: UCRL-15673, 1985\]

-   **4.3.5.3.9** **Text and background combinations.** Text and
    background combinations shall provide sufficient contrast to ensure
    legibility. Black characters may appear on white, yellow, light
    gray, matte-finished brass or aluminum, or any bright plated
    backgrounds; white characters may appear on dark backgrounds; other
    acceptable combinations are blue on white, green on white, green on
    red, and red on yellow. \[Source: UCRL-15673, 1985\]

#### 4.3.5.4 Wording

-   **4.3.5.4.1** **Consistency.** Designations and terms used on labels
    shall be consistent with designations and terms in user
    documentation and parts catalogs. \[Source: UCRL-15673, 1985\]

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-   **4.3.5.4.2** **Wording.** The wording of labels should be brief but
    explanatory, using words that are familiar to maintainers.
    Abbreviations and abstract terms should be used only if it can be
    reasonably expected that they will be known to all maintainers.
    \[Source: UCRL-15673, 1985\]

-   **4.3.5.4.3** **Instructions.** Labels containing a series of steps
    to be carried out should list the steps, not present them in
    paragraph form. \[Source: UCRL-15673, 1985\]

#### 4.3.5.5 Markings

-   **4.3.5.5.1** **Number of color codes.** If color-coding is used,
    the number of different colors shall not exceed nine. \[Source:
    UCRL-15673, 1985\]

-   **4.3.5.5.2** **Recommended colors.** If color-coding is used, the
    colors shall be distinguishable by both color-normal and
    color-deficient persons. Colors meeting this criterion are given in
    Exhibit 4.3.5.5.2. \[Source: UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272141 .anchor}**Exhibit 4.3.5.5.2**
> Recommended colors

-   **4.3.5.5.3** **Arrows.** Arrows used in labels or markings should
    be clearly recognizable and easily identifiable from a distance.
    Sharp angles and a tapered overall shape are preferable to wide
    angles and a relatively uniform overall shape (see Exhibit
    4.3.5.5.3). \[Source: UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272142 .anchor}**Exhibit** **4.3.5.5.3** Good
> and bad arrows

## 4.4 Access openings

This section contains rules for access openings, that is, openings in a
case, cover, panel, or door through which a maintainer requires visual
or physical access or both, to perform maintenance tasks. Rules are
given for properties of the openings, how to ensure both visual and
physical access, the size, shape, and location of the openings, and
their labeling. Rules regarding covers for access openings are given in
Section 4.5.

### 4.4.1 General

-   **4.4.1.1** **When an access opening is required.** An access
    opening shall be provided whenever a maintenance task would
    otherwise require removing a case or covering, opening a fitting, or
    dismantling a unit. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F,
    1999; MIL-STD-1800A, 1990; UCRL-15673, 1985\]

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-   **4.4.1.2** **Number of openings.** One large opening should be
    provided rather than several small ones. \[Source: NASA-STD-3000A,
    1989; UCRL-15673, 1985\]

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-   **4.4.1.3** **Prevention of injury or damage.** The edges of access
    openings shall be either (a) sufficiently rounded and smoothly
    finished or (b) covered or coated sufficiently to prevent injury to
    the maintainer\'s person, clothing, and equipment. \[Source: AFSC DH
    1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990; NASA-STD-3000A, 1989\]

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-   **4.4.1.4** **Uncovered openings.** When environmental, operational,
    and safety conditions permit, openings should be left uncovered.
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; UCRL-15673,
    1985\]

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-   **4.4.1.5** **Unacceptability of rivets.** Riveted panels or doors
    shall not be used to cover access openings. Quick-action fasteners
    shall be used except in cases when the panel or door is subjected to
    stress or pressure, in which case screws shall be used. \[Source:
    MIL-HDBK-759B, 1992\]

### 4.4.2 Access

-   **4.4.2.1** **Visual and physical access.** If a maintainer must see
    what he or she is doing inside the opening, then either the opening
    shall be large enough and positioned so that the maintainer has the
    necessary view, or separate openings shall be provided for visual
    and physical access. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999; MIL-STD-1800A, 1990; NASA-STD-3000A, 1989; UCRL-15673, 1985\]

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-   **4.4.2.2** **Visual access only.** If a maintenance task requires
    only visual access, the access opening should be designed and
    positioned so that the maintainer can see whatever is needed without
    removing panels or other components. Such openings should not
    compromise personnel safety. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

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-   **4.4.2.3** **Physical access only.** Physical access without visual
    access, that is, access in which the maintainer cannot see what he
    or she is doing inside an access opening, shall not be provided
    without approval of the acquisition program office. \[Source:
    MIL-STD-1472F, 1999\]

### 4.4.3 Size

-   **4.4.3.1** **Accommodation.** An access opening shall be large
    enough to accommodate whatever combination of components, tools,
    body parts, clothing, and movements is required to perform the task.
    \[Source: MIL-STD-1472F, 1999\]

-   **4.4.3.2** **Dimensions for one- or two-finger access.** Dimensions
    of openings intended to allow access by one or two fingers shall
    equal or exceed those given in Exhibit 4.4.3.2. \[Source:
    MIL-STD-1472F, 1999\]

> **Exhibit 4.4.3.2** Minimum dimensions of openings designed for access
> by one or two fingers without visual access \[Source: MIL-STD-1472F,
> 1999; AFSC DH 1-3, 1980\]

-   **4.4.3.3** **Dimensions for one hand or arm access.** Dimensions
    for openings intended to allow access by one hand or one arm shall
    equal or exceed those given in Exhibit 4.4.3.3. \[Source:
    MIL-STD-1472F, 1999; AFSC DH 1-3, 1980\]

**Exhibit 4.4.3.3** Minimum dimensions of openings designed for access
by one hand or arm without visual access.

-   **4.4.3.4 Dimensions for two-hand access.** Dimensions of openings
    intended to allow access by two hands shall equal or exceed those
    given in Exhibit 4.4.3.4. \[Source: MIL-STD-1472F, 1999; AFSC DH
    1-3, 1980\]

> []{#__RefHeading___Toc27272145 .anchor}**Exhibit 4.4.3.4** Minimum
> dimensions of openings designed for access by two hands without visual
> access

### 4.4.4 Shape

As with size, the shape of an access opening is influenced by (a) the
body appendages and equipment that will pass through the opening, (b)
the movements the maintainer will perform inside the opening, and (c)
the maintainer\'s need for visual access through the opening. The shape
need not be a conventional shape such as a circle or rectangle.

-   **4.4.4.1** **Shape appropriate to task.** The shape of an access
    opening shall allow the maintainer to perform those tasks requiring
    access through the opening. \[Source: AFSC DH 1-3, 1980;
    MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; NASA-STD-3000A, 1989;
    UCRL-15673, 1985\]

### 4.4.5 Location 

-   **4.4.5.1** **On accessible surfaces.** Access openings shall be
    located on equipment surfaces that are accessible when the equipment
    is in its normal operating position. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.4.5.2** **Near related displays, controls, and connectors.**
    Access openings shall be located within easy view and reach of any
    test points, displays, controls, or connectors that require access
    in performing a particular maintenance task. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.4.5.3** **Away from hazards.** Access openings shall be located
    at a safe distance or shielded from any hazards such as high
    voltages or dangerous moving parts to which the maintainer might be
    exposed. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.4.5.4** **Comfortable for maintainer.** Access openings shall be
    located so that they do not require undue bending, stretching or
    other awkward body postures of the maintainer while he or she
    performs required tasks. \[Source: MIL-HDBK-759B, 1992;
    MIL-STD-1472F, 1999; UCRL-15673, 1985\]

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-   **4.4.5.5** **Easy removal of components.** Access openings should
    be located so that heavy or bulky components can be pulled out
    rather than lifted out. \[Source: MIL-HDBK-759B, 1992; UCRL-15673,
    1985\]

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-   **4.4.5.6** **Conformance with related items.** If work stands or
    carts are used in the maintenance tasks, access openings shall
    conform to the height of the stands or carts. \[Source: UCRL-15673,
    1985\]

-   **4.4.5.7** **Free of obstructions.** Access openings shall be
    located so that it is not necessary to remove any components or
    wires to reach them. \[Source: MIL-HDBK-759B, 1992; UCRL-15673,
    1985\]

-   **4.4.5.8** **Impervious to environmental conditions.** Access
    openings shall be located so that environmental conditions such as
    rain, snow, and ice neither interfere with access nor damage
    components when the access is open. \[Source: MIL-HDBK-759B, 1992\]

### 4.4.6 Labeling and marking

The rules in this section apply only to labeling and marking access
openings; if the opening has a cover, the relevant rules are given in
Paragraphs 4.5.4 and 4.5.5.

-   **4.4.6.1** **Identification of opening.** Each access opening shall
    be labeled with a name, number, letter or other symbol and referred
    to by that identification in maintenance instructions. \[Source:
    AFSC DH 1-3, 1980; UCRL-15673, 1985\]

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-   **4.4.6.2** **Identification of accessible components and
    maintenance tasks.** Each access opening should be labeled with
    identifiers for the maintainable components accessible through the
    opening. This labeling may also include information about equipment
    to be used in the maintenance tasks and procedural information about
    the tasks themselves. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F,
    1999; UCRL-15673, 1985\]

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-   **4.4.6.3** **Warning labels.** If any hazardous condition exists
    inside an access opening (such as high voltages or dangerous moving
    parts), the opening shall have a conspicuous warning label advising
    the maintainer of the hazard and stating any necessary precautions.
    \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999; UCRL-15673, 1985\]

## 4.5 Cases, covers, guards, and shields 

This section contains rules governing the size, mounts, shape, location,
fasteners, interlocks, and labels of cases, covers, guards, and shields.

**Definitions.** A **case** is the part of a unit of equipment that
encloses and protects the equipment from its surroundings. It may also
serve to protect the surroundings - including users - from the
equipment. A **cover** is a part of a unit of equipment that closes an
access opening. A **guard** is an enclosure or barrier intended to
prevent inadvertent or unauthorized operation of a control. A **shield**
is an enclosure or barrier intended to protect components that are
susceptible to damage or to protect maintainers from possible injury.

Covers, guards, and shields may take a variety of forms, including (a)
hinged doors or caps, (b) sliding doors or caps, (c) removable doors or
caps, (d) removable panels, and (e) physical barriers.

Hinged doors and caps allow the fastest and easiest access; they require
few fasteners and, depending upon their orientation, may not need
support in the open position. They do require \"swinging\" space, and
they may intrude on the user\'s workspace.

Sliding doors or caps are particularly useful where \"swinging\" space
is limited. They do not generally provide a tight seal.

Removable doors or caps require little space for opening and, once
removed, do not interfere with working space. Handling them does take
time and effort.

Removable panels can give access to large portions of a unit of
equipment. They do not require \"swinging\" space, but they may be
awkward to handle or susceptible to damage.

### 4.5.1 General

-   **4.5.1.1 Fasteners.** Cases and covers should be fastened in
    accordance with Section 4.6 Fasteners.

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-   **4.5.1.2** **Preferred type of cover.** The cover of an access
    opening shall be appropriate to the type of access required and the
    prevailing environmental conditions as outlined in Exhibit 4.5.1.2.
    \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999; NASA-STD-3000A,
    1989; UCRL-15673, 1985\]

[]{#__RefHeading___Toc27272146
.anchor}![](media/image10.png){width="6.395833333333333in"
height="2.8201388888888888in"} **Exhibit 4.5.1.2** Type of covering for
type of access and environmental conditions

-   **4.5.1.3** **Accessibility.** Cases shall be designed so that they
    can be opened, removed, and replaced without dismantling the
    equipment or associated equipment and items. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.5.1.4 How to open.** It shall be clear to the user how to open a
    case or cover, either through a property of the case or cover
    itself, such as its shape, or by the provision of instructions on or
    near the case or cover. \[Source: AFSC DH 1-3, 1980\]

-   **4.5.1.5 Lift case, not equipment.** The case for a unit of
    equipment shall be designed so that the case is lifted off the
    equipment, not so that the equipment is lifted out of the case.
    \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999\]

-   **4.5.1.6 Ease of removal and replacement.** Cases shall be easy to
    open, remove, and replace. \[Source: AFSC DH 1-3, 1980\]

-   **4.5.1.7 Accessibility upon opening or removal.** Cases and units
    of equipment shall be designed so that when a case is opened or
    removed, all portions of the equipment that are relevant to the user
    task are accessible. The opened case shall not obscure or interfere
    with any controls, displays, test points, service points, or
    connections relevant to the user task. \[Source: MIL-HDBK-759B,
    1992\]

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-   **4.5.1.8 Minimizing need for removal.** Cases and equipment should
    be designed to minimize the need to remove the case. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

    **Example.** Adjustment controls, test points, and service points
    might be made accessible without requiring opening or removing the
    case. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.5.1.9** **Ease of opening.** Cases, covers, and fasteners should
    be selected or designed so that their combination makes the easiest
    to remove, open, and close while meeting the closure and structural
    requirements of the equipment. \[Source: AFSC DH 1-3, 1980;
    UCRL-15673, 1985\]

    **Example.** A case or cover that is opened relatively frequently
    but that does not need to be sealed tightly might be a hinged door
    with a quick-release latch. \[Source: AFSC DH 1-3, 1980; UCRL-15673,
    1985\]

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-   **4.5.1.10** **Fastened-unfastened indication.** Cases and covers
    shall be designed or mounted so that it is clear whether or not they
    are fastened when they are in place. \[Source: AFSC DH 1-3, 1980;
    MIL-STD-1472F, 1999; UCRL-15673, 1985\]

    **Example.** Cases and covers might be spring loaded so that they
    stay open when they are not fastened. \[Source: AFSC DH 1-3, 1980;
    MIL-STD-1472F, 1999; UCRL-15673, 1985\]

-   **4.5.1.11** **Handles or grasp areas.** If a case or cover is heavy
    or difficult to open, remove, or replace, it shall have one or more
    handles, grasp areas, lifting eyes, or a combination of these.
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990; UCRL-15673, 1985\]

-   **4.5.1.12** **Accommodate gloves.** If present, handles and grasp
    areas shall accommodate any gloves or other special clothing the
    user might be expected to wear. \[Source: MIL-HDBK-759B, 1992;
    MIL-STD-1472F, 1999; MIL-STD-1800A, 1990; UCRL-15673, 1985\]

-   **4.5.1.13** **Shift in balance of equipment.** Hinged or sliding
    doors shall not unbalance equipment when they are opened. \[Source:
    MIL-HDBK-759B, 1992\]

    **Discussion.** If an imbalance would otherwise result, the case or
    cover or the equipment might have some sort of prop or support to
    prevent the unbalance. \[Source: MIL-HDBK-759B, 1992\]

-   **4.5.1.14** **Stops and retaining devices.** Attached cases and
    covers shall have stops or retaining devices that hold them in both
    the open and closed positions. These stops and retainers shall be
    appropriate to the ambient environment. \[Source: MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

    **Example.** An access door on equipment out of doors and subject to
    wind would need a more secure restraint than one on equipment
    located indoors. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.5.1.15** **Ventilation holes.** If a case, cover, or shield
    requires ventilation holes, the holes shall be small enough to
    prevent inadvertent insertion of objects that might touch high
    voltage sources or moving parts. \[Source: AFSC DH 1-3, 1980\]

-   **4.5.1.16** **Rounded edges.** The corners and edges of cases and
    covers shall be smooth or rounded so that they do not injure the
    user or damage his or her clothing. \[Source: AFSC DH 1-3, 1980;
    MIL-STD-1472F, 1999; UCRL-15673, 1985\]

-   **4.5.1.17** **Small removable covers.** Small removable caps or
    covers that might be susceptible to dropping or loss, perhaps with
    resulting damage to components inside the opening, shall be attached
    to the equipment or structure. \[Source: MIL-HDBK-759B, 1992;
    MIL-STD-1472F, 1999; UCRL-15673, 1985\]

-   **4.5.1.18 Alignment aids.** If a case is heavy or awkward to
    handle, or if it encloses delicate, sensitive components, the case
    or the equipment shall have guide pins, tracks, or some other
    alignment device to help guide the case while it is being opened,
    removed, or replaced. \[Source: MIL-HDBK-759B, 1992\]

-   **4.5.1.19 Sealing material.** If the equipment design includes
    sealing material between the case and the base to which it is
    attached, the sealing material and its mounting method shall be
    selected so that the material is not damaged while the case is being
    opened, removed, or replaced. \[Source: AFSC DH 1-3, 1980;
    MIL-HDBK-759B, 1992\]

-   **4.5.1.20 Accessible with equipment in installed position.** When
    maintenance tasks require that a cover be opened with the equipment
    in its installed position, both the cover and its fasteners shall be
    located so that they are visually and physically accessible with the
    equipment in that position. \[Source: AFSC DH 1-3, 1980;
    MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

### 4.5.2 Size

-   **4.5.2.1** **Size of covers.** Cases and covers shall be as small
    and light in weight as possible while meeting the closure,
    structural, and ease of maintenance requirements of the equipment.
    \[Source: UCRL-15673, 1985\]

    **Discussion.** It is most desirable that covers be openable,
    removable, and transportable with one hand; next most desirable that
    they require handling by only one person; least desirable, that they
    require handling by two or more people. \[Source: UCRL-15673, 1985\]

-   **4.5.2.2** **Precise movements not required.** Cases shall be
    sufficiently larger than the items they cover so that they can be
    opened, removed, and replaced without requiring precise movements on
    the part of the user. \[Source: MIL-STD-1472F, 1999\]

-   **4.5.2.3** **Clearance between case and components.** Cases shall
    be large enough that they do not damage internal wiring or
    components when they are opened, removed, or replaced. \[Source:
    AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999\]

### 4.5.3 Shape 

-   **4.5.3.1** **Appropriate to opening.** The shape of a case or cover
    shall be appropriate to the opening or item it covers and shall
    provide the degree of closure required. \[Source: MIL-HDBK-759B,
    1992\]

-   **4.5.3.2** **Proper orientation.** If a removable access cover
    requires a particular orientation, the cover shall be designed to
    prevent attachment in any other orientation. \[Source: UCRL-15673,
    1985\]

    **Example.** This might be accomplished by: (a) giving the cover an
    asymmetric or irregular shape, (b) including alignment guides or
    pins, or (c) arranging the holes for fasteners asymmetrically.
    \[Source: UCRL-15673, 1985\]

### 4.5.4 Hinged covers

-   **4.5.4.1** **Safe operation.** Hinged cases and covers shall be
    designed so that opening and closing them will not interfere with,
    damage, or have the potential for harmful contact with wires or
    other components. \[Source: UCRL-15673, 1985\]

-   **4.5.4.2** **Self-supporting.** Hinged cases and covers shall have
    stops or retainers that hold them in the open position. These stops
    shall also prevent the case or cover from swinging into or falling
    on fragile equipment, from swinging into the users themselves, and
    from springing the hinges. \[Source: MIL-HDBK-759B, 1992;
    MIL-STD-1472F, 1999; NASA-STD-3000A, 1989; UCRL-15673, 1985\]

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-   **4.5.4.3** **Operable with one hand.** The user should be able to
    open and close a hinged case or cover using only one hand. \[Source:
    MIL-STD-1472F, 1999\]

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-   **4.5.4.4** **Noninterference of open cover with accessibility.**
    Hinged and sliding covers shall be located so that when they are
    open, they do not interfere with access to the openings themselves,
    or to related controls, displays, test points, and the like.
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; UCRL-15673,
    1985\]

### 4.5.5 Sliding doors and caps 

-   **4.5.5.1** **Safe operation.** Sliding doors and caps shall be
    designed so that opening and closing them will not interfere with,
    damage, or have the potential for harmful contact with wires or
    other components. \[Source: UCRL-15673, 1985\]

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-   **4.5.5.2** **Positive locking.** Sliding doors and caps should lock
    in the closed position, giving the user feedback, perhaps with a
    \"snap\" action or an audible click, when they are closed. They
    should also lock and give feedback in the open position unless they
    are removed when opened. \[Source: UCRL-15673, 1985\]

-   **4.5.5.3** **Non-jamming.** Sliding doors and caps should not bind
    or jam while being opened or closed. \[Source: UCRL-15673, 1985\]

-   **4.5.5.4** **Easy hand operation.** Sliding doors and caps should
    be easy to open and close without the use of tools. \[Source:
    UCRL-15673, 1985\]

### 4.5.6 Interlocks

Interlocks are distinguished from lockouts and tagouts.

**Definitions.** **Interlocks** are devices (for example, switches)
connected with a cover, shield, or case that disable the associated
internal hazard (usually electrical) when the cover, shield, or case is
opened. OSHA regulations discuss lockout and tagout procedures to be
used in the workplace during maintenance or operations to protect from
electrical hazards. A **lockout** uses a mechanical means to disable a
control or switch in its safe position (for example, electricity
disconnected) and to prevent its activation without the use of undue
force or tools. **Tagouts** are tags that are attached to a control or
place of hazard to identify the required control condition and hazard
associated with an ongoing mode of operation or maintenance.

