UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Utilities Service

**BULLETIN 1751F-810**

**SUBJECT:** **[Electrical Protection of Digital and
Lightwave]{.underline}**

**[Telecommunications Equipment]{.underline}**

**TO:** Telecommunications Borrowers

RUS Telecommunications Staff

**EFFECTIVE DATE:** Date of Approval

**EXPIRATION DATE:** Seven years from effective date

**OFFICE OF PRIMARY INTEREST:** Central Office Equipment Branch,
Telecommunications Standards Division

**PREVIOUS INSTRUCTION:** This bulletin replaces Telecommunications
Engineering and Construction Manual (TE&CM) 810, [Electrical Protection
of Electronic Analog and Digital Central Office Equipment]{.underline},
number 6, issued September 3, 1983 and Addendum number 1 issued
September 1984.

**FILING INSTRUCTION:** Discard TE&CM and Addendum 1 and replace with
this bulletin. File this bulletin with 7 CFR 1751 and RUSNET.

**PURPOSE:** To provide information in the design, installation and
operation of RUS borrowers\' telecommunications systems. In particular,
this bulletin now covers electrical protection practices recommended for
digital and lightwave telecommunications systems. These practices were
originally developed for application to digital switching systems housed
in central office type buildings.

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Administrator Date

**TABLE OF CONTENTS**

1\. GENERAL

1.1 Electronic Telecommunications Systems

1.2 The Basic Grounding System

1.3 The Single Point Grounding System

1.4 Equipment Manufacturers

1.5 Resistance

2\. DEFINITIONS

2.1 Introduction

2.2 Building Structural Ground

2.3 Cable Entrance Ground Bar (CEGB)

2.4 Central Office Ground Field (COGF)

2.5 Collocated Switching Systems

2.6 Electrostatic Discharge (ESD) Protection

2.7 Fuse Link

2.8 Green Wire Ground

2.9 Ground Loop

2.10 Ground Window Bar (GWB)

2.11 Insulating Joints

2.12 Intermediate Ground Bar (IGB)

2.13 Isolated Ground Zone (IGZ)

2.14 Main Distributing Frame (MDF)

2.15 Master Ground Bar (MGB)

2.16 MDF Ground Bar (MDFGB)

2.17 MDF Protector Assembly

2.18 Metallic Water Pipe

2.19 Multigrounded Neutral (MGN)

2.20 Personnel Discharge Plates

2.21 Single Point Grounding

2.22 Surge Absorbers (A)

2.23 Surge Producers (P)

3\. SINGLE POINT GROUNDING

3.1 Single Point Grounding

3.2 Surge Potentials

3.3 The Purpose of Single Point Grounding

3.4 MGB Bonding Configuration

3.5 Potential of IGZ Equipment

4\. MASTER GROUND BAR (MGB)

4.1 Purpose and General Description

4.2 Surge Producers (P Section of MGB)

4.3 Surge Absorbers (The A section of MGB)

4.4 Non-IGZ Grounds (N section of MGB)

4.5 IGZ Grounds (I Section of MGB)

4.6 Ground Resistance Objective

5\. CENTRAL OFFICE GROUND WINDOW BAR (GWB)

5.1 Equipment Ground Originating inside IGZ

5.2 Equipment Located in a Remote Area

5.3 Connect Each GWB to the MGB

5.4 The Conductors Terminating on the GWB

5.5 The Frame Grounds of Equipment Located inside IGZ

6\. ISOLATED GROUND ZONE (IGZ)

6.1 Introduction

6.2 Installation and Bonding of Metal Framework

7\. MAIN DISTRIBUTING FRAME (MDF)