-   **4.5.6.1** **Protection from hazards.** If a hazardous condition
    (such as a high voltage or moving parts) exists inside a case or
    behind a cover or shield, that cover or shield shall have an
    interlock that disables the hazard (including both ac and dc power
    sources) when the case, cover, or shield is removed or opened.
    \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990; UCRL-15673, 1985\]

    **Discussion.** Human protection from hazardous conditions with
    unexpected energization or release of stored energy is treated in
    OSHA 29 CFR 1910.301 -308, 331 -335, and 399. The OSHA 29 CFR
    1910.333 requires the simultaneous use of both tagout and lockout in
    the workplace; OSHA 29 CFR 1910.333 states that interlocks shall not
    be the sole means of de-energizing circuits of equipment and are not
    substitutes for lockout and tagout procedures and practice.
    \[Source: AFSC DH 1-3, 1980; 29 CFR 1910.301 -308, 331 -335, and
    399; MIL-STD-1472F, 1999; MIL-STD-1800A, 1990; UCRL-15673, 1985\]

-   **4.5.6.2 Interlock override switch.** If a task requires that a
    user work on hazardous equipment that is equipped with a disabling
    interlock, the equipment shall have an interlock override switch
    that permits manual bypassing or overriding of the interlock when
    the cover is open. This override switch shall automatically reset to
    the non-by-pass position, which is the safe operating position when
    the case or cover is replaced. OSHA 29 CFR 1910.333 requires that
    only qualified personnel be allowed to disable an interlock.
    \[Source: AFSC DH 1-3, 1980; 29 CFR 1910.333; UCRL-15673, 1985\]

-   **4.5.6.3** **Labeling covers with interlocks.** If a case, cover,
    or shield has an interlock, a label stating the presence of both the
    hazard and the interlock shall be placed on the equipment or the
    case or cover so that it is visible both when the case or cover is
    in place and when it is open or removed. \[Source: MIL-STD-1472F,
    1999\]

**Discussion.** It is important that the label contain sufficient
information, particularly if there is more than one hazard present. For
example, if there is the possibility of an electrical hazard from either
ac or dc current, such as when a system has battery backup, the label
must inform of both hazards.

### 4.5.7 Labeling and marking

-   **4.5.7.1** **Method of opening.** If the method for opening a case
    or cover is not obvious, a label with opening instructions shall be
    attached to the outside of the cover itself or to the equipment
    adjacent to the cover. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F,
    1999; UCRL-15673, 1985\]

-   **4.5.7.2** **Hazard labels.** If a hazardous condition exists
    inside a case or behind a cover, a label fully describing the hazard
    shall be attached to the case or cover itself or adjacent to the
    case or cover. Opening or removing the case or cover shall neither
    remove nor visually obstruct the label. \[Source: MIL-STD-1472F,
    1999\]

-   **4.5.7.3** **Instructional labels.** If instructions are provided
    regarding maintenance tasks, components, test points or service
    points inside a case or opening, they shall be oriented horizontally
    with respect to the user and attached to the case or cover itself or
    adjacent to the equipment so that the instructions remain visible
    when the case or cover is opened. \[Source: MIL-STD-1472F, 1999\]

## 4.6 Fasteners

This section contains general rules for fasteners and specific rules for
nuts and bolts, screws, the heads of bolts and screws, latches and
catches, and other fastening devices. Additional rules are provided for
quick fastening and releasing devices, captive fasteners, and the
labeling and marking of fasteners.

**Definition.** **Fasteners** are devices that join, attach, and mount
parts, components, cases, covers, and units of equipment. They include
quick fastening and releasing devices, screws, bolts, latches, catches,
rivets, retainer rings, and retainer chains. \[Source: MIL-HDBK-759B,
1992; UCRL-15673, 1985\]

Fasteners are available in a wide variety of types and sizes, and new
types appear frequently. Designers are advised to review the varieties
available before selecting fasteners for a particular application.
Factors influencing the choice of a fastener for a particular
application include (a) the stress and environmental factors the
fastener must withstand, (b) the tools and clearance required to fasten
and release the fastener, (c) the frequency with which the fastener will
be fastened and released, (d) the speed with which the fastener must be
fastened and released, (e) the types and varieties of other fasteners
used in that and related applications, and (f) the clothing, especially
gloves or mittens, the maintainer may be expected to wear. \[Source:
MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

### 4.6.1 General

-   **4.6.1.1** **Fastener security.** Fasteners shall hold securely in
    their operating environment, for example, withstanding the effects
    of vibration, wind gusts, and pressure. \[Source: MIL-STD-1472F,
    1999\]

-   **4.6.1.2** **Number and ease of opening.** The fasteners for a
    given application shall be the fewest in number and the simplest to
    operate that meet the closure, structural, and ease of maintenance
    requirements for the application. \[Source: MIL-STD-1472F, 1999\]

-   **4.6.1.3** **Common fasteners.** To the extent possible, fasteners
    shall be interchangeable throughout a given application. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.1.4** **Self-alignment.** Fasteners shall be selected or
    designed so that they are easily aligned with their retaining
    catches, nuts, blocks, or inserts. This alignment shall occur
    without binding and without damage to fastener threads or
    receptacles. \[Source: UCRL-15673, 1985\]

-   **4.6.1.5** **Operable by hand or common hand tools.** Fasteners
    shall be operable by hand if possible; otherwise, by common hand
    tools. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

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-   **4.6.1.6** **Open-closed indication.** Fasteners should give a
    clear indication that they are open (unfastened) or closed
    (fastened). \[Source: MIL-STD-1472F, 1999\]

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-   **4.6.1.7** **Hole size.** The holes through which fasteners pass
    shall be large enough to permit inserting or \"starting\" the
    fasteners even when parts are not perfectly aligned. \[Source: AFSC
    DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1800A, 1990\]

-   **4.6.1.8** **Fastener variety.** The variety of fasteners used in a
    particular application (that is, the number of different types and
    sizes) shall be the minimum that meets the requirements for closure,
    structure, and ease of maintenance. These requirements include such
    aspects as stress, bonding, pressure, temperature, and shielding.
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; UCRL-15673,
    1985\]

    **Discussion.** Minimizing the variety of fasteners simplifies the
    stocking of spare parts and reduces the danger that maintainers will
    damage fasteners or equipment by using the wrong tool or the wrong
    fastener. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999;
    UCRL-15673, 1985\]

-   **4.6.1.9** **When different fasteners are required.** If removal or
    insertion of a wrong fastener could result in damage to equipment or
    a change in calibration settings, distinguishably different
    fasteners shall be used. \[Source: MIL-STD-1472F, 1999\]

-   **4.6.1.10** **Different fasteners must be distinguishable.** If
    more than one type of fastener is required for a unit of equipment:

a.  The different types shall be easily distinguishable from each other,
    for example, screws with different threads might also be different
    in physical size. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992\]

b.  The fastener-receptacle interface shall permit the maintainer to
    distinguish the intended location of each fastener easily. \[Source:
    MIL-STD-1472F, 1999\]

-   **4.6.1.11** **Location of fasteners.** Fasteners shall be located
    so that they

a.  are easily accessible to the maintainer without requiring the
    removal of other parts or units,

b.  can be operated with little or no interference from other
    structures,

c.  do not interfere with each other or with other components,

d.  are not hazardous to maintainers or potentially damaging to wires or
    hoses, and

e.  have adequate clearance to permit easy hand or tool operation.
    \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.1.12** **Strength of hand-operated fasteners.** Fasteners that
    are normally operated by hand shall be strong enough to withstand
    being operated with a tool. \[Source: MIL-HDBK-759B, 1992\]

-   **4.6.1.13** **Painted or coated fasteners.** If fasteners are
    painted or coated, the paint or coating shall not adversely affect
    their removal or installation. \[Source: NASA-STD-3000A, 1989\]

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-   **4.6.1.14** **Precise torque requirements.** Equipment should be
    designed so that precise torque on fasteners is not required. If it
    is required, fasteners that incorporate torque indications should be
    used. \[Source: AFSC DH 1-3, 1980\]

    **Examples.** Some examples of fasteners that incorporate torque
    indications are (a) nuts that break away, (b) crushable washers that
    give a visual indication that correct torque has been reached,
    and (c) tools that crimp the nut and achieve the correct torque.
    \[Source: AFSC DH 1-3, 1980\]

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-   **4.6.1.15** **Torqued fasteners.** If a precise torque is required,
    the fastener shall be located so that the torquing tool can be
    applied directly, without the use of irregular extensions. \[Source:
    MIL-STD-1472F, 1999\]

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-   **4.6.1.16** **Quick-action fasteners.** Fasteners for cases and
    covers should be of the quick-action type, requiring only part of a
    turn or a snap action to fasten and unfasten. \[Source: AFSC DH 1-3,
    1980; MIL-HDBK-759B, 1992\]

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-   **4.6.1.17** **Covers as structural members.** When covers serve as
    stress-bearing structural members, their fasteners shall be large
    and strong enough to withstand the stress. \[Source: UCRL-15673,
    1985\]

### 4.6.2 Number

-   **4.6.2.1** **Minimum that meets requirements.** The number of
    fasteners used in a particular application shall be the minimum
    number that meets the closure, structural, and ease of maintenance
    requirements. A few large fasteners are preferable to many small
    fasteners as long as they meet the requirements. \[Source: AFSC DH
    1-3, 1980; MIL-STD-1472F, 1999; MIL-STD-1800A, 1990; UCRL-15673,
    1985\]

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-   **4.6.2.2** **Mounting.** No more than four fasteners should be used
    to mount a unit of equipment. \[Source: UCRL-15673, 1985\]

-   **4.6.2.3** **Minimize by using hinges, catches, latches, and quick
    fastening and releasing devices.** Hinges, catches, latches, and
    quick fastening and releasing devices should be used to reduce the
    number of fasteners whenever they meet the other relevant
    requirements. \[Source: MIL-STD-1472F, 1999; UCRL-15673, 1985\]

-   **4.6.2.4** **Minimize by using tongue-and-slot design.**
    Tongue-and-slot design should be used whenever possible in covers
    and cases to minimize the number of fasteners. \[Source: AFSC DH
    1-3, 1980; MIL-STD-1472F, 1999\]

### 4.6.3 Types

This section lists rules for different types of fasteners. For ease of
maintenance, the order of preference by type is: (a) quick fastening and
releasing devices, (b) latches and catches, (c) captive fasteners, (d)
screws, and (e) nuts and bolts. \[Source: MIL-HDBK-759B, 1992\]

#### 4.6.3.1 Nuts and bolts  {#nuts-and-bolts .unnumbered}

Nuts and bolts are relatively time consuming to install and remove. The
maintainer usually has to have access to both ends of the bolt, to use
both hands, and to make fairly precise movements in starting the nut.
Often the maintainer has to use two tools, one for the bolt and one for
the nut. The inclusion of washers increases the number of parts to
handle and possibly lose.

-   **4.6.3.1.1** **Bolt length.** Bolts shall be no longer than
    necessary for their given application. If a nut is used, at least
    two threads of the bolt shall extend beyond the nut when the nut is
    tightened. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

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-   **4.6.3.1.2 Bolt threads.** The bolt threads should be no finer than
    strength requirements dictate. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.6.3.1.3** **Turns to tighten.** The number of turns to tighten a
    bolt should be less than 10. \[Source: UCRL-15673, 1985\]

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-   **4.6.3.1.4** **Hexagonal nuts.** Hexagonal nuts shall be used in
    high-torque applications. \[Source: UCRL-15673, 1985\]

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-   **4.6.3.1.5** **Wing and knurled nuts.** Wing nuts or knurled nuts
    should be used in low-torque applications. \[Source: MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

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-   **4.6.3.1.6** **Left-hand threads.** Left-hand threaded nuts and
    bolts shall be used only when conditions require them, for example
    to prevent loosening due to rotation. When used, they shall be coded
    by marking, shape, or color so that they are easily distinguishable
    from right-hand threaded nuts and bolts. \[Source: UCRL-15673, 1985;
    MIL-HDBK-759B, 1992\]

-   **4.6.3.1.7** **Lock washers.** Lock washers or other restraining
    measures shall be used to prevent nuts and bolts from loosening
    under vibration. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.6.3.1.8** **Removal and replacement with one hand or tool.**
    Nuts and bolts that are removed and replaced frequently or that are
    relatively inaccessible should be mounted so that they can be
    removed and replaced with one hand or one tool. A recess should be
    provided to hold either the bolt or the nut. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.6.3.1.9** **Bolt mounting.** Bolts should be mounted with their
    heads up so that they remain in position if the nut falls off.
    \[Source: UCRL-15673, 1985\]

#### 4.6.3.2 Screws  {#screws .unnumbered}

Screws are relatively time consuming to insert and remove, and their
threads and slots are susceptible to damage. However, screwdrivers
usually require less operating space than wrenches, and they usually
require the use of only one hand. In addition, screws are usually used
alone, as opposed to bolts, which are usually used in combination with
washers and nuts. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.3.2.1 Number of turns.** The number of turns to tighten or
    loosen a screw should be less than ten. \[Source: UCRL-15673, 1985\]

-   **4.6.3.2.2** **Slot depth.** Screw heads should have deep slots
    that will resist damage. \[Source: MIL-HDBK-759B, 1992; UCRL-15673,
    1985\]

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-   **4.6.3.2.3** **\"Straight-in\" screwdriver orientation.** Screws
    shall be used only when screwdrivers can be used in a
    \"straight-in\" orientation; the use of offset screwdrivers shall
    not be required. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.3.2.4** **Blind operation.** If a screw must be operated in a
    position in which the maintainer cannot see its head, a guide shall
    be provided to help position the screwdriver. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

    **Example.** The screw might be located at the bottom of a
    cylindrical hole so that the hole guides the screwdriver to the
    screw head. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

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-   **4.6.3.2.5** **Screws for pressurized enclosures.** Fine-thread
    screws should be used for pressurized units. \[Source: UCRL-15673,
    1985\]

-   **4.6.3.2.6** **Countersunk screws.** Screws should be countersunk
    when a smooth surface is required. \[Source: UCRL-15673, 1985\]

-   **4.6.3.2.7** **Screws for thin panels.** Flat-head screws should
    not be used on panels less than 2.4 mm (3/32 in) thick. \[Source:
    UCRL-15673, 1985\]

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-   **4.6.3.2.8** **Self-tapping screws.** If a unit of equipment
    requires more than one size of self-tapping screw, the different
    sizes shall be kept to a minimum, and they shall all have the same
    type of head. \[Source: UCRL-15673, 1985\]

#### 4.6.3.3 Screw and bolt heads

Combination-head bolts and screws are preferable to other bolts and
screws because they allow operation with both wrenches and screwdrivers.
Slotted hexagonal heads are preferable to slotted knurled heads.
Combination-head bolts and screws reduce the likelihood of damaged slots
and stuck fasteners. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.3.3.1** **Same heads for screws and bolts.** To the extent
    possible, all bolts and screws on a given unit of equipment shall
    have the same size and type of head so that maintainers can operate
    all of them with a single tool. \[Source: MIL-STD-1472F, 1999\]

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-   **4.6.3.3.2** **Combination-head bolts and screws.**
    Combination-head bolts and screws should be used, preferably those
    having slotted, hexagonal heads. Exhibit 4.6.3.3.2 shows examples of
    combination-head bolts and screws; the combinations illustrated are
    slotted-hexagonal and slotted-knurled. \[Source: MIL-STD-1472F,
    1999\]![](media/image11.png){width="1.8083333333333333in"
    height="1.4215277777777777in"}

> []{#__RefHeading___Toc27272147 .anchor}**Exhibit 4.6.3.3.2** Examples
> of combination-head bolts and screws

-   **4.6.3.3.3** **Straight-slot and cross-recess type internal
    fasteners.** Non-combination straight-slot or cross-recess type
    internal fasteners shall not be used except to fasten wood.
    \[Source: MIL-STD-1472F, 1999\]

-   **4.6.3.3.4** **Internal-wrenching fasteners \-- where to use.**
    Internal-wrenching fasteners shall not be used except where a flat,
    smooth surface is required and where the fasteners are protected
    from the accumulation of foreign material such as dirt, ice, or
    snow. Exhibit 4.6.3.3.4 shows an example of an internal-wrenching
    bolt and an internal-wrenching nut. \[Source: MIL-STD-1472F, 1999;
    UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272148 .anchor}**Exhibit 4.6.3.3.4** Example
> of an internal-wrenching bolt and nut

![](media/image12.png){width="1.520138888888889in"
height="1.8020833333333333in"}

-   **4.6.3.3.5 High-torque fasteners.** If a torque of more than 14 Nùm
    (10 ft-lb) is required, fasteners shall have external hexagonal or
    double-hexagonal heads. If external-wrenching heads cannot meet
    functional or personnel safety requirements, or in situations in
    which the fastener is protected from accumulation of foreign
    material, internal-wrenching fasteners may be used. \[Source:
    MIL-STD-1472F, 1999\]

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-   **4.6.3.3.6** **Low-torque fasteners.** If a torque of less than 14
    Nùm (10 ft-lb) is required, fastener heads should be one of the
    following types: (a) combination-head, (b) hexagonal, external-grip
    head, (c) hexagonal internal-grip heads, or (d) Torq-set. \[Source:
    MIL-STD-1472F, 1999\]

#### 4.6.3.4 Latches and catches

Latches and catches can be operated quickly and easily. They do not
require the use of tools, and they have good holding power, but they
cannot be used where smooth surfaces are required. \[Source: UCRL-15673,
1985\]

-   **4.6.3.4.1** **Positive catch.** Latches and catches shall have a
    positive catch. \[Source: UCRL-15673, 1985\]

-   **4.6.3.4.2** **Visual indication.** Latches and catches shall give
    a clear visual indication that the latch or catch is engaged.
    \[Source: UCRL-15673, 1985\]

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-   **4.6.3.4.3** **Spring-loading of catches.** Catches should be
    spring-loaded so that they lock on contact rather than requiring
    some other action by the maintainer. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

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-   **4.6.3.4.4** **Nonhazardous.** The spring action or snap-down force
    in a catch or latch shall not be so strong that it could injure the
    maintainer. \[Source: MIL-HDBK-759B, 1992\]

-   **4.6.3.4.5** **Associated handles**. If a latch or catch is
    associated with a handle, the release mechanism for the latch or
    catch shall be located on or near the handle so that release and
    opening can be accomplished with one hand. \[Source: MIL-HDBK-759B,
    1992\]

-   **4.6.3.4.6** **Preventing inadvertent operation.** Latches and
    catches shall be located and positioned so that it is not likely
    that they will be operated inadvertently under normal operating
    conditions. \[Source: UCRL-15673, 1985\]

#### 4.6.3.5 Other fastening devices

This section gives rules for a variety of additional fastening devices,
including cotter pins and keys, retainer rings, safety wire, rivets,
retainer chains, and washers.