7.1 Special Grounding Considerations

7.2 Transmission Equipment Termination and Protection

7.3 Entrance and Tip Cables

7.4 Protection

7.5 Current Limitation

7.6 Heat Coils

8\. GROUNDING CONDUCTOR SIZING, ROUTING AND TERMINATING

8.1 Sizing of Protective Grounding Conductors

8.2 The Planning and Installation of the Wiring

8.3 Stenciling and Tagging

9\. POWER SERVICE PROTECTION

9.1 The Minimum Protection for AC Power Serving Central Office Buildings

9.2 Power Arrester Protecting Kilowatt-hour Meter

9.3 Protecting the AC Power Service Entering a Central Office

10\. RADIO OR MICROWAVE INSTALLATIONS

10.1 Radio or Microwave Towers Which Are Located on or in Close
Proximity to CO Buildings

10.2 Protection of the Tower and Associated Equipment

10.3 Bonding Tower Ground to CO Grounding Systems

11\. ELECTROSTATIC & ELECTROMAGNETIC FIELD EFFECTS

11.1 Static Electricity

11.2 FET, MOS, and CMOS

11.3 The Accumulation of Electrostatic Discharge by a Human Body

11.4 Electrostatic Conditions That Produce Equipment Problems

12\. GENERAL ENVIRONMENTAL AND HANDLING REQUIREMENTS FOR ELECTROSTATIC
SENSITIVE EQUIPMENT

12.1 Proper Environmental and Handling Considerations

12.2 Environmental Conditions

12.3 Precautions for Maintenance Procedure

12.4 Precautions for Operating Motor Driven Devices

12.5 Precautions for Memory Devices

13\. DISCHARGE PLATES

13.1 For Protection of Static Sensitive Equipment

13.2 Installation of Electrostatic Discharge Plates

13.3 Supplemental Discharge Plates

13.4 Warning Signs

APPENDIX A: VOLTAGE FROM SURGE CURRENTS

APPENDIX B: PROTECTION FOR ANTENNA INSTALLATIONS

APPENDIX C: ANALYSIS OF CENTRAL OFFICE GROUNDING SYSTEMS

TABLES

Table C-1 67

FIGURES

Figure 1: Master Ground Bar

Figure 2: Outdoor Grounding Conductor Connections

Figure 3: Entrance and Tip Cable Arrangements

Figure 4: Cable Entrance without Vault

Figure 5: Typical Power Service Protector Installation

Figure 6: Central Office Protection Grounding

Figure 7: Maximum Conductor Length to Meet the Grounding

Conductor Resistance Objectives

Figure B1: Self Supporting Tower

Figure B2: Bonding Details

Figure B3: Guyed Tower Installation

Figure B4: Pole Mounted Installation

Figure B5: Building Mounted Installation

Figure B6: Tower Ground Kit Installation

Figure B7: Entrance Grounding at Central Office

Figure C1: Master Grounding Bar Currents

Figure C2: CEGB Currents

Figure C3: Undesirable Reduction in Size of Grounding

Conductor

Figure C4: Undesirable Butt-Splice

INDEX:

Protection

Protection, Central Office

**ABBREVIATIONS**

**A** Surge Absorbers, a section of the MGB

Amperes, unit for measuring the flow of current

**ANSI** American National Standards Institute

**CATV** Community Antenna Television, cable

**CEGB** Cable Entrance Ground Bar

**CMOS** Complementary Metal Oxide Semiconductor

**CO** Central Office

**COGF** Central Office Ground Field

**ESD** Electrostatic Discharge

**FET** Field Effect Transistor

**GWB** Ground Window Bar

**I** IGZ Grounds section of the MGB

**IGB** Intermediate Ground Bar

**IGZ** Isolated Ground Zone

**MCM** One Thousand Circular Mils

**MDF** Main Distributing Frame

**MDFB** MDF Ground Bar

**MGB** Master Ground Bar

**MGN** Multigrounded Neutral

**MOS** Metal Oxide Semiconductor

**µH** Microhenries

**N** Non-IGZ Grounds section of the MGB

**NEC** National Electrical Code

**NFPA** National Fire Protection Association

**P** Surge Producers section of the MGB

**PVC** Polyvinyl Chloride

**RUS** Rural Utilities Service

**SCR** Silicon Controlled Rectifier

**TE&CM** Telecommunications Engineering and Construction Manual

**DEFINITIONS**

Section 2 of this bulletin is devoted to definitions.

**1.** **GENERAL**

**1.1** **[Electronic Telecommunications Systems]{.underline}** may
experience malfunctions, erratic operation, and total disruption of
service as a result of excessive induced transient voltages which may be
introduced through incoming circuits, the central office grounding
system, or by electrostatic action. Electronic switching systems are
vulnerable to voltage sources because of the fragile nature of
semiconductor components and their fast transient response
characteristics. Semiconductors typically have low breakdown voltage
ratings and can be permanently damaged by excessive voltage and current
spikes.

**1.2** **[The Basic Grounding System]{.underline}** discussed in this
section is primarily designed for single floor office buildings and
outside cabinets which are typical in RUS financed systems.