-   **4.6.3.5.1** **Integral fasteners not allowed.** Fasteners shall
    not be an integral part of the equipment\'s housing; studs are an
    example of disallowed integral fasteners. \[Source: MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

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-   **4.6.3.5.2** **Cotter pins and keys.** Criteria for cotter pins and
    keys are:

a.  Pins and keys should fit snugly, but not so tightly that they cannot
    be slid in and out by hand.

b.  The heads of cotter keys should be large enough so that they do not
    slip through the hole and so that they are easy to remove. \[Source:
    UCRL-15673, 1985\]

-   **4.6.3.5.3** **Retainer rings.** Criteria for retainer rings are:

a.  Retainer rings shall be easy to remove and replace when worn.

b.  When possible, retainer rings shall lock into position with a
    positive snap.

c.  Twist-to-lock retainer rings shall have spring tension to prevent
    their loosening. \[Source: UCRL-15673, 1985\]

-   **4.6.3.5.4** **Pin-and-hook fasteners.** If a cover must have the
    ability to survive nuclear, biological and chemical hazards, it
    should be attached with pins and hooks rather than hinges. \[Source:
    MIL-STD-1472F, 1999\]

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-   **4.6.3.5.5** **Safety wire.** Criteria for safety wire are:

a.  Safety wire shall be used only when self-locking fasteners or
    fasteners with cotter pins are not adequate to withstand the
    expected vibration or stress.

b.  When safety wire is used, it shall be easy to remove and replace.

c.  If a visible means of detecting that a fastener has become loosened
    or has changed position is required, safety wire shall be used.
    \[Source: UCRL-15673, 1985\]

-   **4.6.3.5.6** **Rivets.** Rivets are permanent fasteners that are
    difficult and time consuming to remove and replace; they are not
    ordinarily used on parts that might require removal. Criteria for
    rivets are:

a.  Rivets shall not be used to attach hinges, latches, catches, or
    other quick fastening and releasing devices.

b.  Rivets shall be of softer material than the pieces they fasten.

c.  The holes for shear rivets shall be drilled to close tolerances.
    Maintenance instructions shall specify these tolerances and the
    sizes of plug gauges and reamers to be used. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.3.5.7** **Retainer chains.** Retainer chains or locking bars
    are used to (a) keep hatches or doors from opening too far or from
    springing their hinges, (b) convert doors or covers into shelves for
    the maintainer, (c) prevent small covers or caps from being
    misplaced, (d) secure small, special tools at the location where
    they will be used, (e) secure objects that might otherwise fall and
    get lost, and (f) secure objects that might otherwise fall and
    injure the maintainer or damage the equipment. Criteria for retainer
    chains are:

a.  Only link, sash, or woven-mesh type chains should be used. Bead-link
    chain should not be used; it is more breakable than other types.

b.  Retainer chains should be attached with screws or bolts so that they
    can be disconnected easily if necessary.

c.  Each end of a retainer chain should have an eyelet.

d.  Retainer chains for filler caps should be attached externally, not
    internally.

e.  Chains should not be used where they might interfere with moving
    parts.

f.  If chain covers are required, they should be flexible and durable.
    \[Source: UCRL-15673, 1985\]

-   **4.6.3.5.8** **Washers.** Criteria for washers are:

a.  Washers should fit tightly against the underside of the fastener
    head.

b.  Washers should fit the shaft snugly, but should be easy to remove.

c.  Split-ring washers should be used with static loads in excess of 55
    g (2 oz).

d.  Lock washers should be used with lock nuts for maximum locking
    action. \[Source: UCRL-15673, 1985\]

#### 4.6.3.6 Quick fastening and releasing devices 

> Quick fastening and releasing devices are quick (by definition) and
> easy to use. They require no tools; they can be operated with only one
> hand; and they are good for securing plug-in components, small
> components, and covers. However, they have relatively low holding
> power, and they cannot be used where a smooth surface is required.
> \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.3.6.1** **Frequent access.** Quick fastening and releasing
    devices shall be used for components that must be dismantled or
    removed frequently. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999; UCRL-15673, 1985\]

-   **4.6.3.6.2 Tools not required.** Quick fastening and releasing
    devices shall fasten and release easily without the use of tools.
    \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.3.6.3** **Single motion.** Quick fastening and releasing
    devices shall operate with a single motion of the hand, for example,
    requiring no more than one complete revolution. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.6.3.6.4 Visual indication of state.** Quick fastening and
    releasing devices shall give a clear visual indication that they are
    fastened or released. \[Source: MIL-HDBK-759B, 1992; UCRL-15673,
    1985\]

-   **4.6.3.6.5** **Minimum turns.** Wherever bolts or screws are used,
    they shall be selected so that fastening them requires only the
    minimum number of turns necessary to meet the closure and structural
    requirements of the application. \[Source: AFSC DH 1-3, 1980;
    MIL-STD-1472F, 1999\]

    **Discussion.** Bolts and screws requiring more than the minimum
    number of turns may be excepted from this rule if they are used to
    reduce the variety of fasteners in a unit of equipment. \[Source:
    AFSC DH 1-3, 1980; MIL-STD-1472F, 1999\]

#### 4.6.3.7 Captive versus removable

Captive fasteners can be time consuming and difficult to operate. They
usually require the use of a tool, but the tool can usually be operated
with one hand. The fasteners stay in place, thus saving handling time
and avoiding the possible loss of parts. \[Source: MIL-HDBK-759B, 1992;
UCRL-15673, 1985\]

-   **4.6.3.7.1** **When to use.** Captive fasteners shall be used
    whenever dropping a fastener or a related part, such as a washer or
    bolt, might cause damage or excessive loss of time. \[Source:
    UCRL-15673, 1985; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999\]

-   **4.6.3.7.2** **Operation and replacement.** Captive fasteners shall
    be operable by hand or with common hand tools, and they shall be
    easily replaceable if damaged. \[Source: UCRL-15673, 1985;
    MIL-HDBK-759B, 1992\]

-   **4.6.3.7.3** **\"Quarter-turn\" fasteners.** If \"quarter-turn\"
    type fasteners are used, they shall be self-locking and
    spring-loaded. \[Source: UCRL-15673, 1985; MIL-HDBK-759B, 1992\]

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-   **4.6.3.7.4** **Access covers.** Access covers that are removed
    frequently should have captive fasteners. \[Source: MIL-STD-1472F,
    1999\]

-   **4.6.3.7.5 Small removable pins, caps, and covers.** Small
    removable pins, caps, and covers should be attached, probably with a
    retainer chain, so that they are not lost or dropped into the
    equipment. \[Source: MIL-HDBK-759B, 1992\]

-   **4.6.3.7.6** **Mounting bolts.** Mounting bolts should be
    semi-permanently captive, perhaps by means of \"snap-on\" collars.
    \[Source: AFSC DH 1-3, 1980\]

### 4.6.4 Labeling, marking, and coding

-   **4.6.4.1** **Mounting bolts.** Bolts that mount units of equipment
    should be color coded or perhaps embossed with the letter \"M\" to
    distinguish them from other visible fasteners. \[Source: AFSC DH
    1-3, 1980\]

-   **4.6.4.2** **Fasteners requiring torquing.** Fasteners that require
    a precise torque should have labels on or near the fasteners stating
    the required torque value and the torquing sequence. \[Source:
    MIL-STD-1472F, 1999\]

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-   **4.6.4.3** **Durability of marking.** If fasteners are marked or
    coded, the marks or codes shall withstand exposure to any chemicals,
    fuels, weather, or other adverse conditions in their ambient
    environment. \[Source: UCRL-15673, 1985\]

-   **4.6.4.4** **Consistent coding.** If a coding system for fasteners
    is used, it shall be consistent throughout a unit of equipment and
    for similar or related units of equipment. \[Source: UCRL-15673,
    1985\]

## 4.7 Connectors

This section contains rules for various types of connectors (plug-in,
threaded, and quick-action), for the location and accessibility of
connectors, and for alignment aids. It also contains rules specific to
electrical connectors and to fluid and gas connectors, including rules
for their labeling and marking.

**Definition.** A **connector** is a piece of hardware that joins or
attaches lines or cables to other lines or cables or to units of
equipment. The term is used rather loosely to refer to either of the two
parts that mate with each other and to the plug that mates with a
receptacle.

### 4.7.1 General

-   **4.7.1.1** **Fluid and gas line connectors.** Connectors utilized
    for fluid and gas lines should comply with Paragraph 4.8.2 Fluid and
    gas lines.

-   **4.7.1.2** **Connector gaskets and seals.** Gaskets and seals used
    in connectors should comply with Paragraph 4.8.2.4 Gaskets and
    seals.

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-   **4.7.1.3** **Fast, easy operation.** Connectors shall be selected
    or designed to permit fast, easy maintenance operations, including
    such tasks as testing, servicing, removing, and replacing units of
    equipment and components. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.1.4** **Safety.** Connectors shall be selected or designed to
    ensure the safety of maintainers and equipment from pressures,
    contents, or voltages during the release of connectors. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.7.1.5** **Hand or common tool operation.** Connectors shall be
    selected or designed to permit operation by hand or by common hand
    tools. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.7.1.6** **Compatibility.** Connectors shall be selected or
    designed to be compatible with their associated lines and cables,
    fasteners, mounting, environmental extremes, and maintenance
    routines. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.1.7** **Protection of connectors.** If a connector is
    susceptible to damage, it shall be protected by one or more of the
    following measures: (a) recessing the receptacle, (b) recessing
    delicate parts such as pins and keys within the connector, and (c)
    providing a protective cap, insert, cover, case, or shield.
    \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.1.8** **Captive covers.** If a connector has a protective
    cover, the cover shall be of the captive type. \[Source:
    MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

### 4.7.2 Types

#### 4.7.2.1 Distinctive

-   **4.7.2.1.1 Distinctive types.** Connectors for lines serving
    different functions, for example, fuel lines and water lines, or
    electrical power lines and radio-frequency signal lines, shall be
    distinctively different and physically incompatible. \[Source:
    UCRL-15673, 1985\]

-   **4.7.2.1.2** **Preventing mismatching.** Connectors serving the
    same or similar functions shall be selected or designed to minimize
    the likelihood of mismatching. \[Source: MIL-HDBK-759B, 1992\]

    **Discussion.** Preferably this will be accomplished by making the
    connectors physically incompatible, for example, by using connectors
    of different sizes or by using alignment pins or keys. If that is
    not feasible, coding by color may be acceptable.

#### 4.7.2.2 Plug-in

Plug-in connectors are the easiest and least time consuming to use, but
they have low holding power. They are particularly convenient where
frequent connection and disconnection is required. \[Source:
MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.7.2.2.1** **When to use.** If a line or cable is likely to be
    connected and disconnected frequently, and if the line or cable is
    not likely to be pulled accidentally, plug-in connectors should be
    used. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.7.2.2.2** **Preventing damage.** Plug-in connectors and their
    receptacles shall be selected or designed so that the plug cannot be
    inserted into a receptacle that it does not match. An attempt to
    insert a plug into a non-matching receptacle shall damage neither
    the plug nor the receptacle. \[Source: MIL-STD-1800A, 1990\]

#### 4.7.2.3 Threaded

Threaded connectors provide very secure connections, particularly when
locked in place with set screws, retainers, or safety wires. They are
more time consuming to connect and disconnect than plug-in connectors.

-   **4.7.2.3.1** **Ease of operation.** Threaded connectors shall be
    selected or designed so that they meet the holding requirements for
    the connection with a minimum number of turns. \[Source:
    MIL-HDBK-759B, 1992\]

#### 4.7.2.4 Quick-action

Quick-action connectors are, as their name implies, quick and easy to
use. They include connectors that operate in one of the following ways:
(a) by a snap action, (b) by rotating the connector up to one complete
turn, (c) by triggering a latch or spring device, and (d) by removal of
an external pin. Exhibit 4.7.2.4 illustrates a common type of
quick-action connector, one that operates with a quarter-turn rotation.
\[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272149 .anchor}**Exhibit 4.7.2.4** Example of
> a quick-action connector

\[Source: UCRL-15673, 1985\]

-   **4.7.2.4.1** **When to use.** Quick-action connectors shall be used
    when units of equipment or components must be connected or
    disconnected frequently or when connection and disconnection must be
    completed quickly, provided that they meet all other requirements
    for the connection, such as holding or sealing. \[Source: AFSC DH
    1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990; UCRL-15673, 1985\]![](media/image13.png){width="1.35in"
    height="0.5020833333333333in"}

-   **4.7.2.4.2** **Self-locking.** Quick-action connectors shall have
    self-locking catches that prevent loosening and ensure a secure
    connection. \[Source: UCRL-15673, 1985\]

### 4.7.3 Location and accessibility

-   **4.7.3.1** **Visual and physical access.** Connectors shall be
    located so that maintainers can see, reach, connect, and disconnect
    them easily and safely. \[Source: MIL-HDBK-759B, 1992;
    NASA-STD-3000A, 1989\]

-   **4.7.3.2** **Unobstructed access.** Connectors shall be located so
    that they can be seen and reached without the disassembly or removal
    of other equipment or components. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.3.3** **Relative accessibility.** The connectors that are
    connected and disconnected most frequently shall be the most
    accessible. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.3.4** **Full access.** The rear of plug connectors shall be
    accessible for testing and servicing. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990; NASA-STD-3000A, 1989\]

-   **4.7.3.5** **Protected from dislodging and damage.** Connectors
    shall be located so that they are not dislodged or damaged by the
    movement of people or objects in their vicinity. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.7.3.6** **Minimum spacing.** The space between a connector and
    any other connector or obstruction shall be sufficient to permit the
    connector to be grasped as firmly as necessary for connecting and
    disconnecting it. This spacing shall be at least:

a.  25 mm (1 in) if the connector is operated with bare fingers,

b.  32 mm (1.25 in) if the connector is operated with gloved fingers,

c.  64 mm (2.5 in) if the connector must be \"gripped firmly,\" and

-   75 mm (3 in) if the connector is operated with mittened hands.
    \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999\]

d.  

-   **4.7.3.7** **Measuring clearance.** Clearance shall be measured
    from the outermost portion of the connector, that is, from the
    backshell, strain relief clamp, dust cover, or electromagnetic
    interference shield or radio frequency interference shield, if they
    exist, and it shall permit a rotation of at least 270°. \[Source:
    AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999\]

-   **4.7.3.8** **Space for wrench.** If the connector requires high
    torque, there shall be enough space around it to permit use of a
    wrench. \[Source: MIL-STD-1472F, 1999\]

### 4.7.4 Alignment aids

-   **4.7.4.1** **Preventing misalignment.** Wherever a particular
    orientation of a connector is required, the connector and its
    receptacle shall be provided with an aligning device, such as a pin
    or key way, that prevents the connector from being inserted in any
    but the correct orientation. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

    **Discussion.** Even when an alignment device is provided, care must
    be taken to ensure that the connector is not symmetric, which would
    permit connection 180° from the correct orientation.

-   **4.7.4.2** **Alignment before contact.** Alignment devices shall
    ensure that correct alignment is achieved before electrical contact
    is made. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990;
    UCRL-15673, 1985; MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

-   **4.7.4.3** **Aligning the alignment devices.** If a unit of
    equipment has more than one connector having the same sort of
    alignment device, all of those connectors shall be oriented so that
    the alignment device is in the same relative position. For example,
    all alignment keys might be at the top. \[Source: MIL-STD-1472F,
    1999\]

-   **4.7.4.4** **Alignment coding.** If a connector has an alignment
    device, the connector shall be durably marked or coded to show the
    position of the alignment device. Methods for marking or coding
    include painted stripes and arrows. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990; MIL-HDBK-759B, 1992; AFSC DH 1-3, 1980\]

-   **4.7.4.5** **Alignment of drawer connectors.** If a module or unit
    of equipment is mounted in a drawer with a connector at the back
    that mates with a connector in the rack, guide pins or other
    alignment devices shall be provided to ensure proper mating.
    \[Source: MIL-STD-1472F, 1999\]

### 4.7.5 Electrical connections

#### 4.7.5.1 Plugs and receptacles

-   **4.7.5.1.1** **Fast, easy connection.** Unless precluded by other
    requirements, electrical connectors should be of the plug-in or
    quick-action types. \[Source: UCRL-15673, 1985\]

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-   **4.7.5.1.2** **Prevention of insertion errors.** Electrical plugs
    shall be selected or designed so that it is physically impossible to
    insert a plug in the wrong receptacle or to insert it into a
    receptacle the wrong way. Some ways in which this can be
    accomplished are:

a.  Use plugs with polarized prongs or prongs of different sizes,

b.  Use plugs having different numbers of pins or different
    configurations of pins, and

c.  Use plugs of different sizes. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.7.5.1.3** **Alignment.** Alignment of electrical connections
    should comply with Paragraph 4.7.4 Alignment aids.

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-   **4.7.5.1.4** **Few plugs, many contacts.** Where applicable,
    electrical connections shall be accomplished by using few connectors
    with many contacts rather than many connectors with few contacts.
    \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.7.5.1.5** **\"Hot\" leads.** Wiring shall be routed through
    plugs and receptacles so that \"hot\" leads are not exposed in
    either the plug or the receptacle when they are disconnected.
    \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.5.1.6** **\"Cold\" plugs.** Wiring shall be routed so that
    receptacles are \"hot,\" and plugs are \"cold\" when they are
    disconnected. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.7.5.1.7** **Electrical charges.** Disconnected plugs and leads
    shall not expose maintainers to stored electrical charges. \[Source:
    UCRL-15673, 1985\]

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-   **4.7.5.1.8** **Self-locking or latching.** Electrical connectors
    should be self-locking or should have safety catches; they should
    not require safety wire. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

-   **4.7.5.1.9** **Insertion force.** Electrical connectors should
    require low insertion forces to minimize the possibility of damaging
    the connector or injuring the maintainer. \[Source: MIL-HDBK-759B,
    1992\]

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-   **4.7.5.1.10** **Durability.** Plugs and pins shall be selected or
    designed to withstand rough use. \[Source: UCRL-15673, 1985\]

-   **4.7.5.1.11** **Non-shorting contacts.** Connectors shall be
    selected or designed so that electrical contacts cannot be shorted
    by external objects. \[Source: AFSC DH 1-3, 1980; UCRL-15673, 1985\]

-   **4.7.5.1.12** **Pin identification.** Each pin on each plug shall
    be clearly identified or coded, using labels or color or shape
    coding. \[Source: AFSC DH 1-3, 1980; UCRL-15673, 1985\]

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-   **4.7.5.1.13** **Test points.** If test points are required to
    measure inputs or outputs that cannot be easily checked otherwise,
    they should be provided: (a) on the plug itself, or (b) on an
    adapter that can be inserted between the plug and the receptacle.
    \[Source: AFSC DH 1-3, 1980; UCRL-15673, 1985\]

-   **4.7.5.1.14** **Disassembly by hand or using common hand tools.**
    The disassembly of connectors to change pin connections should not
    require the use of special tools; it should be possible by hand or
    with the use of common hand tools. \[Source: AFSC DH 1-3, 1980;
    UCRL-15673, 1985\]

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-   **4.7.5.1.15** **Drawer module connectors.** Units of equipment that
    are mounted in drawers and that do not require that connections be
    maintained when the drawer is extended, shall be provided with plugs
    mounted on the back of the drawer. These plugs shall have alignment
    guides that allow the unit to be slid back into place and mate with
    receptacles in the cabinet to accomplish whatever electrical
    interconnections among the drawer, other equipment in the cabinet,
    and external connections are required. \[Source: MIL-STD-1800A,
    1990\]

#### 4.7.5.2 Wire connections

This section contains rules governing the arrangement and attachment of
individual wires. Lugs and crimp-on devices are preferable to wire-wrap,
pig tailing, and soldering for connecting or splicing individual wires.
Soldering provides the most secure connection, but it is also the most
time-consuming.

-   **4.7.5.2.1** **Spacing of leads.** It shall be easy for maintainers
    to perform any necessary operations on leads, in particular
    connecting and disconnecting them. This may be accomplished by
    adequate spacing of the terminals to which they are attached, or by
    ensuring that the leads are long enough that the maintainer can
    separate them. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.7.5.2.2** **Extra wire length.** If wires terminate in lugs or
    crimp-on devices, the wires shall be long enough to permit at least
    six replacements of the devices. Exhibit 4.7.5.2.2 illustrates an
    example of a crimp-on splice. \[Source: UCRL-15673, 1985\]

> **Exhibit** **4.7.5.2.2** Example of a crimp-on splice.
>
> []{#refheading___toc27272150 .anchor}\[Source: UCRL-15673, 1985\]

![](media/image14.png){width="1.8444444444444446in"
height="0.7902777777777777in"}

-   **4.7.5.2.3** **Clamping insulation.** Lugs and crimp-on devices
    shall clamp the insulation as well as the conductor. \[Source:
    UCRL-15673, 1985\]

-   **4.7.5.2.4** **Compatibility of lugs with terminals.** Lugs shall
    be compatible with terminal post requirements. \[Source: UCRL-15673,
    1985\]

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-   **4.7.5.2.5** **U-lugs.** U-lugs should be used rather than O-lugs
    (or eye lugs). Exhibit 4.7.5.2.5 illustrates a U-lug and an O-lug.
    \[Source: UCRL-15673, 1985; AFSC DH 1-3, 1980\]

> []{#__RefHeading___Toc27272151 .anchor}**Exhibit** **4.7.5.2.5**
> Examples of a U-lug and an O-lug \[Source: UCRL-15673, 1985\]
> ![](media/image15.png){width="1.8423611111111111in"
> height="0.8451388888888889in"}

-   **4.7.5.2.6** **Soldered connections.** Soldered connections shall
    be compatible with terminal post requirements. \[Source: UCRL-15673,
    1985\]

-   **4.7.5.2.7** **Spacing of terminals.** Terminals to which wires are
    to be soldered shall be far enough apart that work on one terminal
    does not damage neighboring terminals or nearby parts. \[Source:
    UCRL-15673, 1985; AFSC DH 1-3, 1980\]

-   **4.7.5.2.8** **Length of terminals.** Terminals or other
    connections to which wires are soldered shall be long enough that
    soldering does not damage anything nearby. \[Source: UCRL-15673,
    1985; AFSC DH 1-3, 1980\]

-   **4.7.5.2.9** **Soldered wires.** The end of a wire soldered to a
    terminal shall extend beyond the solder so that the wire will be
    easy to remove. \[Source: UCRL-15673, 1985\]

-   **4.7.5.2.10** **Wire wrapping or pig tailing.** Wire wrapping and
    pig tailing shall not be used unless authorized by the acquisition
    program office. \[Source: UCRL-15673, 1985\]

### 4.7.6 Labeling, marking, and coding

Coding and identifying connectors and associated parts can expedite
maintenance and troubleshooting procedures by keying the connectors to
references in job instructions and by identifying replaceable parts for
ordering. Labeling, marking and coding can also provide appropriate
warnings and cautions.

-   **4.7.6.1** **Matching connectors or plugs and receptacles.** Each
    connector or plug and its corresponding connector or receptacle
    shall be labeled or coded so that the two parts are easily matched.
    \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.6.2** **Non-interchangeable connectors.** Non-interchangeable
    connectors shall be labeled or coded so that they are clearly
    distinguishable. Coding methods include shape, size, and color.
    \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.6.3** **Matching wires to terminals or pins.** Each wire in a
    connector or receptacle shall be clearly identified with its
    terminal post or pin. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.6.4** **Identification of terminals on terminal strips or
    blocks.** Terminals on terminal strips or blocks shall be identified
    on the terminal strip or block itself or on the chassis, adjacent to
    the terminals (same as paragraphs 4.8.1.8.3 and 4.9.5.8). \[Source:
    AFSC DH 1-3, 1980; UCRL-15673, 1985\]

-   **4.7.6.5** **Location of labels and codes \-- connectors.** Labels
    and codes on connectors shall be located, in order of
    preference, (a) directly on the connector, (b) on plates permanently
    attached to the connector, or (c) on tabs or tapes attached to the
    connector. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.6.6** **Location of labels and codes \-- receptacles.** Labels
    and codes for receptacles shall be located, in order of
    preference, (a) directly on the receptacle, (b) on the surface or
    panel immediately adjacent to the receptacle or, if it is recessed,
    adjacent to its access opening. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.6.7** **Consistency of labels and codes.** Labels and codes on
    connectors shall be consistent with labels and codes on associated
    items, such as pins, terminals, and receptacles. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.7.6.8** **Warnings and cautions.** If any hazard to maintainers
    or equipment exists in the connection or disconnection of a
    connector, the connector shall be labeled or coded with an
    appropriate warning or caution. \[Source: MIL-HDBK-759B, 1992\]

-   **4.7.6.9** **Marking electrical connections.** Marking adjacent to
    plugs, jacks and other electrical connectors shall identify the
    connected circuits to preclude cross connections. \[Source:
    MIL-HDBK-454\]

## 4.8 Lines and cables

General rules for lines and cables are given in this section. The
routing and mounting of electrical cables (including extension and
mock-up cables) and fluid and gas lines are covered, as well as their
labeling and marking.