**1.3** **[The Single Point Grounding System]{.underline}** described in
this bulletin meets the protection requirements of most central office
and other electro-optic equipment manufacturers. The described methods
should be followed unless there are compelling reasons for change (see
1.4).

**1.4** **[Equipment Manufacturers]{.underline}** may request grounding
systems that exceed those recommended herein. Manufacturers might
include a rigid, low maximum resistance requirement for the equipment
site ground fields or various forms of extraordinary lightning
protection.

**1.5** **[Resistance]{.underline}:** This bulletin is based on
resistance since this is a primary parameter that is readily understood.
However, the essential factor in telecommunications service system
protection is the grounding system impedance, especially the reactance
component, of the grounding conductors. The general guidelines presented
in this bulletin are based on providing a system having a relatively low
overall grounding system impedance to the flow of lightning and power
fault currents.

**2. DEFINITIONS**

**2.1** **[Introduction]{.underline}:** The following terms are defined
as an aid to understanding their usage in this bulletin. These terms are
commonly used for describing telecommunications grounding systems.
Different terms have been used by individual manufacturers and operating
companies other than those commonly used. Such terms are included in
parentheses at the end of each definition, where applicable.

**2.2 [Building Structural Ground]{.underline}:** A grounding electrode
source provided by structural steel and/or reinforcing steel rods
contained within the building walls, roofs, floors, footing, or
foundations.

**2.3** **[Cable Entrance Ground Bar (CEGB)]{.underline}:** A copper
ground bar to terminate incoming telephone cable shields on a common
connection point. The bar is normally located close to the entrance
location (Cable Vault Ground Bar).

**2.4** **[Central Office Ground Field (COGF)]{.underline}:** A series
of interconnected ground rods, buried perimeter cable or a metallic well
casing for a low impedance path to earth ground (Central Office Ground
Grid).

**2.5** **[Collocated Switching Systems]{.underline}:** Two or more
switching (i.e. central office or remote switching terminals) systems at
a single location.

**2.6 [Electrostatic Discharge (ESD) Protection]{.underline}:** The
protection of electronic components from static voltage discharges.
Static charges are commonly generated by personnel or air moving in a
work area where the relative humidity is low.

**2.7** **[Fuse Link]{.underline}:** A section of fine wire in a
conductor designed to melt during a current-surge condition. This
element normally protects from currents which could otherwise heat
conductors and start fires.

**2.8** **[Green Wire Ground]{.underline}:** A normally non-current
carrying conductor that protects personnel and equipment. The green
color code distinguishes the lead from the current carrying grounded
conductors (neutrals) which are natural, gray or white (Equipment
Grounding Conductor).

**2.9 [Ground Loop]{.underline}:** Ground loops exist when there is more
than one electrical path to a ground connection. Such parallel paths to
ground are normally not a problem if associated with nonsensitive
circuitry located outside the Isolated Ground Zone (IGZ.) Ground loops
are undesirable for equipment located in the IGZ.

**2.10** **[Ground Window Bar (GWB)]{.underline}:** A copper bar for the
common connection of all equipment located in the IGZ. See Green Wire
Ground, Master Ground Bar.

**2.11** **[Insulating Joints]{.underline}:** Nonconducting inserts in
metal framework of equipment located in the IGZ. These inserts insulate
the IGZ equipment from outside ground connections.

**2.12** **[Intermediate Ground Bar (IGB)]{.underline}:** A copper bar,
insulated from its support, used to connect a ground wire from the
Master Ground Bar (MGB) (see Ground Window Bar) to several racks or
frames of equipment, usually in the non-IGZ area, but [not]{.underline}
to include battery (+) from the main power board.

**2.13** **[Isolated Ground Zone (IGZ)]{.underline}:** A dedicated area
within an office building where all equipment is electrically insulated
from all external grounds except through a single ground connection
between the GWB and the MGB. The isolated area should preferably extend
a minimum of six feet (two meters) on all sides from the equipment
frames and framework and where practical be separated from other
equipment by permanent walls. The IGZ will normally house sensitive
electronic components (Isolated Area).

**2.14 [Main Distributing Frame (MDF)]{.underline}:** A distributing
frame where outside plant cables are terminated on vertical protection
assemblies. Cable pairs are also cross-connected on this frame to
equipment terminated on horizontal blocks.