**Definitions.** A **cable** is a number of lines bound together within
a single, permanent sheath. A **line** is any single length of pipe,
wire, or tubing.

Lines and cables most often end in connectors, and some rules regarding
connectors are also given in this section. See Section 4.7 for detailed
information pertaining to connectors. \[Source: UCRL-15673, 1985\]

### 4.8.1 Electrical

#### 4.8.1.1 General

-   **4.8.1.1.1** **Selection.** Lines and cables shall be selected,
    designed, bound, mounted, and routed to:

a.  preclude wear out, breakage, or damage,

b.  facilitate logical and efficient divisions of maintenance
    responsibilities, and

c.  allow maintainers to quickly and easily: (a) troubleshoot, test,
    check, and isolate malfunctions, (b) remove, repair, and replace
    other units of equipment and components, and (c) connect and
    disconnect lines and cables. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.8.1.1.2** **Insulation.** Clear plastic insulation should be
    used where possible to allow rapid detection of internal breaks.
    \[Source: UCRL-15673, 1985\]

-   **4.8.1.1.3** **Minimization.** Lines and cables should be designed
    to minimize the number of:

a.  types and varieties of lines and cables,

b.  different lengths of otherwise identical lines or cables, and

c.  related connectors, fittings, and fixtures. \[Source: UCRL-15673,
    1985\]

-   **4.8.1.1.4** **Quick-action connections.** When maintenance
    requires that cables be connected or disconnected easily or
    frequently, cables shall terminate in quick-action connectors.
    \[Source: UCRL-15673, 1985\]

-   **4.8.1.1.5** **Cable \"fan out\".** The wires in cables shall \"fan
    out as seen in Exhibit 4.8.1.1.5 so that the individual wires can be
    attached to junction boxes, terminal blocks, or other mounts. Each
    attachment point shall be easily identifiable and easy to reach with
    test probes. \[Source: MIL-HDBK-759B, 1992\]

[]{#__RefHeading___Toc27272152 .anchor}**Exhibit 4.8.1.1.5** Fanning out
cables \[Source: UCRL-15673, 1985\]

-   **4.8.1.1.6 Preformed cables.** Preformed cables should be used
    wherever possible to minimize wiring errors and allow for the use of
    more flexible and efficient assembly methods. Spare lines should be
    included to allow for growth and to speed wiring time as agreed upon
    by the acquisition agency. \[Source: UCRL-15673, 1985\]

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-   **4.8.1.1.7** **Harnesses.** When harnesses are used, they shall

a.  be designed, fabricated, and installed as units,

b.  be held securely with lacing twine or other means acceptable to the
    user, and

c.  keep the individual conductors essentially parallel, so they do not
    intertwine, though twisted pairs may be used when required.
    \[Source: UCRL-15673, 1985; AFSC DH 1-3, 1980\]

-   **4.8.1.1.8** **Protection.** Shields or other protection shall be
    provided for easily damaged conductors such as waveguides,
    high-frequency cables, and insulated high-voltage cables. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.8.1.1.9** **Exposed cables.** Exposed cables shall be protected
    from mechanical damage. For example, armored cables might be used
    where damage is likely. \[Source: MIL-HDBK-759B, 1992\]

-   **4.8.1.1.10** **Special purpose cables.** Cables intended for use
    in the presence of nuclear, biological, or chemical hazards shall be
    sealed. \[Source: MIL-STD-1800A, 1990\]

-   **4.8.1.1.11** **Insect protection.** If damage from termites is
    likely, line and cable insulation shall be protected by coating it
    with compounds of creosote, antimony, or other acceptable mixtures.
    \[Source: UCRL-15673, 1985\]

-   **4.8.1.1.12** **Fluid protection.** All electrical lines and cables
    shall be protected from oil, grease, fuel, hydraulic fluid, water or
    cleaning solvents. These may damage insulation and may result in
    injury to personnel. \[Source: MIL-HDBK-759B, 1992\]

-   **4.8.1.1.13** **Storage space.** If long electrical cables are
    required for auxiliary power or test equipment, storage space shall
    be provided. \[Source: MIL-HDBK-759B, 1992\]

    **Example.** Often, a storage compartment is present, but no easy
    means is provided for coiling the wire into a shape and size that
    will permit storage. A simple means of accomplishing this is a cable
    winder, a device around which the cable can be wrapped (see Exhibit
    4.8.1.1.13).

**Exhibit** **4.8.1.1.13** Cable
winder![](media/image18.png){width="2.247916666666667in"
height="1.5277777777777777in"}

-   **4.8.1.1.14** **Use of grommets.** When cables must pass over sharp
    edges, insulation shall be protected from fraying or other damage by
    grommets or equivalent means. \[Source: MIL-STD-1472F, 1999;
    MIL-HDBK-759B, 1992\]

#### 4.8.1.2 Length of cables and leads

[]{.mark}

-   **4.8.1.2.1** **Length of cables.** If a circuit might be affected
    by differences in the length of a cable, the length of cables should
    be the same for each installation of a given type of electronic
    equipment. \[Source: MIL-HDBK-759B, 1992; AFSC DH 1-3, 1980\]

    **Discussion.** Even if a unit can be adjusted to compensate for
    differences in the length of the cable, using different lengths of
    cable means that an adjustment made on the bench might be out of
    tolerance when the unit is installed. \[Source: MIL-HDBK-759B, 1992;
    AFSC DH 1-3, 1980\]

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-   **4.8.1.2.2** **Extra cable.** Cables shall be long enough so that a
    unit of equipment can be moved to a convenient place for maintenance
    activities; extension cables shall be provided if necessary.
    \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999; MIL-STD-1800A,
    1990\]

-   **4.8.1.2.3** **Accessibility.** Cables shall be long enough so that
    units of equipment mounted in drawers and on slide-out racks can be
    worked on without breaking electrical connections. \[Source: AFSC DH
    1-3, 1980; MIL-STD-1800A, 1990\]

-   **4.8.1.2.4** **Cable length and connectors.** Cables shall be long
    enough so that connectors can be easily connected and disconnected.
    \[Source: AFSC DH 1-3, 1980; NASA-STD-3000A, 1989\]

-   **4.8.1.2.5** **Length of leads.** Lead lengths shall be as short as
    possible but long enough to allow all of the following that apply
    (see Paragraph 4.7.5.2.1):

a.  easy connection and disconnection, with enough slack to back the
    wire away from the point of attachment to facilitate removal of the
    unit,

b.  sufficient slack for at least two (preferably six) replacements of
    terminal fittings, electrical considerations permitting,

c.  movements of parts to which they may be attached (doors, covers, and
    the like) without undue stress or bending,

d.  connection, disconnection, or movement without requiring a bending
    radius of less than six times the diameter of the lead, and

e.  movement of the units that are difficult to handle in their mounted
    position to a more convenient position for connection or
    disconnection. \[Source: UCRL-15673, 1985; MIL-HDBK-759B, 1992\]

-   **4.8.1.2.6** **Slack.** Leads or cables to moving parts, doors, and
    covers shall have adequate slack and protection so that they

a.  permit movement, such as pulling out a drawer for maintenance,
    without breaking the electrical connection,

b.  fold out of the way when the part is moved,

c.  are not pinched or otherwise damaged when the part is returned to
    its original position (see Exhibit 4.8.1.2.6 for the use of springs
    and cable mechanisms to prevent pinching),

d.  do not chafe or break under the repeated flexing required. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

[]{#__RefHeading___Toc27272154 .anchor}**Exhibit 4.8.1.2.6** Recoiling
slack cable

> \[Source: MIL-HDBK-759B,
> 1992\]![](media/image19.png){width="2.390277777777778in"
> height="2.452777777777778in"}

#### 4.8.1.3 Routing and mounting

-   **4.8.1.3.1** **Routing considerations.** Lines and cables shall be
    routed so they will not be

a.  pinched or stressed by loose objects, doors, lids, covers, sliding
    drawers, or roll-out racks,

b.  walked on or rolled over by heavy traffic,

c.  used for hand- or foot-holds (a protective guard should be placed
    over the cables where the possibility of such use exists), or

d.  bent or sharply twisted. \[Source: AFSC DH 1-3, 1980\]

-   **4.8.1.3.2** **Combining lines.** The layout and routing of lines
    shall be determined during design and made as simple and
    functionally logical as possible by combining lines into cables
    (preferable) or combining lines into harnesses if cables are not
    used. \[Source: UCRL-15673, 1985\]

-   **4.8.1.3.3** **Segregate conductors.** Conductors shall be
    segregated into cables or harnesses according to their functions and
    relationships to replaceable equipment. \[Source: UCRL-15673, 1985\]

-   **4.8.1.3.4** **Routing over pipes.** Electrical wires and cables
    shall be mounted above, rather than under, pipes or fluid
    containers. \[Source: MIL-HDBK-759B, 1992\]

-   **4.8.1.3.5** **Lightly insulated wires.** Lines and cables that are
    lightly insulated shall be at least 19 mm (0.75 in) from a potential
    ground. \[Source: UCRL-15673, 1985\]

-   **4.8.1.3.6** **Protection.** Raceways, stuffing tubes, conduit,
    junction boxes, and insulation shall be provided as necessary to
    obtain the required degree of protection, security of mounting, and
    ease of maintenance. \[Source: UCRL-15673, 1985\]

-   **4.8.1.3.7** **Visual and physical access.** Lines, cables, and
    wire harnesses shall be routed so that they are readily accessible
    for inspection and repair, especially at points of connection,
    splicing, and testing. \[Source: NASA-STD-3000A, 1989;
    MIL-STD-1800A, 1990; MIL-STD-1472F, 1999; MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.8.1.3.8** **Unobstructed access.** Lines and cables shall be
    accessible without requiring disassembly or removal of other
    equipment or components. \[Source: MIL-HDBK-759B, 1992\]

-   **4.8.1.3.9 Replacement.** Lines and cables shall be accessible
    throughout their route for removal and replacement if they are
    damaged. \[Source: MIL-HDBK-759B, 1992\]

-   **4.8.1.3.10** **Areas to avoid.** High voltage lines and cables
    shall be routed away from sensitive equipment, high temperature
    sources, work areas, controls, and the like. \[Source: UCRL-15673,
    1985; MIL-HDBK-759B, 1992\]

-   **4.8.1.3.11** **Ease of maintenance.** Line and cable routing shall
    facilitate maintenance by ensuring that each unit of equipment can
    be moved to a convenient place for maintenance activities; extension
    cables shall be provided if necessary. \[Source: MIL-HDBK-759B,
    1992\]

-   **4.8.1.3.12** **Non-obstruction.** Line and cable routing shall not
    obstruct visual or physical access to equipment for operation or
    maintenance. \[Source: MIL-STD-1472F, 1999\]

-   **4.8.1.3.13** **Remote switches.** Lines and cables shall not be
    routed through remote switches that may be turned on and off
    inadvertently while maintenance is being performed. \[Source:
    UCRL-15673, 1985\]

-   **4.8.1.3.14 Cables within racks.** Cables shall not be terminated
    or mounted on the front of cabinets, control panels, display panels,
    or on the face of equipment racks. Test cables are an exception to
    this criterion (see Paragraph 4.8.1.6.4). \[Source: NASA-STD-3000A,
    1989; UCRL-15673, 1985\]

-   **4.8.1.3.15** **Shortest route.** Lines and cables shall be routed
    over the shortest runs allowable by lead, mounting, and other
    requirements. \[Source: UCRL-15673, 1985\]

#### 4.8.1.4 Leads

-   **4.8.1.4.1** **No weight-bearing.** Leads shall be mounted so that
    they do not bear the weight of cables, harnesses, or other
    components. \[Source: UCRL-15673, 1985\]

-   **4.8.1.4.2** **Support.** Leads shall be mounted so that they are
    supported at splices and points of connection. \[Source: UCRL-15673,
    1985\]

-   **4.8.1.4.3** **Orientation.** Where possible, leads shall be
    mounted so that they are oriented in a way that prevents erroneous
    connection or \"crossing\". \[Source: UCRL-15673, 1985\]

-   **4.8.1.4.4** **No flexing.** Leads shall be mounted so that they do
    not allow flexing at weak areas, for example, at splices, solder
    points, points where the conductor is bare or crimped, or points
    where strands are tinned together. \[Source: UCRL-15673, 1985\]

-   **4.8.1.4.5** **Signal checks.** Signal flow checks shall be made
    possible by the appropriate arrangement, location, and mounting of
    leads. \[Source: UCRL-15673, 1985\]

#### 4.8.1.5 Clamps and mounting plates

-   **4.8.1.5.1** **Snug fit.** Clamps and mounting plates shall fit
    snugly without deforming or crimping the line or cable. \[Source:
    UCRL-15673, 1985\]

-   **4.8.1.5.2** **Spacing.** Clamps and mounting plates shall be
    operable by hand or with common hand tools. \[Source: MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

-   **4.8.1.5.3** **Special clamps.** Quick-release clamps (hinged or
    spring) shall be used if cables are removed frequently. Hinged
    clamps are preferable for non-overhead mounting, because they
    support the weight of the line during maintenance, freeing the
    maintainer\'s hands for other tasks. Exhibit 4.8.1.5.3 shows these
    two types of clamps. \[Source: MIL-HDBK-759B, 1992\]

    **Discussion.** For overhead mounting, a spring clamp with a hinged,
    locking latch over the clamp\'s open side is preferable because it
    would help prevent accidents. \[Source: MIL-HDBK-759B, 1992\]

> []{#__RefHeading___Toc27272155 .anchor}**Exhibit 4.8.1.5.3**
> Quick-release clamps, hinged and spring

-   **4.8.1.5.4** **Placement.** Clamps and mounting plates shall be
    located at both ends of bends where the bending radius is 75 mm (3
    in) or less. \[Source: UCRL-15673, 1985\]

-   **4.8.1.5.5** **Unsupported cable.** Lengths of cable or wire longer
    than 300 mm (12 in) shall be attached to the equipment chassis by
    means of clamps, unless contained in wiring ducts or cable
    retractors. \[Source: MIL-STD-1800A, 1990; MIL-HDBK-759B, 1992\]

-   **4.8.1.5.6** **Visibility of clamps.** All clamps shall be visible
    when equipment is installed. \[Source: MIL-STD-1800A, 1990\]

-   **4.8.1.5.7** **Mechanically-mounted clamps.** If a wire or cable is
    not routed through a wiring duct or conduit, it shall be attached
    with mechanically-mounted (not adhesive) cable clamps. \[Source:
    MIL-STD-1472F, 1999\]

    **Discussion.** Mechanically-mounted clamps can ensure the correct
    routing of electrical cables within and between units of equipment.
    They can also (a) ensure that cables do not hinder or obstruct
    equipment maintenance, (b) prevent chafing due to contact with an
    adjacent surface, and (c) facilitate the mating of cables with their
    associated equipment. \[Source: MIL-STD-1472F, 1999\]

-   **4.8.1.5.8** **Non-conductive.** Clamps and mounting plates that
    secure a conductor shall be nonconductive or properly insulated.
    \[Source: UCRL-15673, 1985\]

#### 4.8.1.6 Test and extension cables

Test and extension cables need to be planned, designed, and provided to
increase the efficiency and ease of maintenance.

-   **4.8.1.6.1** **Easy access.** Test and extension cables shall allow
    equipment and components to be moved to a convenient location for
    the performance of maintenance activities. \[Source: UCRL-15673,
    1985\]

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-   **4.8.1.6.2** **Multiple related functions.** Test and extension
    cables should serve as many related functions as possible, but
    should avoid the possibility of misuse or misconnection. \[Source:
    UCRL-15673, 1985\]

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-   **4.8.1.6.3** **Support equipment.** Test and extension cables shall
    permit support equipment to be placed in a convenient location.
    \[Source: UCRL-15673, 1985\]

-   **4.8.1.6.4** **Noninterference.** If it is essential that test
    cables terminate on control and display panels, the panel test
    receptacles shall be located so that the test cables will not
    visually or physically interfere with operational controls and
    displays. \[Source: NASA-STD-3000A, 1989; MIL-STD-1800A, 1990\]

-   **4.8.1.6.5** **Storage provisions.** Adequate storage shall be
    provided for test and extension cables. For example, racks, hooks,
    or cable winders might be provided within the storage place.
    \[Source: UCRL-15673, 1985\]

-   **4.8.1.6.6** **Handling devices for cable.** Reels or reel carts
    shall be provided for handling large, heavy, or long lines of cable
    (see Exhibit 4.8.1.6.6). \[Source: UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272156 .anchor}**Exhibit 4.8.1.6.6** An
> example of a line and cable reel cart.
>
> \[Source: UCRL-15673, 1985\]

![](media/image22.png){width="0.9694444444444444in"
height="0.8236111111111111in"}

-   **4.8.1.6.7** **Automatic rewind.** If reels or reel carts are
    provided, they should rewind the cable automatically, when possible.
    \[Source: UCRL-15673, 1985\]

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-   **4.8.1.6.8** **Mobile support.** If especially large lines or
    cables must be moved frequently, wheels or other mobile supports
    shall be provided. Exhibit 4.8.1.6.8 shows an example of a line and
    cable mobile support. \[Source: UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272157 .anchor}**Exhibit 4.8.1.6.8** Line and
> cable mobile support
> ![](media/image23.png){width="1.4291666666666667in" height="0.8125in"}
>
> \[Source: UCRL-15673, 1985\]

#### 4.8.1.7 Bench mockup cables

-   **4.8.1.7.1** **Extension cables.** Bench mockups shall have
    extension cables for all units so that the units can be removed from
    the bench mockup for the performance of maintenance activities.
    \[Source: AFSC DH 1-3, 1980\]

-   **4.8.1.7.2** **Connectors on mockup cables.** Bench mockup cables
    shall have connectors that require only a strong push or pull to
    connect or disconnect them. Bench mockup cables are not subject to
    strong vibration or shock, but they are connected and disconnected
    frequently. \[Source: AFSC DH 1-3, 1980\]

-   **4.8.1.7.3** **Coverings.** Mockup cables shall have an extra-heavy
    covering (for example, vinyl tubing) to protect them from wear
    resulting from frequent connection and disconnection. \[Source: AFSC
    DH 1-3, 1980\]

-   **4.8.1.7.4** **Checking signal flow.** Bench mockup cables,
    including extension cables for units of equipment, shall have test
    points to check the signal flow through each wire. \[Source: AFSC DH
    1-3, 1980\]

    **Examples.** One method for accomplishing this is to provide test
    points at the connector; another is to provide test points on the
    junction boxes or terminal strips. \[Source: AFSC DH 1-3, 1980\]

#### 4.8.1.8 Labeling, marking, and coding

-   **4.8.1.8.1** **Coding wire.** Insulated wire, cables, and
    electrical connectors shall be color- or number-coded in accordance
    with standards (for example, MIL-STD-195 and MIL-STD-681) acceptable
    to the acquisition agency. \[Source: UCRL-15673, 1985;
    MIL-HDBK-759B, 1992\]

    **Discussion.** Number-coded wire, cables, and electrical connectors
    are preferred so that maintainers who have problems discriminating
    various colors may be able to identify these items. \[Source:
    UCRL-15673, 1985; MIL-HDBK-759B, 1992\]

-   **4.8.1.8.2** **Identification.** Cables shall be labeled to
    indicate the equipment with which they are associated and the
    connectors with which they mate. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

-   **4.8.1.8.3** **Identification of terminals on terminal strips or
    blocks.** Terminals on terminal strips or blocks shall be identified
    on the terminal strip or block itself or on the chassis, adjacent to
    the terminals (same as Paragraphs 4.7.6.4 and 4.9.5.8). \[Source:
    AFSC DH 1-3, 1980; UCRL-15673, 1985\]

-   **4.8.1.8.4** **Methods of color-coding.** Conductors shall be
    color-coded using, in order of preference, (a) solid-color
    insulation, (b) solid-color insulation with a colored-stripe tracer,
    or (c) color braid insulation with a woven tracer. \[Source:
    MIL-HDBK-759B, 1992\]

    **Explanation.** Exhibit 4.8.1.8.4 shows 12 different pattern
    variations based on different insulation and tracer colors. For more
    than 12 wires, see MIL-STD-686C. If a wire\'s color-coding is
    susceptible to becoming obscured, wires may be coded with numbered
    metal tags. \[Source: MIL-HDBK-759B, 1992\]

> []{#__RefHeading___Toc27272158 .anchor}**Exhibit 4.8.1.8.4**
> Electrical cable coding

-   **4.8.1.8.5** **Cables within a sheath.** Cables containing
    individually insulated conductors with a common sheath shall be
    coded. The coding shall be repeated every 300 mm (12 in) along their
    entire length. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

-   **4.8.1.8.6** **Coding for orientation.** Coding by such means as
    color or labels shall identify the correct item and its proper
    orientation or replacement. \[Source: MIL-STD-1472F, 1999\]

### 4.8.2 Fluid and gas lines

#### 4.8.2.1 General

-   **4.8.2.1.1** **Connectors.** Fluid and gas line connectors should
    comply with Section 4.7 Connectors.

-   **4.8.2.1.2** **Use of flexible tubing.** Flexible tubing should be
    used instead of rigid lines because it allows more flexibility in
    handling, can be backed-off easily, and is easier to thread through
    equipment when replacement is required. \[Source: MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

-   **4.8.2.1.3** **Use of flexible hose.** Flexible hose should be used
    rather than pipes or tubing where minimum space is available for
    removing, handling, or replacing lines. \[Source: UCRL-15673, 1985\]

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-   **4.8.2.1.4** **Quick-action connectors.** Quick-action connectors
    shall be used on lines that require frequent disconnection.
    \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

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-   **4.8.2.1.5** **Preventing leakage.** When quick-action connectors
    are used, self-sealing features should be provided to prevent
    leakage of fluid when the line is disconnected. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

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-   **4.8.2.1.6** **Leakage tests.** Fluid and gas line connectors shall
    be located and installed so that leakage tests can be performed
    easily and without danger to the maintainer or the equipment.
    \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.8.2.1.7** **Control of leakage and spillage.** Fluid and gas
    line connectors shall be selected or designed so that leakage and
    accidental spillage during connection and disconnection are
    prevented or controlled so that they do not injure maintainers or
    damage equipment. \[Source: MIL-HDBK-759B, 1992; NASA-STD-3000A,
    1989\]

-   **4.8.2.1.8** **Standardized fittings.** To avoid the possibility of
    mismating connectors during service or maintenance, fittings shall
    be standardized so that lines that differ in the substances they
    carry cannot be interchanged. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.8.2.1.9** **Avoiding spraying fluids.** Lines shall be kept from
    spraying or draining fluid on personnel or equipment during
    disconnection by locating connections away from work areas and
    sensitive components, shielding sensitive components where required,
    and providing drains and bleed fittings so lines can be drained or
    depressurized before they are disconnected. \[Source: MIL-HDBK-759B,
    1992; UCRL-15673, 1985\]

-   **4.8.2.1.10** **High-pressure lines.** Systems that contain liquids
    or high pressure gases (pressures exceeding 125 psi) shall be
    provided with isolation or disconnect valves to permit isolation for
    servicing and to aid in leak detection. \[Source: NASA-STD-3000A,
    1989\]

-   **4.8.2.1.11 Cutoff** **valves.** Cutoff valves shall be provided at
    appropriate locations in the system to permit isolation or drainage
    of the system for maintenance and during emergencies. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.8.2.1.12** **Draining and filling.** Fluid and gas line
    connectors shall be located and installed so that draining, filling,
    and other maintenance involving the connectors or lines can be
    accomplished without jacking up the equipment. \[Source: UCRL-15673,
    1985; MIL-HDBK-759B, 1992\]

-   **4.8.2.1.13** **Avoiding drainage problems.** Drainage problems
    shall be avoided by

a.  designing lines so they can be emptied completely if necessary,

b.  making bends horizontal, rather than vertical, to avoid fluid traps,

c.  avoiding low points or dips in lines that make them difficult to
    drain, and

d.  providing special drains where low points do occur. \[Source:
    MIL-HDBK-759B, 1992\]

-   **4.8.2.1.14** **Exposure to noise and vibration.** Fluid and gas
    line connectors shall be located and installed so that maintenance
    activities involving them do not require that the maintainer be
    exposed to extreme noise, vibration, or other danger. \[Source:
    MIL-HDBK-759B, 1992\]

#### 4.8.2.2 Routing and mounting

-   **4.8.2.2.1 Accessibility.** Fluid and gas lines mounted in cable
    trays shall be located for ready access. \[Source: NASA-STD-3000A,
    1989\]

-   **4.8.2.2.2** **Connectors for rigid lines.** Connectors for rigid
    fluid and gas lines shall be located and installed so that it is not
    necessary to back the lines off or remove other equipment or
    components to connect or disconnect the connectors. \[Source:
    UCRL-15673, 1985\]

    **Discussion.** Protruding gaskets or seals might be susceptible to
    damage, and that damage might spread internally, destroying the
    seal. Tapered nylon or Teflon washers of appropriate size can be
    employed to prevent extrusion. \[Source: UCRL-15673, 1985\]

-   **4.8.2.2.3** **Areas to avoid.** High-pressure lines and cables
    shall be routed away from sensitive equipment, high temperature
    sources, work areas, controls, and the like. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.8.2.2.4** **Fuel lines.** Fuel lines shall be routed below
    electrical cables and hot pipes. \[Source: MIL-HDBK-759B, 1992\]

-   **4.8.2.2.5** **Heat resistant liners.** If fluid and gas lines are
    likely to become extremely hot, clamps and mounting plates shall be
    lined with heat resistant material so the maintainer will not be
    burned. \[Source: UCRL-15673, 1985; MIL-HDBK-759B, 1992\]

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-   **4.8.2.2.6** **Avoiding line kinking.** Fluid and gas lines should
    be installed and mounted with sufficient clearance from surrounding
    equipment and structures to allow the maintainer to disconnect and
    remove the lines without bending or kinking them. \[Source:
    UCRL-15673, 1985\]

#### 4.8.2.3 Clamps and supports

-   **4.8.2.3.1** **External service supports.** Unmounted lines
    attached to equipment (for example, lines from external service or
    test equipment or lines attached for other purposes) shall have
    supports capable of withstanding

a.  the initial surges of pressure through the line,

b.  the weight of external extensions, and

c.  the wear and tear of handling and repeated connection and
    disconnection. \[Source: UCRL-15673, 1985; MIL-HDBK-759B, 1992\]

-   **4.8.2.3.2** **Spring clamps.** Spring clamps shall be used to
    mount tubing and fluid pipes that may require frequent removal and
    replacement. \[Source: MIL-HDBK-759B, 1992\]

    **Discussion.** For overhead mounting, a spring clamp shall be used
    with a hinged-locking latch over the clamp\'s open side to prevent
    accidents (see Paragraph 4.8.1.5.3). \[Source: MIL-HDBK-759B, 1992\]

#### 4.8.2.4 Gaskets and seals

-   **4.8.2.4.1** **Replaceable, renewable gaskets and seals.** Gaskets
    and seals used in connectors for fluid and gas lines shall be
    selected and installed to be replaceable or to have renewable
    wearing surfaces; it shall not be necessary to discard the connector
    when the seal is damaged or worn. \[Source: UCRL-15673, 1985\]

-   **4.8.2.4.2** **Repair and replacement of gaskets and seals.**
    Gaskets and seals used in connectors for fluid and gas lines shall
    be easily inserted and removed, without requiring the removal of
    other connector parts or the disassembly of other equipment.
    \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.8.2.4.3** **Ability to identify gaskets and seals.** Part
    numbers for gaskets and seals used in connectors for fluid and gas
    lines shall be easily identifiable. This may be accomplished through
    labeling, coding, marking, or user documentation. \[Source:
    UCRL-15673, 1985\]

-   **4.8.2.4.4** **Life expectancy of gaskets and seals.** Job
    instructions shall state the life expectancy of gaskets and seals
    and recommend when they should be changed. \[Source: UCRL-15673,
    1985; MIL-HDBK-759B, 1992\]

-   **4.8.2.4.5** **Prevent entrance of air.** If the entrance of air
    into a disconnected fluid or gas line would create a maintenance
    problem, as for example, in a hydraulic line, gaskets and seals used
    in connectors shall prevent the entrance of air when the line is
    disconnected. \[Source: UCRL-15673, 1985\]

-   **4.8.2.4.6** **Tightening to offset shrinkage.** If a gasket or
    seal used in a fluid or gas line connector is subject to shrinkage,
    the connector shall permit tightening to offset the shrinkage.
    \[Source: UCRL-15673, 1985\]

-   **4.8.2.4.7** **Visibility.** Gaskets and seals used in fluid and
    gas line connectors shall be visible after they are installed so
    that maintainers can see that the gasket or seal is present as
    illustrated in Exhibit 4.8.2.4.7. \[Source: MIL-HDBK-759B, 1992\]

> []{#__RefHeading___Toc27272159 .anchor}**Exhibit 4.8.2.4.7**
> Externally visible seals

\[Source: MIL-HDBK-759B, 1992\]

-   **4.8.2.4.8 Non-protrusion.** Gaskets and seals used in fluid and
    gas connectors shall not protrude beyond the coupling. \[Source:
    UCRL-15673, 1985\]

    **Discussion.** Protruding gaskets and seals might be susceptible to
    damage, and that damage might spread internally, destroying the
    seal. Tapered nylon or Teflon washers of appropriate size can be
    employed to prevent extrusion. \[Source: UCRL-15673, 1985\]

#### 4.8.2.5 Labeling, marking, and coding

-   **4.8.2.5.1** **Fluid conductor coding.** Fluid conductors shall be
    either color coded (see Exhibit 4.8.2.5.1), or coded by metal tags.
    Metal tags shall be used where adverse conditions (such as grease or
    mud) could obscure colors; otherwise, color-coding shall be used.
    \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272160 .anchor}**Exhibit** **4.8.2.5.1**
> Color-coding of fluid conductors

-   **4.8.2.5.2** **Valve color-coding.** Valves shall be color-coded in
    accordance with the substances they control or the function they
    perform. Exhibit 4.8.2.5.2 lists recommended color codes for valves
    by substance. \[Source: UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272161 .anchor}**Exhibit** **4.8.2.5.2** Valve
> color-coding scheme

-   **4.8.2.5.3** **Hydraulic and pneumatic line coding.** Hydraulic and
    pneumatic lines shall be coded based on arrangement, size, shape,
    and color as necessary. Exhibit 4.8.2.5.3 lists color-codes for
    lines by function. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

[]{#__RefHeading___Toc27272162 .anchor}**Exhibit** **4.8.2.5.3**
Hydraulic and pneumatic line color-coding scheme

-   **4.8.2.5.4** **Label contents.** Codes and labels shall be provided
    on, or adjacent to, the line as necessary to indicate the direction
    of flow. \[Source: MIL-HDBK-759B, 1992\]

-   **4.8.2.5.5** **Valve position labeling.** Labels or other marking
    devices shall be provided to clearly designate the position of a
    valve control. Exhibit 4.8.2.5.5 illustrates labeling of a valve
    control. \[Source: UCRL-15673, 1985\]

    **Example.** A rider, as illustrated in Exhibit 4.8.2.5.5, may be
    attached to the shaft to indicate the fully opened and fully closed
    positions. \[Source: UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272163 .anchor}**Exhibit** **4.8.2.5.5** Value
> position labeling ![](media/image26.png){width="1.1604166666666667in"
> height="1.6152777777777778in"}
>
> \[Source: UCRL-15673, 1985\]

## 4.9 Packaging, layout, and mounting of internal components

Criteria and rules for the packaging, layout, mounting, labeling, and
marking of internal modules, components, and parts are given in this
section. All of these can affect the ease or difficulty of maintenance
activities.

**Definitions.** A **module** is an assemblage of two or more
interconnected parts or components that comprise a single physical and
functional entity (for example, a printed circuit board). It is this
singular functionality that defines a module. A **component** is a
subdivision of a unit of equipment that can be treated as an object by
the maintainer, but which can be further broken down into parts. A
mounting board together with its mounted parts is an example of a
component. A **part** is an object that cannot normally be broken down
further without destroying its designated use. Fuses, resistors, and
capacitors are examples of parts. **Packaging** is the grouping of
functions, components, and parts into units or modules; **layout** is
the physical arrangement of the parts and components that make up a
module or a unit of equipment; **mounting** is the positioning and
attachment of parts, components, and modules.

### 4.9.1 General

-   **4.9.1.1** **Accessibility.** Parts and modules on which
    maintenance is performed shall be positioned so that the maintainer
    has complete visual and physical access. \[Source: MIL-HDBK-759B,
    1992\]

-   **4.9.1.2** **Minimize tool requirements.** Parts and modules shall
    be packaged, laid out, and mounted so that maintenance activities
    require a minimum number and variety of tools, preferably only
    common hand tools. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.9.1.3** **Minimize maintainer movement.** Parts and modules
    should be packaged, laid out, and mounted so that a minimum of
    movement is required of the maintainer in carrying out maintenance
    activities. \[Source: UCRL-15673, 1985\]

-   **4.9.1.4** **Organized by maintenance specialty.** Parts and
    modules should be packaged, laid out, and mounted so that
    maintenance activities by one maintenance specialist do not require
    removal or handling of equipment or components maintained by another
    specialist. \[Source: UCRL-15673, 1985\]

### 4.9.2 Packaging

Dividing a unit of equipment into a number of separate modules has
several advantages: it can permit specialization by maintainers; it can
speed up corrective maintenance; and it can make working on
malfunctioning units easier.

**Definition.** **Modularization** is the separation of equipment into
physically and functionally distinct units that can be easily removed
and replaced.

#### 4.9.2.1 Modularization

-   **4.9.2.1.1** **Modularization.** Units of equipment should be
    divided into as many modules as are electrically and mechanically
    practical and feasible for maintenance (see also Paragraph 4.1.2.7).
    \[Source: AFSC DH 1-3, 1980; MIL-STD-1800A, 1990; UCRL-15673, 1985\]

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-   **4.9.2.1.2** **Single function.** A module shall contain only parts
    that contribute to a single function; it shall not provide multiple,
    divergent functions. \[Source: UCRL-15673, 1985\]

-   **4.9.2.1.3** **Physical and functional interchangeability.** If
    modules are physically interchangeable, they shall also be
    functionally interchangeable; if they are not functionally
    interchangeable, they shall not be physically interchangeable.
    \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; NASA-STD-3000A,
    1989\]

-   **4.9.2.1.4** **Ability to distinguish non-interchangeable
    modules.** Non-interchangeable modules shall be distinguishably
    different in appearance, and this difference shall be apparent when
    the module is in its installed position. \[Source: NASA-STD-3000A,
    1989\]

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-   **4.9.2.1.5** **Unreliable components.** If a module contains some
    parts that are significantly less reliable than the remaining parts,
    the unreliable parts should be accessible without removal of the
    module. \[Source: UCRL-15673, 1985\]

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-   **4.9.2.1.6** **Maintenance in installed location.** Modules shall
    be designed so that required maintenance can be performed with the
    module in its installed position, without requiring disconnection,
    disassembly, or removal of other modules. \[Source: MIL-HDBK-759B,
    1992\]

-   **4.9.2.1.7** **Testing.** Modules shall be designed to permit
    testing when they are removed from their installed position, and
    they shall require little or no calibration immediately after
    installation. \[Source: UCRL-15673, 1985\]

#### 4.9.2.2 Modularization methods

The breaking up of a unit of equipment into modules is done in
accordance with one or more of the following methods: (a) logical flow
packaging, (b) circuit packaging, or (c) component packaging.

In logical flow packaging, circuits, parts, and components are packaged
and arranged in correspondence with their functional relationships.

In circuit packaging, all parts of a single circuit or logically related
group of parts, and only that circuit or group, are placed in a separate
module.

In component packaging, similar parts or components are located
together; for example, all the fuses or all the relays might be grouped
together.

-   **4.9.2.2.1 Modularization method.** The modularization of equipment
    should be done using one or more of the following methods, in this
    order of preference: (a) logical flow packaging, (b) circuit
    packaging, (c) component packaging. \[Source: UCRL-15673, 1985\]

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-   **4.9.2.2.2** **Logical flow packaging.** If logical flow packaging
    is used:

a.  Circuits and parts shall be packaged and located in an arrangement
    that parallels their functional relationships.

b.  A module shall be designed so that only single input and output
    checks are necessary to isolate a fault in the module.

c.  The unidirectional signal flow within a module shall be clearly
    indicated. \[Source: UCRL-15673, 1985\]

-   **4.9.2.2.3** **Circuit packaging.** If circuit packaging is used:

a.  All parts of a given circuit or group of logically related parts
    shall be located in a single module.

b.  A module shall contain only one circuit or group of related parts.

c.  The circuit shall be packaged as a single terminal board or plug-in
    module when possible.

d.  Circuits shall be grouped to minimize crisscrossing of signals among
    modules. \[Source: UCRL-15673, 1985\]

-   **4.9.2.2.4** **Component packaging.** If component packaging is
    used:

a.  Similar components should be grouped in one location, for example,
    all fuses or all relays.

b.  Inexpensive components should be placed on separate plug-in boards
    that can be discarded upon failure.

c.  Similar parts that are likely to require replacement at
    approximately the same time should be grouped together.

d.  Components requiring the same maintenance activity should be grouped
    together; for example test points or components requiring a
    particular cleaning method. \[Source: UCRL-15673, 1985\]

-   **4.9.2.2.5** **Printed circuit boards.** If printed circuit boards
    are used:

a.  Printed circuit boards shall be designed and mounted for ease of
    removal and foolproof replacement.

b.  Plug-in printed circuit boards shall be structurally rigid and easy
    to remove and replace, providing finger access and gripping aids if
    necessary.

c.  Feedback shall be provided to the maintainer when plug-in printed
    circuit boards are securely connected.

d.  Printed circuit boards shall be identified in accordance with
    MIL-STD-130, and references for parts mounted on the board shall be
    provided in accordance with MIL-HDBK-454, Requirement 67. \[Source:
    MIL-STD-1472F, 1999\]

### 4.9.3 Layout

Rules governing the layout of the parts that make up a module fall into
three general categories: (a) accessibility of the parts, (b) logical or
functional grouping of parts, and (c) protection of maintainers and
equipment from hazards.

#### 4.9.3.1 Accessibility

-   **4.9.3.1.1** **No interference from other parts.** Modules shall be
    laid out so that all parts can be removed and replaced without
    interference from and without removal of other parts. \[Source: AFSC
    DH 1-3, 1980; MIL-HDBK-759B, 1992\]

-   **4.9.3.1.2** **No stacking of parts.** The parts that make up a
    module shall be mounted in an orderly, flat, two-dimensional array;
    they shall not be stacked one on top of another. \[Source: AFSC DH
    1-3, 1980; MIL-STD-1472F, 1999\]

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-   **4.9.3.1.3** **Consistent orientation.** If a module has more than
    one part of the same type that must be inserted in a particular
    orientation (connectors, for example), all those parts should be
    oriented in the same direction. \[Source: AFSC DH 1-3, 1980;
    MIL-HDBK-759B, 1992\]

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-   **4.9.3.1.4** **Spacing of parts.** The parts that make up a module
    shall be positioned so that any required tools (such as test probes
    or soldering irons) can be used without difficulty. \[Source: AFSC
    DH 1-3, 1980; MIL-HDBK-759B, 1992\]

-   **4.9.3.1.5** **Separation of parts and wiring on printed circuit
    boards.** On printed circuit boards, all parts shall be mounted on
    one side of the board, and all wiring (including printed circuits)
    shall be placed on the other side. \[Source: AFSC DH 1-3, 1980;
    UCRL-15673, 1985\]

-   **4.9.3.1.6** **Frequently inspected component parts.** Parts that
    require frequent visual inspection (fuses, for example) shall be
    located where they can be seen easily without the removal of panels,
    covers, or other modules. \[Source: AFSC DH 1-3, 1980\]

-   **4.9.3.1.7** **High failure-rate parts.** Parts that have a high
    failure rate, such as fuses, shall be located where they can be seen
    and replaced without the removal of other parts. \[Source: AFSC DH
    1-3, 1980; MIL-STD-1472F, 1999\]

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-   **4.9.3.1.8** **Indicator lights.** If a module contains indicator
    lights, it should be possible to change the lights from the front
    panel, that is, without opening or removing the module. \[Source:
    UCRL-15673, 1985\]

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-   **4.9.3.1.9 Shutoff switches.** If the module contains emergency
    shutoff switches, they shall be positioned within easy reach of the
    maintainer, and they shall be located or guarded to prevent
    inadvertent operation. \[Source: MIL-STD-1800A, 1990\]

-   **4.9.3.1.10** **Visual and physical accessibility.** Test points,
    adjustment points, and cable and line connectors shall be located
    where the maintainer can see them easily and perform any required
    operations on them without interference. \[Source: UCRL-15673,
    1985\]

#### 4.9.3.2 Grouping of parts

-   **4.9.3.2.1** **Grouping maintenance displays.** All maintenance
    displays relevant to a particular task shall be grouped together and
    located where they are easily visible to the maintainer. \[Source:
    UCRL-15673, 1985\]

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-   **4.9.3.2.2** **Separating maintenance and operational displays.**
    If a module contains both maintenance and operational displays, the
    maintenance displays should be separated from the operational
    displays. \[Source: UCRL-15673, 1985\]

#### 4.9.3.3 Hazard protection

-   **4.9.3.3.1** **Avoidance of damage to parts and wiring.** The parts
    and wiring of a module shall be located and arranged so that they
    are not damaged when the module or the unit of equipment of which
    they are part is opened and closed. \[Source: AFSC DH 1-3, 1980\]

-   **4.9.3.3.2** **Avoidance of damage from handling.** Parts that are
    susceptible to damage during maintenance activities shall be located
    or shielded so that they will not be damaged during these
    activities. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999;
    UCRL-15673, 1985\]

-   **4.9.3.3.3** **Avoidance of damage from the environment.** Parts
    shall be positioned so that they are not likely to be damaged by
    oil, other fluids, dirt, or static electricity. \[Source:
    UCRL-15673, 1985\]

-   **4.9.3.3.4** **Protecting maintainers from heat and electrical
    shock.** If a module contains parts that retain heat or electrical
    potential after power is turned off, those parts shall be located
    where maintainers will not touch them during maintenance activities,
    or they shall be shielded to protect the maintainers. In addition,
    heat-producing parts shall be shielded to protect maintainers from
    injury. \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

-   **4.9.3.3.5** **Bleeder networks.** Parts that retain electrical
    potential after power is turned off shall be equipped with bleeder
    networks. \[Source: MIL-HDBK-759B, 1992\]

-   **4.9.3.3.6** **Separating internal controls from hazardous
    voltages.** Internal controls such as switches and adjustment
    controls shall not be located where maintainers might come into
    contact with hazardous voltages while operating the controls.
    \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.9.3.3.7** **High current switching devices.** High current
    switching devices shall be shielded to prevent maintainers from
    coming into contact with them. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

### 4.9.4 Mounting

This section describes several methods for mounting modules and gives
criteria and rules pertaining to these methods; it then lists more
general criteria and rules. In mounting a particular module, it may be
appropriate to use more than one of these methods simultaneously.
\[Source: UCRL-15673, 1985\]

-   **4.9.4.1** **Foldout mounting.** Foldout mounting should be used
    whenever feasible. Exhibit 4.9.4.1 gives an example of foldout
    mounting. \[Source: MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272164 .anchor}**Exhibit** **4.9.4.1** Example
> of foldout mounting construction
>
> \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992, 5.6.18.11;
> UCRL-15673, 1985\]

![](media/image27.png){width="3.99375in" height="1.1in"}

-   **4.9.4.2** **Prevention of damage with foldout mounting.** If
    foldout mounting is used, parts and wiring shall be positioned so
    that they are not damaged during opening and closing. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.9.4.3** **Support for hinged mounting.** If a module is mounted
    on hinges, some sort of brace or support shall be provided to hold
    the module in the \"out\" or \"open\" position. \[Source:
    MIL-HDBK-759B, 1992; UCRL-15673, 1985\]

-   **4.9.4.4** **Rests and stands.** If a module contains parts that
    might be damaged when it is moved into position for maintenance
    activities, the module shall include rests or stands that protect
    those parts. These rests and stands shall be integral parts of the
    construction of the module. \[Source: MIL-HDBK-759B, 1992;
    UCRL-15673, 1985\]

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-   **4.9.4.5** **Characteristics of straps and brackets.** Straps and
    brackets should:

a.  be thick and rounded enough so that there are no sharp edges, and

b.  be shorter than the part or module they hold so that they provide a
    clamping action. \[Source: UCRL-15673, 1985\]

c.  

-   **4.9.4.6** **Shock mounts.** Shock mounts should be used as
    appropriate to

a.  eliminate vibration that would make displays and markings difficult
    to read,

b.  reduce noise levels that might be hazardous to maintainers, and

c.  reduce levels of vibration that might be hazardous to maintainers or
    equipment. \[Source: UCRL-15673, 1985\]

-   **4.9.4.7** **Preventing mounting errors by physical design.**
    Modules shall be designed so that it is physically impossible to
    mount them incorrectly. \[Source: MIL-HDBK-759B, 1992;
    MIL-STD-1472F, 1999; NASA-STD-3000A, 1989; UCRL-15673, 1985\]

    **Discussion.** Incorrect mounting includes reversal, mismating, and
    misaligning. Measures to prevent incorrect mounting include (a) the
    incorporation of keys or other aligning devices, (b) the provision
    of asymmetrical mounting brackets, and (c) the provision of
    asymmetrical mounting holes. Exhibit 4.9.4.7 illustrates all three
    of these measures. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999; NASA-STD-3000A, 1989; UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272165 .anchor}**Exhibit 4.9.4.7** Error-free
> mounting provisions
> ![](media/image28.png){width="1.4409722222222223in" height="1.5875in"}
>
> \[Source: UCRL-15673, 1985\]

-   **4.9.4.8** **Mounting and orientation of similar items.** Parts and
    modules that are similar shall use the same mounting method and be
    mounted with the same orientation. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

-   **4.9.4.9** **Accessibility.** Parts and modules that are intended
    to be removed and replaced by maintainers shall be mounted so that
    they can be removed without the removal of other parts or modules
    and without interference from other parts or modules. \[Source: AFSC
    DH 1-3, 1980; MIL-HDBK-759B, 1992; MIL-STD-1800A, 1990\]

-   **4.9.4.10 Controls.** Modules shall be mounted so that it is not
    necessary to disconnect controls that may be needed for maintenance.
    \[Source: AFSC DH 1-3, 1980\]

-   **4.9.4.11** **Common hand tools.** Modules shall be designed so
    that they are replaceable by hand or with common hand tools.
    \[Source: AFSC DH 1-3, 1980\]

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-   **4.9.4.12** **Front access.** Modules designed to be replaceable
    should be accessible through the front surface of the equipment
    rather than the back. \[Source: AFSC DH 1-3, 1980; NASA-STD-3000A,
    1989\]

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-   **4.9.4.13** **Orientation of modules within cases.** If a module
    has a case, the proper orientation of the module within its case
    shall be obvious, preferably through the physical design of the
    case, rather than through labeling. \[Source: MIL-STD-1472F, 1999\]

-   **4.9.4.14** **\"Plug-in\" connectors.** Electrical connections
    between modules shall be made using plug-in connectors unless
    special requirements, such as holding power or sealing, dictate
    another type. \[Source: AFSC DH 1-3, 1980; MIL-STD-1472F, 1999;
    NASA-STD-3000A, 1989\]

### 4.9.5 Labeling and marking

-   **4.9.5.1** **When to use labels and markings.** Labels or markings
    shall be used to

a.  outline and identify functional groups of parts,

b.  identify each part by name or symbol,

c.  indicate direction of current or signal flow to aid troubleshooting,
    and

d.  if applicable, identify the value and tolerance level of parts.
    \[Source: UCRL-15673, 1985; MIL-HDBK-759B, 1992\]

-   **4.9.5.2** **Location of labels and markings \-- consistency.**
    Labels and markings shall be placed consistently in relation to the
    parts to which they refer. This placement may be on or immediately
    adjacent to the part. \[Source: UCRL-15673, 1985\]

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-   **4.9.5.3** **Location of labels and markings \-- eye level.** If
    the part being labeled or marked will be below eye level in its
    installed position, the label or mark should be above the item; if
    the item will be above eye level, the label or mark should be below
    the item. \[Source: UCRL-15673, 1985\]

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-   **4.9.5.4** **Visibility.** Labels and markings shall be placed so
    that the maintainer can see them without having to move or remove
    anything. \[Source: UCRL-15673, 1985\]

-   **4.9.5.5** **Orientation.** Labels shall be horizontally oriented
    so that the maintainer can read them while the module is in its
    installed position. \[Source: UCRL-15673, 1985\]

-   **4.9.5.6** **Electrical parts.** Small electrical parts that are
    attached to mounting boards (resistors and capacitors, for example)
    shall be labeled or marked on the mounting boards. \[Source:
    UCRL-15673, 1985\]

-   **4.9.5.7** **Parts requiring identification.** The following parts
    shall be identified with labels or markings on the parts themselves
    or on the chassis or board adjacent to the part:

a.  all parts identified by designations in drawings, schematics, and
    parts descriptions of the module,

b.  all wires, sockets, plugs, receptacles, and similar parts having
    designations in wiring diagrams of the module,

c.  all replaceable mechanical parts,

d.  all semi-fixed electrical items, such as fuses and ferrule-clipped
    resistors, and

e.  items having critical polarity or impedance ratings. \[Source:
    UCRL-15673, 1985\]

-   **4.9.5.8** **Identification of terminals on terminal strips or
    blocks.** Terminals on terminal strips or blocks shall be identified
    on the terminal strip or block itself or on the chassis, adjacent to
    the terminals (same as Paragraphs 4.7.6.4 and 4.8.1.8.3). \[Source:
    UCRL-15673, 1985\]

-   **4.9.5.9** **Identification of terminals on parts.** Each terminal
    of a part having terminals (transformers, relays, and capacitors,
    for example) shall be identified adjacent to the terminal. \[Source:
    UCRL-15673, 1985\]

-   **4.9.5.10** **Identification of parts accessible from both sides.**
    Receptacles for plugs and other parts that are accessible from both
    sides of a board or panel shall be identified on both sides.
    \[Source: UCRL-15673, 1985\]

-   **4.9.5.11** **Adequacy of markings.** Markings shall be sufficient
    to identify a part or component. \[Source: UCRL-15673, 1985\]

-   **4.9.5.12** **Durability of markings.** Markings shall be durable
    enough to last the life of the equipment. \[Source: UCRL-15673,
    1985\]

-   **4.9.5.13** **Marking stacked parts.** If parts or modules are
    stacked, marking shall permit identification of the individual parts
    or modules. \[Source: UCRL-15673, 1985\]

-   **4.9.5.14** **Marking enclosed parts.** If a part or module is
    enclosed or shielded, the marking shall be placed outside the
    enclosure or shield. \[Source: UCRL-15673, 1985\]

-   **4.9.5.15** **Designation of parts.** The alphanumeric designation
    of a part shall be in accordance with MIL-STD-16 or comparable
    industry standards. \[Source: UCRL-15673, 1985\]

-   **4.9.5.16** **Hazard warnings.** If any hazard exists in connection
    with a part or module, a warning or caution label shall be provided
    on the part or module, on the case or cover, or both. \[Source:
    UCRL-15673, 1985\]

-   **4.9.5.17** **Labeling symmetric parts.** Parts that are symmetric
    in shape shall be labeled or marked to indicate the proper
    orientation for mounting. \[Source: MIL-HDBK-759B, 1992\]

-   **4.9.5.18** **Auxiliary information for parts.** Parts to which
    auxiliary information applies (for example, values and tolerances of
    resistors and capacitors) shall be labeled with that information.
    Preferably this information will be in alphanumeric, not coded,
    form. \[Source: UCRL-15673, 1985; MIL-HDBK-759B, 1992\]

## 4.10 Adjustment controls

Rules for accessing, adjusting, sequencing, and reading adjustment
controls are given in this section.

-   **4.10.1** **Controls and feedback.** Each adjustment control shall
    provide feedback. This feedback might be visual, audible, or
    tactile. \[Source: MIL-STD-1800A, 1990\]

-   **4.10.2** **Simultaneous access to controls and displays.**
    Maintainers shall have simultaneous access to an adjustment control
    and its associated display or other source of feedback; that is,
    they shall be able to observe the effects of adjustments as they are
    made. \[Source: UCRL-15673, 1985\]

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-   **4.10.3** **Location of adjustment controls.** All the adjustment
    controls for a module or unit of equipment should be located on a
    single surface, preferably the front panel or face of the equipment.
    \[Source: MIL-HDBK-759B, 1992\]

-   **4.10.4** **Differentiating maintenance controls from operational
    controls.** When maintenance and operation of a unit of equipment
    are performed by different sets of people, the maintenance and
    operational controls should not appear on the same panel. If
    maintenance and operational controls do appear on the same panel,
    the maintenance controls should be grouped and separated from the
    operational controls. If appropriate, the maintenance controls might
    also be guarded with removable covers so as not to interfere with
    the operator\'s performance. \[Source: UCRL-15673, 1985;
    NASA-STD-3000A, 1989\]

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-   **4.10.5** **Independence of adjustment controls.** Where possible
    and practical, the adjustment of one control shall be independent of
    the adjustments of other controls. \[Source: MIL-HDBK-759B, 1992\]

-   **4.10.6** **Sequential adjustments.** If the adjustment of one
    control affects the adjustment of another, the controls shall be
    arranged in sequential order, and labeled or marked to indicate the
    order of adjustment. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.10.7** **Functionally related adjustments.** If a maintenance
    task consists of adjusting several functionally related variables, a
    single control with a switch for selecting the particular function
    should be provided so that the maintainer can select the functions
    in sequence and make adjustments with the same control. \[Source:
    UCRL-15673, 1985\]

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-   **4.10.8** **Direct readings.** If a maintenance task requires
    adjusting a control to achieve a certain value or range of values,
    the display shall permit direct reading of the value or range; the
    maintainer shall not have to convert or transform the reading.
    \[Source: MIL-HDBK-759B, 1992\]

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-   **4.10.9** **Knob adjustments preferred to screwdriver
    adjustments.** Knob adjustments should be used rather than
    screwdriver adjustments, especially if the adjustment is made more
    than once a month. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F,
    1999; MIL-STD-1800A, 1990\]

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-   **4.10.10** **Screwdriver adjustments \-- preventing slipping.**
    When screwdriver adjustment controls are used, a positive means,
    such as guides or slots, shall be provided to prevent the
    screwdriver tip from slipping. \[Source: MIL-STD-1800A, 1990\]

-   **4.10.11** **Screwdriver guides.** If a screwdriver adjustment must
    be made without the maintainer being able to see the control, or if
    the control is located near a high voltage, screwdriver guides shall
    be provided (see Paragraph 4.6.3.2.4). \[Source: MIL-STD-1472F,
    1999; UCRL-15673, 1985\]

-   **4.10.12** **Use of mirrors or flashlights.** Maintainers shall not
    have to use mirrors or flashlights in making adjustments. \[Source:
    MIL-STD-1472F, 1999\]

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-   **4.10.13** **Remote adjustments.** If it is not practical to
    provide access to an internally located control, a remote control
    should be provided. \[Source: UCRL-15673, 1985\]

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-   **4.10.14** **Degree of adjustment.** Controls shall accommodate the
    degree of adjustment required, that is, gross adjustment, fine
    adjustment, or both. \[Source: MIL-STD-1800A, 1990\]

-   **4.10.15** **Mechanical stops.** Adjustment controls intended to
    have a limited range of motion shall have mechanical stops; these
    stops shall be capable of withstanding a force or torque 100 times
    greater than the resistance to movement within the range of
    adjustment. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

-   **4.10.16** **Preventing inadvertent adjustment.** Adjustment
    controls shall be located and mounted so that they cannot be
    adjusted inadvertently by the maintainer. \[Source: MIL-HDBK-759B,
    1992\]

-   **4.10.17** **Critical or sensitive adjustments.** Critical or
    sensitive adjustments shall incorporate features, such as locking
    devices, to prevent inadvertent or accidental adjustment. If a
    locking device is used, operation of the locking device shall not
    change the adjustment setting. \[Source: MIL-STD-1472F, 1999\]

-   **4.10.18** **Hand or arm support.** If an adjustment control or the
    maintainer will be subjected to disturbing vibration during
    adjustment, a suitable hand or arm support shall be provided.
    \[Source: MIL-STD-1472F, 1999\]

-   **4.10.19** **Avoidance of hazards.** Adjustment controls shall not
    be located close to dangerous voltages, moving machinery, or other
    hazards. If a hazardous location cannot be avoided, the controls
    shall be appropriately labeled, shielded, and guarded. \[Source:
    MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

## 4.11 Fuses and circuit breakers

This section contains rules for systems that use fuses and circuit
breakers. General rules address the selection and use of fuses and
circuit breakers, the design characteristics of fuses, push-pull circuit
breakers, toggle bat and legend switch circuit breakers, and the
labeling of fuses and circuit breakers.

### 4.11.1 General

-   **4.11.1.1** **Selection of fuses and circuit breakers.** Fuses and
    circuit breakers should be selected so that they are appropriate to
    the particular function they will perform. Exhibit 4.11.1.1 lists
    the appropriate type of fuse or circuit breaker for a variety of
    functions. \[Source: MIL-HDBK-759B, 1992\]

    **Discussion.** When selecting fuses or circuit breakers, consider
    the suitability of each to perform a particular function. There are
    two types of circuit breakers, thermal air and magnetic air. Thermal
    air circuit breakers are used primarily for overcurrent circuit
    protection. They are best adapted to dc circuits up to 250 volts,
    and to ac circuits up to 600 volts in capacities up to 600 amperes.
    Magnetic air circuit breakers may be used to provide protection in
    event of overcurrent, undercurrent, reverse current, low voltage,
    and reverse phase. \[Source: MIL-HDBK-759B, 1992\]

> []{#__RefHeading___Toc27272166 .anchor}**Exhibit 4.11.1.1** General
> comparison of fuses and circuit breakers

-   **4.11.1.2** **Location of fuses and circuit breakers.** Fuses and
    circuit breakers shall be grouped in a minimum number of
    centralized, readily accessible locations for removal, replacement,
    and resetting. \[Source: NASA-STD-3000A, 1989; MIL-STD-1472F, 1999;
    MIL-HDBK-759B, 1992\]

-   **4.11.1.3** **Verification of an open circuit.** An indication
    shall be provided when a fuse or circuit breaker has opened a
    circuit. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990;
    NASA-STD-3000A, 1989\]

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-   **4.11.1.4** **Individual fused units.** Fuses or circuit breakers
    should be provided so that each unit of a system is separately fused
    and adequately protected from harmful variations in voltages.
    \[Source: MIL-HDBK-759B, 1992\]

### 4.11.2 Fuses

-   **4.11.2.1** **Using fuses.** Fuses shall conform to MIL-F-15160.
    \[Source: MIL-HDBK-759B, 1992\]

-   **4.11.2.2** **Worker safety.** Fuse installations shall be designed
    so that only the \"cold\" terminal of the fuse can be touched by
    maintenance personnel. \[Source: MIL-HDBK-759B, 1992\]

-   **4.11.2.3** **Safeguarding the circuit.** Fuses shall be provided
    that safeguard the circuit if the wrong switch or jack position is
    used. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.11.2.4** **Quick-disconnect fuse holders.** Fuse holder cups or
    caps should be of the quick-disconnect type rather than the screw-in
    type; they should be knurled and large enough to be removed easily
    by hand. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.11.2.5** **No special tools for fuse replacement.** Fuse
    replacement shall not require special tools, unless they are needed
    for safety. \[Source MIL-STD-1800A, 1990; MIL-STD-1472F, 1999; AFSC
    DH 1-3, 1980\]

-   **4.11.2.6** **No other components to be removed.** Fuses shall be
    located so they can be replaced without removing any other
    components. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1800A, 1990; AFSC
    DH 1-3, 1980; MIL-STD-1472F, 1999\]

-   **4.11.2.7** **Spare fuse provisions.** Spare fuses and holders for
    them shall be provided and located near fuse holders. Labels
    adjacent to these spare fuse holders shall contain the word
    \"SPARE\" and shall state the fuse values and functions. \[Source:
    MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

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-   **4.11.2.8** **Anticorrosion precautions.** A silicon electrical
    lubricating compound should be applied to the fuse and the interior
    of the fuse holder. The exterior of the fuse holder (except contact
    surfaces) should be coated with fungicidal varnish. Sealed fuses
    should be used. \[Source: MIL-HDBK-759B, 1992\]

### 4.11.3 Push-pull circuit breakers

-   **4.11.3.1** **Push-pull circuit breaker specifications.** Push-pull
    actuated circuit breaker dimensions, displacement, and separation
    shall conform to Exhibit 4.11.3.1. \[Source: MIL-STD-1472F, 1999\]

> []{#__RefHeading___Toc27272167 .anchor}**Exhibit 4.11.3.1** Push-pull
> circuit breaker specifications
> ![](media/image29.png){width="6.490972222222222in"
> height="6.479166666666667in"}
>
> \[Source: MIL-STD-1472F; MIL-HDBK-759B, 1992; MIL-STD-1800A\]

-   **4.11.3.2** **Power switches.** Push-pull type circuit breakers
    shall not be used as power switches. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

### 4.11.4 Toggle bat and legend switch circuit breakers

Toggle bat and legend switch actuated circuit breakers may be used to
control electrical power. \[Source: MIL-STD-1472F, 1999\]

-   **4.11.4.1** **Toggle bat specifications.** Dimensions, resistance,
    displacement, and separation for toggle bat actuated breakers shall
    comply with Exhibit 4.11.4.1. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

[]{#__RefHeading___Toc27272168
.anchor}![](media/image30.png){width="6.444444444444445in"
height="6.986111111111111in"}**Exhibit 4.11.4.1** Toggle bat
specifications

-   **4.11.4.2 Legend switch specifications.** Legend switch actuated
    breakers shall comply with the dimension, displacement, separation,
    and resistance criteria shown in Exhibit 4.11.4.2. \[Source:
    MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

[]{#__RefHeading___Toc27272169 .anchor} **Exhibit 4.11.4.2** Legend
switch specifications. \[Source: MIL-STD-1472F;
MIL-STD-1800A\]![](media/image31.png){width="6.461111111111111in"
height="5.386805555555555in"}

### 4.11.5 Labeling and marking

-   **4.11.5.1** **Fuses and circuit breakers.** Fuses and circuit
    breakers shall be permanently labeled or marked. The labeling or
    marking shall be legible in the anticipated ambient illumination
    range for the maintainer\'s location. \[Source: MIL-STD-1472F, 1999;
    MIL-STD-1800A, 1990\]

-   **4.11.5.2** **Fuse ratings.** A fuse\'s rating shall be indicated
    adjacent to the fuse. The rating shall be in whole numbers, common
    fractions, such as 1/2, or whole numbers and common fractions, such
    as 2 1/2. \[Source: MIL-HDBK-759B, 1992; MIL-STD-1472F, 1999\]

-   **4.11.5.3** **Circuits.** The area of equipment served by a fuse or
    circuit breaker shall be identified. \[Source: MIL-STD-1800A, 1990;
    MIL-STD-1472F, 1999\]

## 4.12 Test points and service points

Strategically placed test points make signals available to maintenance
personnel for checking, adjusting, and troubleshooting. Test points are
recommended for units of equipment that are not completely
self-checking. \[Source: UCRL-15673, 1985\]

**Definition.** **Test points** are a means for conveniently and safely
determining the operational status of equipment and for isolating
malfunctions. **Service points** are a means for lubricating, filling,
draining, charging, and performing other service functions. They permit
the routine performance of these services on all equipment and
components requiring them. \[Source: UCRL-15673, 1985\]

### 4.12.1 Adjustment controls

-   **4.12.1.1** **Location.** An adjustment control associated with a
    test point shall be located near the test point and shall provide a
    signal at the test point that indicates clearly when the correct
    adjustment has been achieved. \[Source: AFSC DH 1-3, 1980;
    MIL-HDBK-759B, 1992\]

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-   **4.12.1.2** **Individual adjustment controls.** A test point should
    not have more than one associated adjustment control. \[Source: AFSC
    DH 1-3, 1980\]

### 4.12.2 Location and arrangement

-   **4.12.2.1** **Test points for units of equipment.** A test point
    should be provided for each input to and output from a unit of
    equipment. \[Source: AFSC DH 1-3, 1980\]

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-   **4.12.2.2** **Arranging test points.** When testing complexity
    warrants, test points shall be arranged on a single control panel or
    on a series of functionally autonomous panels. \[Source: UCRL-15673,
    1985; AFSC DH 1-3, 1980\]

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-   **4.12.2.3** **Tracing signals.** Test points should be provided to
    permit the systematic tracing of signals and voltages through a unit
    of equipment. These test points allow a maintainer to determine the
    point at which signals or voltages in a malfunctioning unit are out
    of tolerance. \[Source: AFSC DH 1-3, 1980\]

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-   **4.12.2.4** **Test and service point accessibility.** All test and
    service points shall be physically and visually accessible to the
    maintainer. \[Source: MIL-HDBK-759B, 1992\]

-   **4.12.2.5** **Proximity to associated controls and displays.** Test
    and service points shall be located in physical and visual proximity
    to the controls and displays used to make the adjustments associated
    with the points. \[Source: NASA-STD-3000A, 1989; MIL-STD-1472F,
    1999; UCRL-15673, 1985\]

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-   **4.12.2.6** **Test and service point location.** Test and service
    points should be provided, designed, and located in accordance with
    their frequency of use and any applicable time limits on maintenance
    activities. \[Source: MIL-HDBK-759B, 1992\]

-   **4.12.2.7** **Minimizing testing and servicing.** Requirements for
    periodic or repetitive testing and servicing of components should be
    avoided if possible by using sealed bearings, oil impregnated
    bushings, highly reliable components, and the like. \[Source:
    UCRL-15673, 1985\]

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-   **4.12.2.8** **Minimizing test and service points.** To reduce the
    number of test and service points required, built-in indicators,
    center reading meters, pressure gauges, direct reading fluid level
    gauges, and the like shall be used for quick checks, thus avoiding
    the need for auxiliary equipment. \[Source: UCRL-15673, 1985\]

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-   **4.12.2.9** **Avoid isolated test or service points.** Isolated
    test or service points should be avoided; such points are likely to
    be overlooked or neglected. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.12.2.10** **Compatibility of test and service points.** Test and
    service points shall be designed for compatibility with checking,
    troubleshooting, and servicing procedures and with test and service
    equipment. \[Source: UCRL-15673, 1985\]

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-   **4.12.2.11** **Distinctive connections.** Distinctively different
    connectors or fittings should be provided for each type of test or
    service equipment (for example, grease and oil fittings should be
    distinctively different from each other) to minimize the likelihood
    of error. \[Source: MIL-HDBK-759B, 1992\]

-   **4.12.2.12** **Avoid separate accessories.** Separate funnels,
    strainers, adapters, and other accessories should be avoided. Where
    practical, these accessories should be built into the equipment or
    the service equipment, so that they need not be handled separately.
    \[Source: MIL-HDBK-759B, 1992\]

-   **4.12.2.13** **Terminal strips.** If special test points are not
    provided on electrical equipment, cables should be fanned out on
    terminal strips as illustrated in Exhibit 4.12.2.13. \[Source:
    UCRL-15673, 1985\]

> []{#__RefHeading___Toc27272170 .anchor}**Exhibit 4.12.2.13** Terminal
> strips![](media/image32.png){width="2.2472222222222222in"
> height="1.0506944444444444in"}

### 4.12.3 Drain points

-   **4.12.3.1** **Drain provisions.** Drains shall be provided on all
    fluid tanks and systems, fluid filled cases, filter systems, float
    chambers, and other items that are likely to contain fluid that
    would otherwise be difficult to remove. \[Source: UCRL-15673, 1985\]

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-   **4.12.3.2** **Minimization.** The number of types and sizes of
    drain fittings should be minimized and standardized throughout the
    system. \[Source: UCRL-15673, 1985\]

-   **4.12.3.3** **Valves and petcocks versus drain plugs.** Whenever
    practical, valves or petcocks should be used rather than drain
    plugs. \[Source: UCRL-15673, 1985\]

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-   **4.12.3.4** **Drain plugs**. Drain plugs shall require only common
    hand tools for operation, and their placement shall ensure adequate
    tool and work clearance for operation. \[Source: UCRL-15673, 1985\]

-   **4.12.3.5 Labels.** Drain cocks or valves shall be clearly labeled
    to indicate open and closed positions, and the direction of movement
    required for opening. \[Source: UCRL-15673, 1985\]

-   **4.12.3.6** **Drain cock motions.** Drain cocks shall always close
    with clockwise motion and open with counterclockwise motion.
    \[Source: UCRL-15673, 1985\]

-   **4.12.3.7** **Instruction labels.** Instruction labels shall be
    provided, as necessary, to ensure that the fluid system is properly
    prepared prior to draining. \[Source: UCRL-15673, 1985\]

-   **4.12.3.8** **Accessibility.** As applicable, drain points shall be
    designed, located, and installed

a.  where they are reachable and operable by the maintainer,

b.  so that fluid will not drain or spill on equipment or personnel,

c.  at the lowest point in the system if complete draining is required,

d.  to permit selective draining or bleeding to facilitate maintenance
    procedures,

e.  to permit drainage directly into a waste container without use of
    separate adapters or piping, and

f.  so that fuel or other combustible fluids cannot run down to or
    collect in hazardous areas. \[Source: UCRL-15673, 1985\]

### 4.12.4 Accessibility

-   **4.12.4.1** **Test and service point accessibility.** Test and
    service points shall be easily accessible for checking and
    troubleshooting. Recommended minimum clearances are 19 mm (0.75 in)
    when only finger control is required, and 75 mm (3 in) when the
    gloved hand is used. \[Source: UCRL-15673, 1985; AFSC DH 1-3, 1980\]

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-   **4.12.4.2** **Test probe guides.** Suitable guides for test probes
    should be provided when test points are located internally.
    \[Source: UCRL-15673, 1985\]

-   **4.12.4.3** **Test accesses.** Test accesses should be provided for
    mechanical components likely to wear. For example, brake assemblies
    should be provided with an inspection opening to permit insertion of
    a gauge for determining the clearance between the brake lining and
    drum. \[Source: UCRL-15673, 1985; AFSC DH 1-3, 1980\]

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-   **4.12.4.4** **Test points in plugs.** If appropriate, plugs with
    integral test points for each input and output shall be used. If
    dust or moisture is a factor, an integral sliding cover for the test
    points shall be provided on the plug, as shown in Exhibit 4.12.4.4
    (a). An acceptable alternative is the provision of a test-point
    adapter for insertion between a plug and its receptacle, as shown in
    Exhibit 4.12.4.4 (b). \[Source: AFSC DH 1-3, 1980\]

> []{#__RefHeading___Toc27272171 .anchor}**Exhibit 4.12.4.4 (a)** Test
> plug with sliding cover

\[Source: AFSC DH 1-3, 1980; UCRL-15673,
1985\]![](media/image33.png){width="2.4027777777777777in"
height="0.9444444444444444in"}

> []{#__RefHeading___Toc27272172 .anchor}**Exhibit 4.12.4.4 (b)** Test
> point adapter
>
> \[Source: AFSC DH 1-3, 1980; UCRL-15673,
> 1985\]![](media/image34.png){width="2.25in" height="1.01875in"}

### 4.12.5 Safety

-   **4.12.5.1** **Test point shielding.** All test points shall be
    located or shielded to protect the maintainer against contact with
    high voltages. \[Source: AFSC DH 1-3, 1980\]

-   **4.12.5.2** **Minimum clearance.** Test and service points shall be
    separated by more than a hand\'s width, 114 mm (4.5 in), from the
    nearest hazard. \[Source: UCRL-15673, 1985\]

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-   **4.12.5.3** **Recessed test and service points.** Test and service
    points should be recessed to protect them from damage by personnel,
    dust, moisture, and the like. \[Source: UCRL-15673, 1985\]

-   **4.12.5.4** **High pressure test indicators.** High pressure test
    indicators should be used wherever possible to avoid some of the
    hazards associated with making temporary high pressure connections.
    \[Source: UCRL-15673, 1985\]

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-   **4.12.5.5** **Ground points.** If a good grounding point is not
    available, a special \"ground\" point shall be provided. Connection
    to this special ground point shall be made during tests of a given
    unit. \[Source: UCRL-15673, 1985\]

    **Discussion.** Maintainers may have difficulty if only painted
    surfaces are available for ground connections. \[Source: UCRL-15673,
    1985\]

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-   **4.12.5.6** **Shields around lubrication points.** Shields should
    be provided around lubrication points that may be serviced while
    equipment is operating. \[Source: UCRL-15673, 1985\]

### 4.12.6 Labeling, marking, and coding

-   **4.12.6.1** **Label location.** Labels for test and service points
    shall be in full view of the maintainer making connections or
    adjustments. \[Source: UCRL-15673, 1985\]

-   **4.12.6.2** **Distinguishable marking.** Test and service points
    shall be designed and marked so that they are easily
    distinguishable, for example, by coding them with distinctive
    colors. \[Source: UCRL-15673, 1985; AFSC DH 1-3, 1980\]

-   **4.12.6.3** **Distinguishing test and service points.** If
    color-coding is used, the color of test points shall be clearly
    different from the color of service points. \[Source: UCRL-15673,
    1985\]

-   **4.12.6.4** **Hazardous points.** Hazardous test and service points
    shall be labeled to warn maintainers about any possible injury to
    themselves or damage to internal circuits. \[Source: NASA-STD-3000A,
    1989; UCRL-15673, 1985\]

-   **4.12.6.5** **Identification of test points.** Each test point
    shall be clearly labeled with a number, letter, symbol, or
    description of its function or, at a minimum, with a code number
    keyed to the user documentation. \[Source: NASA-STD-3000A, 1989;
    AFSC DH 1-3, 1980\]

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-   **4.12.6.6** **Luminescent markings.** If test points must be read
    under very low ambient illumination, they should be marked in
    phosphorescent colors. \[Source: AFSC DH 1-3, 1980\]

-   **4.12.6.7** **Tolerance limits.** Each test point should be labeled
    with the tolerance limits of the signal to be measured there.
    \[Source: AFSC DH 1-3, 1980\]

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-   **4.12.6.8** **Internal test and service points.** When a test or
    service point is located internally, its location shall be indicated
    on the cover or adjacent to its access opening on the surface of the
    equipment. \[Source: UCRL-15673, 1985\]

## 4.13 Test equipment

This section contains general rules for test equipment and specific
rules for four types of test equipment: (a) built-in, (b) go, no-go, (c)
automatic, and (d) collating. Bench mockups and storage for test
equipment are also addressed.

**Definitions.** **Built-in test equipment** is an integral part of a
unit of equipment and can range from a simple voltmeter to a complex
automatic checker. **Go, no-go test equipment** provides one of two
alternative answers to any question. For example, it tells whether a
given signal is in or out of tolerance. **Automatic test equipment**
checks two or more signals in sequence without the intervention of a
maintainer. The test usually stops when the first out-of-tolerance
signal is detected. **Collating test** **equipment** presents the
results of two or more checks as a single display; for example, a light
might come on only if a number of different signals are in tolerance.
\[Source: AFSC DH 1-3, 1980; UCRL-15673, 1985\]

### 4.13.1 General

Test equipment is intended to (a) simplify the job of the maintainer,
(b) reduce the preparation or turn-around time for installing,
maintaining, and repairing systems, and (c) reduce total maintenance
costs. It needs to be fast, easy, and safe to use.

The type of test equipment is decided upon in the early stages of
equipment design. Selection of test equipment depends on (a) the mission
and operational characteristics of the equipment, (b) the anticipated
reliability of the equipment, (c) the maintenance concept, (d) the
available personnel, (e) the operational environment, (f) the logistics
support requirements, and (g) the development time and cost. \[Source:
UCRL-15673, 1985\]

#### 4.13.1.1 General characteristics

The following general rules are aimed at simplifying the maintainers\'
job.

-   **4.13.1.1.1** **Test equipment treatment.** Test equipment and
    bench mockups shall be treated like any other equipment with respect
    to design requirements for units, covers, cases, cables, connectors,
    test points, displays, and controls. \[Source: UCRL-15673, 1985,
    1.8.2\]

-   **4.13.1.1.2** **Accuracy of test equipment.** The accuracy of all
    test equipment shall exceed that of the equipment being tested.
    \[Source: NASA-STD-3000A, 1989\]

-   **4.13.1.1.3** **Conversion tables.** Conversion tables shall not be
    used in deciding if equipment is within tolerances. \[Source:
    UCRL-15673, 1985\]

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-   **4.13.1.1.4** **Selector switches.** Selector switches should be
    used rather than multiple plug-in connections as long as the effects
    of switching do not degrade the desired information. \[Source: AFSC
    DH 1-3, 1980; UCRL-15673, 1985\]

    **Discussion.** Selector switches can be used more quickly than
    plug-in connections, and they reduce the likelihood of faulty
    connections. \[Source: AFSC DH 1-3, 1980; UCRL-15673, 1985\]

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-   **4.13.1.1.5** **Maintenance instructions.** Clear operating and
    maintenance instructions shall be prepared and available to the
    maintainer. \[Source: AFSC DH 1-3, 1980\]

-   **4.13.1.1.6** **Storing instructions.** Full instructions shall be
    stored inside the test equipment\'s cover or case, if any, or
    attached to a metal plate containing a checklist for operating the
    equipment. \[Source: AFSC DH 1-3, 1980\]

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-   **4.13.1.1.7** **Labeling.** The outer case and all removable parts
    should be clearly labeled with the equipment identification,
    including its purpose and any precautions that should be observed in
    using it. \[Source: AFSC DH 1-3, 1980\]

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-   **4.13.1.1.8** **Label contents.** The label shall contain all items
    the maintainer must be able to recognize, read, or use. \[Source:
    AFSC DH 1-3, 1980\]

#### 4.13.1.2 Safety

-   **4.13.1.2.1** **Shielding hazardous parts.** Test equipment shall
    be designed so that all exposed moving and cutting parts are
    shielded to prevent maintainer injuries during maintenance tasks.
    \[Source: UCRL-15673, 1985\]

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-   **4.13.1.2.2** **Minimizing hazards.** When possible, fail-safe
    features should be incorporated in test equipment to minimize any
    danger to maintainers or equipment. \[Source: UCRL-15673, 1985\]

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-   **4.13.1.2.3** **Internal controls.** Internal controls shall be
    located away from dangerous voltages. \[Source: UCRL-15673, 1985\]

-   **4.13.1.2.4** **Safeguarding high voltages.** High voltage areas
    shall be insulated or shielded. \[Source: UCRL-15673, 1985\]

-   **4.13.1.2.5** **Warning labels.** Adequate warnings shall be
    provided wherever potential hazards exist. \[Source: UCRL-15673,
    1985\]

#### 4.13.1.3 Ease of use

-   **4.13.1.3.1** **Accessibility.** Adjustment points, test points,
    cables, connectors, and labels for all required maintenance tasks
    shall be visually and physically accessible. Access openings
    necessary to connect test equipment using required tools shall
    accommodate maintainers and equipment as specified in Paragraph
    4.4.3 and Paragraph 4.12.4.1. \[Source: MIL-STD-1800A, 1990\]

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-   **4.13.1.3.2** **Minimizing test equipment.** The number and types
    of test equipment and accessories, such as connectors and test
    cables, should be minimized. \[Source: UCRL-15673, 1985\]

-   **4.13.1.3.3** **Ease of use.** Equipment should be simple to
    operate, have self-checking and calibrating features, and have a
    minimum number of controls and displays. \[Source: AFSC DH 1-3,
    1980; UCRL-15673, 1985\]

-   **4.13.1.3.4** **Reducing the number and complexity of steps.** The
    number and complexity of steps should be reduced by grouping
    controls (such as by sequence or criticality) or by making certain
    operations automatic. \[Source: AFSC DH 1-3, 1980\]

-   **4.13.1.3.5** **Individual operation.** Test equipment should be
    designed for operation by one person. \[Source: UCRL-15673, 1985\]

#### 4.13.1.4 Controls and displays

-   **4.13.1.4.1** **Calibration check.** Test equipment should be
    easily calibrated or equipped with a simple check (for example, a
    go, no-go indicator) to indicate whether or not the test equipment
    is out of calibration or is malfunctioning. \[Source: AFSC DH 1-3,
    1980; UCRL-15673, 1985\]

-   **4.13.1.4.2** **Warm-up indicators.** A warm-up indicator should be
    provided, if applicable, to show when the test equipment is warmed
    up and ready to use. If such a signal cannot be provided, a label
    near the warm-up switch should state clearly how much warm-up time
    is required. \[Source: UCRL-15673, 1985; AFSC DH 1-3, 1980\]

-   **4.13.1.4.3** **Automatic shutoff switches.** If feasible, test
    equipment should have an automatic shutoff. If it is not feasible,
    test equipment should have both warning lights and written warnings
    to remind the maintainer to turn the equipment off when finished.
    \[Source: UCRL-15673, 1985\]

-   **4.13.1.4.4** **Misalignment.** Controls and displays should be
    designed to prevent misalignment that might be caused by vibration,
    service use, or accidental contact. \[Source: UCRL-15673, 1985\]

### 4.13.2 Built-in test equipment (and partially built-in, portable, and test panels)

The advantages of built-in test equipment include (a) being less likely
than portable test equipment to be lost or damaged, (b) being available
when needed, and (c) requiring no special storage facilities.

The disadvantages of built-in test equipment include (a) adding to the
weight and space requirements of the equipment being tested, (b)
requiring more built-in test equipment because a separate item is
usually required for each unit of equipment, (c) transporting built-in
test equipment to a point for convenient calibration may be more
difficult than transporting portable test equipment, and (d) installing
test equipment permanently may increase the complexity of wiring for the
system and may even increase the amount of maintenance activity.

**Definition.** **Built-in test equipment** is an integral part of a
unit of equipment and can range from a simple voltmeter to a complex
automatic checker.

#### 4.13.2.1 Completely built-in test equipment

-   **4.13.2.1.1** **Combining test points.** If a unit of equipment has
    built-in test capabilities, all maintenance tests should be
    performed with the built-in test unit. \[Source: AFSC DH 1-3, 1980\]

-   **4.13.2.1.2** **Efficiency.** If possible, built-in test units
    should be integrated into the equipment for efficient maintenance
    and troubleshooting. \[Source: UCRL-15673, 1985\]

    **Discussion.** If voltages and wave shapes must be checked, for
    example, the test unit might consist of a meter, an oscilloscope,
    and a rotary switch for selecting circuits. \[Source: UCRL-15673,
    1985\]

-   **4.13.2.1.3** **Easy to use.** Meters and oscilloscopes should have
    fixed, preset circuits so that the meter always reads center scale
    and the oscilloscope requires no adjustment. \[Source: UCRL-15673,
    1985\]

-   **4.13.2.1.4** **In-tolerance.** Either an in-tolerance meter
    reading or an in-tolerance waveshape on the oscilloscope should be
    coded for each position of the rotary switch. If more test points
    are needed than can be handled by a single switch, multiple switches
    can be used. \[Source: UCRL-15673, 1985\]

#### 4.13.2.2 Partially built-in test equipment

-   **4.13.2.2.1** **Combining test points.** If possible, all test
    points should be incorporated into one built-in unit of test
    equipment. \[Source: AFSC DH 1-3, 1980\]

-   **4.13.2.2.2** **Test capabilities.** To the extent feasible, all
    the test capabilities described in Paragraph 4.13.2.1 should be
    built-in. \[Source: AFSC DH 1-3, 1980; UCRL-15673, 1985\]

    **Example.** A center-reading meter might be mounted on each major
    component that can be checked by a meter, and a set of test jacks
    might be provided as an outlet for signals requiring an
    oscilloscope. \[Source: AFSC DH 1-3, 1980; UCRL-15673, 1985\]

#### 4.13.2.3 Portable test equipment

-   **4.13.2.3.1** **When to use.** If it is not practical to
    incorporate all test points into one unit of built-in test equipment
    or to provide a center-reading meter and test jacks for an external
    oscilloscope, a portable test unit shall be provided. \[Source: AFSC
    DH 1-3, 1980\]

-   **4.13.2.3.2** **Single connection.** Portable test equipment shall
    connect to its associated unit of equipment or partially built-in
    test equipment through a single, multi-prong connector. \[Source:
    NASA-STD-3000A, 1989; AFSC DH 1-3, 1980; UCRL-15673, 1985\]

-   **4.13.2.3.3** **Internal storage.** Portable test equipment shall
    have enough storage space in its handling case or lid to contain
    leads, probes, spares, and any special tools required for operation.
    \[Source: MIL-STD-1800A, 1990; MIL-STD-1472F, 1999\]

-   **4.13.2.3.4** **Operating instructions.** Instructions for
    operating portable test equipment shall be provided on the face of
    the test equipment, on its case or cover, if any, or in a special
    storage compartment in the test. These instructions shall be easily
    readable by the maintainer while the test equipment is being
    operated. If applicable, the instructions shall include a reminder
    to calibrate the equipment and instructions for calibration.
    \[Source: MIL-STD-1800A, 1990\]

-   **4.13.2.3.5** **Calibration records.** If applicable, a placard
    shall be attached to the equipment for recording calibration
    information, including tolerance check values. \[Source:
    MIL-STD-1472F, 1999\]

#### 4.13.2.4 Built-in test panel

-   **4.13.2.4.1** **When to use.** If built-in, partially built-in, or
    portable test units are not practical, a test panel should be
    provided on the equipment. \[Source: UCRL-15673, 1985\]

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-   **4.13.2.4.2** **Test point connections.** Test points shall permit
    the connection of appropriate test equipment, such as voltage meters
    or oscilloscopes. \[Source: UCRL-15673, 1985\]

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-   **4.13.2.4.3** **Block diagram.** The test points on a test panel
    should be arranged within a miniature block diagram of the system,
    with each block representing components or units of equipment.
    \[Source: UCRL-15673, 1985\]

-   **4.13.2.4.4** **Overlays.** Overlays for the test panel should be
    provided to direct the maintainer to test points that should be
    checked and the order in which they should be checked. Instructions
    should be provided in the user documentation. \[Source: UCRL-15673,
    1985\]

-   **4.13.2.4.5** **Tolerance limits for signals.** Tolerance limits
    for signals should be shown on overlays, and test points should be
    coded on the test panel. Full instructions should be provided in
    user documentation so that they are still available in the event an
    overlay is lost. \[Source: UCRL-15673, 1985\]

### 4.13.3 Test equipment {#test-equipment-1 .unnumbered}

There are three classifications of test equipment: **go/no-go test
equipment**, **automatic test equipment**, and **collating test
equipment.**

**Definition.** **Go, no-go test equipment** provides one of two
alternative answers to any question. For example, it tells whether a
given signal is in or out of tolerance.

**Definition. Automatic test equipment** checks two or more signals in
sequence without the intervention of a maintainer. Testing usually stops
when the first out-of-tolerance signal is detected.

**Definition. Collating test equipment** presents the results of two or
more checks as a single display; for example, a \"test passed\" light
would come on only if all of the relevant signals are in tolerance.

The advantages of go, no-go test equipment include (a) presenting
information in a clear, unambiguous manner and (b) simplifying difficult
tasks, such as balancing circuits and checking complex waveshapes.

The disadvantages include (a) requiring unique circuitry for each signal
value to be tested (sometimes, however, ordinary displays can be
converted to go, no-go displays by appropriate use of reference scales
such as a colored section on a meter dial), (b) increasing the number
and complexity of circuits required, which will probably add to initial
cost and development time and increase the rate of test equipment
breakdown, (c) providing relatively little help to the maintainer in
checking common voltages or simple waveshapes, and (d) requiring a
special model for many or most units of equipment. \[Source: UCRL-15673,
1985\]

An advantage of automatic test equipment is that it can make a rapid
sequence of checks with little or no chance of omitting steps.

Disadvantages of automatic test equipment include: (a) it can be
relatively expensive, large, and heavy, and it may require maintenance
of itself, (b) it can be relatively specialized, with little
versatility, (c) it can require self-checking features to detect test
equipment malfunctioning, which adds to cost and to problems of
maintaining the test equipment, and (d) it will probably require a
special model for each unit of equipment. \[Source: UCRL-15673, 1985;
AFSC DH 1-3, 1980\]

An advantage of collating test equipment is that it reduces the number
of displays the maintainer must read, thereby reducing testing time and,
probably, errors.

The disadvantages are similar to those for go, no-go and automatic test
equipment. \[Source: UCRL-15673, 1985; AFSC DH 1-3, 1980\]

-   **4.13.3.1** **Identification of out-of-tolerance signals.** If
    equipment fails a test performed by collating test equipment, the
    test equipment should indicate which signal(s) are out of tolerance,
    not just that the equipment failed the test. \[Source: UCRL-15673,
    1985\]

### 4.13.4 Bench mockups

-   **4.13.4.1** **Accessibility.** Bench mockup cables should comply
    with 4.8.1.7, Bench mockup cables.

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-   **4.13.4.2** **Accessibility.** Adequate space shall be provided in
    the layout of a mockup to allow the maintainer to perform any
    required maintenance activities on the units. \[Source: AFSC DH 1-3,
    1980\]

-   **4.13.4.3** **Support for test equipment.** Pullout shelves or some
    other method of supporting the test equipment shall be provided
    while the test equipment is being used. \[Source: AFSC DH 1-3,
    1980\]

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-   **4.13.4.4** **Test leads.** Test leads should require no more than
    a fraction of a turn for attachment to the equipment being
    maintained. \[Source: AFSC DH 1-3, 1980\]

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-   **4.13.4.5 Signal values.** The operating instructions for bench
    mockups shall include correct signal values and tolerances for each
    test point. \[Source: AFSC DH 1-3, 1980\]

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-   **4.13.4.6** **Covers.** Transparent, plastic covers should be used
    on mockup units that contain parts whose operation may be checked
    visually, unless a metal cover is needed for electrical shielding.
    \[Source: AFSC DH 1-3, 1980\]

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-   **4.13.4.7** **Easy access.** All mockup units shall be installed so
    that every unit is accessible without removing any other unit.
    \[Source: AFSC DH 1-3, 1980\]

### 4.13.5 Storage space

-   **4.13.5.1** **Available storage.** Storage space shall be provided
    for removable items, for example, test leads. \[Source: AFSC DH 1-3,
    1980\]

-   **4.13.5.2** **Securing accessories.** Fasteners and holders shall
    be provided to hold accessories securely and safely in the storage
    compartment. \[Source: AFSC DH 1-3, 1980\]

-   **4.13.5.3** **Labeling.** A label shall show the intended contents
    of the storage compartment and how they should be stored. \[Source:
    AFSC DH 1-3, 1980\]

-   **4.13.5.4** **Handles.** If test equipment has hinged handles on
    the cover or case, the handles shall be recessed for convenient
    storage. \[Source: AFSC DH 1-3, 1980\]

## 4.14 Tools

Rules for common hand tools and special tools are given in this section.
The tools required by maintainers depend upon the nature of the
maintenance tasks and the characteristics of the equipment. It is highly
desirable that the need for special tools, that is, tools other than the
most common types and sizes of hand tools, be eliminated or at least
minimized. This goal can best be accomplished early in the equipment
design process, but it deserves attention throughout design and
development. \[Source: MIL-HDBK-759B, 1992\]

### 4.14.1 General

-   **4.14.1.1** **Minimize maintenance tools.** Units of equipment
    shall be designed to minimize the numbers and types of auxiliary
    tools required to accomplish maintenance tasks. \[Source:
    NASA-STD-3000A, 1989\]

-   **4.14.1.2** **Use common tools.** Whenever possible, units of
    equipment shall be designed to use common tools for maintenance.
    \[Source: AFSC DH 1-3, 1980; MIL-HDBK-759B, 1992\]

-   **4.14.1.3** **Minimize variety and sizes of tools required.** The
    variety and number of different sizes of tools required shall be
    minimized; ideally, the tools required shall be limited to those
    normally found in a maintainer\'s tool kit. \[Source: MIL-HDBK-759B,
    1992\]

-   **4.14.1.4** **Special tools.** Uncommon or specially-designed tools
    shall be used only when common hand tools do not satisfy the
    requirements or when the special tools provide a significant
    advantage over common hand tools. Special tools shall not be
    required or used without the approval of the acquisition program
    office. \[Source: MIL-STD-1472F, 1999; MIL-STD-1800A, 1990\]

    []{.mark}

### 4.14.2 Common hand tools

-   **4.14.2.1** **Use common test equipment and tools.** Systems and
    units of equipment shall be designed so that maintenance can be
    accomplished with common test equipment and tools. \[Source: AFSC DH
    1-3, 1980\]

-   **4.14.2.2** **Gripping surfaces.** Tool handles shall have adequate
    gripping surfaces. \[Source: MIL-HDBK-759B, 1992\]

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-   **4.14.2.3** **Providing thongs.** If a tool will be used where
    dropping it could cause injury, damage, or significant loss of time,
    the tool should be provided with a thong or other means of
    attachment to the maintainer or the equipment. \[Source:
    MIL-HDBK-759B, 1992\]

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-   **4.14.2.4** **Insulation of handles.** If a tool will be used in
    the vicinity of voltages in excess of 30 volts, the tool handle and
    any other part of the tool the maintainer is likely to touch shall
    be electrically insulated. \[Source: MIL-HDBK-759B, 1992\]

### 4.14.3 Special tools

-   **4.14.3.1** **Reasons for requiring special tools.** Special tools
    shall be required only when common tools cannot be used or when they
    are necessary to facilitate maintenance tasks, reduce time, or
    improve accuracy. \[Source: MIL-HDBK-759B, 1992\]

-   **4.14.3.2** **Availability of special tools.** If a special tool is
    required for the maintenance of a unit of equipment, the tool shall
    be made available at the same time as the equipment. \[Source:
    MIL-HDBK-759B, 1992\]

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-   **4.14.3.3** **Attach to equipment.** If a unit of equipment
    requires a special tool for maintenance, the tool should be mounted
    on or attached to the equipment in a readily accessible location.
    \[Source: MIL-STD-1472F, 1999\]

[]{#__RefHeading___Toc34717537 .anchor}Glossary

**Automatic test equipment** - Checks two or more signals in sequence
without the intervention of a maintainer. The test usually stops when
the first out-of-tolerance signal is detected.

**Built-in test equipment** - An integral part of a unit of equipment
and can range from a simple voltmeter to a complex automatic checker.

**Cable** - A number of lines bound together within a single, permanent
sheath.

**Case** - The part of a unit of equipment that encloses and protects
the equipment from its surroundings. It may also serve to protect the
surroundings - including maintainers - from the equipment.

**Collating test** **equipment** - Presents the results of two or more
checks as a single display; for example, a light might come on only if a
number of different signals are in tolerance.

**Component** - A subdivision of a unit of equipment that can be treated
as an object by the maintainer, but which can be further broken down
into parts. A mounting board together with its mounted parts is an
example of a component.

**Connector** - A piece of hardware that joins or attaches lines or
cables to other lines or cables or to units of equipment. The term is
used rather loosely to refer to either of the two parts that mate with
each other and to the plug that mates with a receptacle.

**Cover** - A part of a unit of equipment that closes an access opening.

**Fasteners** - Devices that join, attach, and mount parts, components,
cases, covers, and units of equipment. They include quick fastening and
releasing devices, screws, bolts, latches, catches, rivets, retainer
rings, and retainer chains.

**Go, no-go test equipment** - Provides one of two alternative answers
to any question. For example, it tells whether a given signal is in or
out of tolerance.

**Guard** - An enclosure or barrier intended to prevent inadvertent or
unauthorized operation of a control.

**Hazardous condition** - The presence of energy or a substance which is
likely to cause death or injury by reason of physical force, shock,
radiation, explosion, flames, poison, corrosion, oxidation, irritation
or other debilitation. Biological and chemical hazards can have
debilitating effects through disease or interference with physiological
functions.

**Hazardous location** - A space within a facility, room, or open
environment where a hazardous condition exists or is accessible or
exposed within the system or equipment located within the space.

**Interlocks** - Devices (for example, switches) connected with a cover,
shield, or case that disable the associated internal hazard (usually
electrical) when the cover, shield, or case is opened. OSHA regulations
discuss lockout and tagout procedures to be used in the workplace during
maintenance or operations to protect from electrical hazards.

**Item** - A nonspecific term used to denote any product, available or
in design or development, including parts, components, modules, and
units of equipment.

**Label** - Alphanumeric information that identifies or describes an
object. Labels can be printed directly on or adjacent to the object, or
they can be printed on a card or plate that is attached to the object or
adjacent to the object.

**Layout** - The physical arrangement of the parts and components that
make up a module or a unit of equipment.

**Limit stops** - Mechanisms that restrict a moving object or part by
stopping it at predetermined (limit) positions.

**Line** - Any single length of pipe, wire, or tubing.

**Lockout** - Uses a mechanical means to disable a control or switch in
its safe position (for example, electricity disconnected) and to prevent
its activation without the use of undue force or tools.

**Marking** - Nonverbal information, such as colors or symbols, that
identifies or describes an object. Marking can appear directly on or
adjacent to the object, or it can be printed on a card or plate that is
attached to the object or adjacent to the object.

**Modularization** - The separation of equipment into physically and
functionally distinct units that can be easily removed and replaced.

**Module** - An assemblage of two or more interconnected parts or
components that comprise a single, physical and functional entity. It is
this singular functionality that defines a module.

**Mounting** - The positioning and attachment of parts, components, and
modules.

**Packaging** - (of a unit of equipment) - The assembling, mounting, and
enclosing of the items it includes.

**Part** - An object that cannot normally be broken down further without
destroying its designated use. Fuses, resistors, and capacitors are
examples of parts.

**Service points** - A means for lubricating, filling, draining,
charging, and performing other service functions. They permit the
routine performance of these services on all equipment and components
requiring them.

**Shield** - An enclosure or barrier intended to protect components that
are susceptible to damage or to protect maintainers from possible
injury.

**Tagouts** - Tags that are attached to a control or place of hazard to
identify the required control condition and hazard associated with an
ongoing mode of operation or maintenance.

**Test points** - A means for conveniently and safely determining the
operational status of equipment and for isolating malfunctions.

**Unit of equipment** - An assemblage of items that may include modules,
components, and parts that are packaged together into a single hardware
package. For example, a computer, its keyboard, and its visual display
are all units of equipment, as are radio transmitters and receivers.

[]{#__RefHeading___Toc34717538 .anchor}References

Department of Defense. (1961). *Electronical and electronic reference
designations* (MIL-STD-16). Philadelphia, PA: Navy Publishing and
Printing Office.

Department of Defense. (1990). *Human engineering performance
requirements for systems* (MIL-STD-1800A). Philadelphia, PA: Navy
Publishing and Printing Office.

Department of Defense. (1990). *Identification coding and application of
hookup and lead wire* (MIL-STD-681). Philadelphia, PA: Navy Publishing
and Printing Office.

Department of Defense. (1990). *Identification, marking, and color
coding of electrical cable and cords*. (MIL-STD-686C). Philadelphia, PA:
Navy Publishing and Printing Office. (Available from
<http://astimage.daps.dla.mil/quicksearch/>)

Department of Defense. (1992). *Human factors engineering design for
army materiel* (MIL-HDBK-759B). Philadelphia, PA: Navy Publishing and
Printing Office.

Department of Defense. (1993). *Identification, marking of U.S. military
property.* (MIL-STD-130H). Philadelphia, PA: Navy Publishing and
Printing Office. (Available from
<http://astimage.daps.dla.mil/quicksearch/>)

Department of Defense. (1995). *General guidelines for electronic
equipment* (MIL-HDBK-454). Philadelphia, PA: Navy Publishing and
Printing Office.

Department of Defense. (1995). *Marking of connections for electrical
assemblies* (MIL-STD-195). Philadelphia, PA: Navy Publishing and
Printing Office.

Department of Defense. (1999). *Design criteria standard -- human
engineering* (MIL-STD-1472 F). Philadelphia, PA: Navy Publishing and
Printing Office.

Department of Energy. (1985). *Human factors design guidelines for
maintainability of department of energy nuclear facilities*
(UCRL-15673). Washington, DC: U.S. Department of Energy.

Department of the Air Force. (1980). *Human factors engineering* (AFSC
DH 1-3). Wright-Patterson AFB, OH: ASD/ENESS.

National Aeronautics and Space Administration. (1989). *Man-systems
integration standards* (NASA-STD-3000A). Houston, TX: National
Aeronautics and Space Administration.

Snook, S.H., Ciriello, V.M. (1991). The design of manual handling tasks:
revised tables of maximum acceptable weights and forces. *Ergonomics,
34* (9): 1197 --1213.

Title 29. *Occupational Health and Safety Standards* (OSHA 29 C.F.R.).
Rehabilitation Act of 1973, 29 U.S.C. § 794 et seq. (amended 1974). Part
1910. Occupational Health and Safety Standards (29 CFR 1910):
Washington, DC.

[]{#__RefHeading___Toc34717539 .anchor}Index

A

Adjustment controls 4-90

C

Circuit breakers 4-92

labeling and marking 4-98

legend switch 4-96

push-pull 4-94

toggle bat 4-96

Connectors

alignment aids 4-54

electrical connections 4-55

gaskets and seals 4-73

labeling, marking, and coding 4-58

location and accessibility 4-53

types 4-52

Covers, guards, and shields

hinged covers 4-37

interlocks 4-38

labeling and marking 4-39

shape 4-37

size 4-36

sliding doors and caps 4-37

Cranes 4-15

D

Designing equipment for handling

alignment aids 4-14

designing for remote handling 4-14

grasp areas 4-13

handles 4-8

shape 4-8

size 4-8

stands and rests 4-13

weight 4-6

Designing equipment for maintenance

connectors 4-51

covers, guards, and shields 4-33

designing equipment for handling 4-5

fasteners 4-40

lines and cables 4-59

minimizing need for maintenance 4-5

E

Electrical

bench mockup cables 4-68

clamps and mounting plates 4-65

labeling, marking, and coding 4-69

leads 4-65

length of cables and leads 4-61

routing and mounting 4-63

test and extension cables 4-67

Electrical connections

plugs and receptacles 4-55

wire connections 4-56

Equipment

arrangement 4-15

mounting 4-15

packaging 4-15

F

Fasteners

labeling, marking, and coding 4-50

number 4-43

types 4-43

Fluid and gas lines

clamps and supports 4-73

labeling, marking, and coding 4-75

routing and mounting 4-72

Fuses 4-92, 4-93

labeling and marking 4-98

H

Handles

dimensions 4-10

handle characteristics 4-9

location 4-12

when handles are needed 4-8

Hazardous conditions 4-4

Hazardous location 4-4

Hoists 4-15

I

Internal components

layout 4-79

mounting 4-79, 4-86

packaging 4-79

J

Jacks 4-15

L

Labeling 4-88

Labeling and marking

location and orientation 4-23

markings 4-26

types of labels 4-22

typographic matters 4-24

wording 4-25

Layout

accessibility 4-82

grouping of parts 4-84

hazard protection 4-85

Lines and cables

electrical 4-59

fluid and gas lines 4-71

M

Modularization 4-80

Module 4-1

Mounting in drawers, on racks, and on hinges

external connectors and interlocks 4-19

restraints and supports 4-19

P

Packaging

modularization 4-80

modularization methods 4-81

Packaging, arrangement, and mounting of equipment

interchangeability, noninterchange-ability 4-17

labeling and marking 4-22

mounting in drawers, on racks, and on hinges 4-18

positioning equipment 4-20

utilization of equipment 4-16

Packaging, layout, and mounting of internal components

layout 4-82\`packaging 4-80

Positioning equipment

physical accessibility 4-20

relative accessibility 4-21

S

Service points 4-98

T

Test equipment 4-104, 4-112

bench mockups 4-110

built in 4-107

built-in 4-108

collating test equipment 4-109

controls and displays 4-106

ease of use 4-105

go/no-go 4-109

partially built in 4-107

portable 4-108

safety 4-105

storage space 4-111

Test points 4-98

Test points and service points 4-98

accessibility 4-101

adjustment controls 4-98

drain points 4-100

labeling, marking, and coding 4-103

location and arrangement 4-99

Tools 4-111

common hand tools 4-112

special tools 4-112

Training 4-4

Types

captive versus removable 4-50

distinctive 4-52

latches and catches 4-46

nuts and bolts 4-43

other fastening devices 4-47

plug-in 4-52

quick fastening and releasing devices 4-49

quick-action 4-53

screw and bolt heads 4-45

screws 4-44

threaded 4-52