**2.15 [Master Ground Bar (MGB)]{.underline}:** A copper bar used as
single point connection for surge producers, surge absorbers, non-IGZ
equipment grounds, and IGZ equipment grounds. The MGB is normally
non-current carrying and isolated from the building/structural ground.

**2.16** **[MDF Ground Bar (MDFGB)]{.underline}:** A copper bar commonly
found at the bottom of the MDF for the connection of tip cable shields
and MDF protector assemblies. It may be used as a connection point for
MDF ironwork (see 7.1.3). The MDFB may be used as an MGB in small
offices (Entrance Cable Protector Bar).

**2.17** **[MDF Protector Assembly]{.underline}:** An assembly
consisting of a protector module and a connector module.

**2.18** **[Metallic Water Pipe]{.underline}:** An outdoor section of
buried metallic water pipe at least 10 feet (3.0 meters) in length and
owned by the telecommunications company.

**2.19** **[Multigrounded Neutral (MGN)]{.underline}:** A continuous
electrical conductor on a power distribution system with multiple direct
connections to earth ground at multiple points. In this system, at least
4 grounds have to be provided in each mile of line (2.5 grounds in each
kilometer of line), not including grounds at individual services. This
multiple grounding arrangement provides a very low impedance path to
earth ground for absorbing lightning and other surges. It also provides
a return path for residual (unbalanced) currents resulting from less
than perfect balance on associated three-phase power distribution
systems.

**2.20 [Personnel Discharge Plates]{.underline}:** Plates found in
equipment areas containing voltage-sensitive electronic equipment. These
plates are connected to ground and are used to discharge body voltages
to ground rather than through sensitive electronic components by
accidental contact.

**2.21** **[Single Point Grounding]{.underline}:** A grounding system
using a single point, usually the MGB, for a zero reference potential to
ground for an entire system. While the voltage at this connection point
may rise above zero volts-to-earth-ground under fault conditions, the
entire system will also rise at the same rate to the same voltage. This
helps minimize any circulating currents between components from
lightning or power surges.

**2.22 [Surge Absorbers (A)]{.underline}:** Paths with a low impedance
connection to a remote earth ground. A grounding element which has a low
impedance path to earth ground is considered a primary surge absorber.
There are three primary surge absorbers: (1) the service system ground
field, (2) the power system multigrounded neutral (MGN), and (3) a
metallic water system.

**2.23** **[Surge Producers (P)]{.underline}:** Connections to metallic
sources of lightning and/or power surges. For example, radio/microwave
towers, telephone cable shields, telephone cable pairs and power system
conductors.

**3.** **SINGLE POINT GROUNDING**

**3.1** **[Single Point Grounding]{.underline}:** There is a need to
control the high voltage differences produced between the ends of single
conductors by fast rising electrical surges. Reference Appendix A for a
discussion of the voltage effects from fast rising surge currents.

**3.2** **[Surge Potentials]{.underline}** need to be equalized through
bonding of central office or other service system ground elements. Among
these ground elements are: Surge Producers (P), Surge Absorbers (A),
Non-IGZ equipment grounds (N), and IGZ equipment grounds (I). See Figure
1, "Master Ground Bar."

**3.3** **[The Purpose of Single Point Grounding]{.underline}** is to
reduce voltage differences and control surge currents. The basic
elements of a single point grounding system include the following:

**3.3.1** [A Master Ground Bar (MGB)]{.underline} with connections
grouped to confine lightning and power surge activity. This is the point
for establishing a common reference plane, with respect to earth ground,
for the entire system.

**3.3.2** [A Ground Window Bar (GWB)]{.underline} establishes a single
location for grounding sensitive electronic equipment within the IGZ.
Section I of the MGB (see Section 4) provides a single-point termination
and ground reference to which the GWB and associated electronic
equipment are bonded.

**3.3.3** [An Isolated Ground Zone (IGZ)]{.underline} surrounds the
electronic switch and other sensitive electronic equipment. The IGZ will
consistently have the same voltage potential as the GWB.

**3.4** **[MGB Bonding Configuration]{.underline}**: A high momentary or
transitory voltage rise that can occur between the point of strike and
point(s) of dissipation would result from direct or indirect lightning
strike to cable or other outside plant connected to the MGB. The MGB
bonding configurations illustrated, in Figure 1 and Figure 6, "Central
Office Protection Grounding," allow high current surges to be
concentrated and dissipated through the P and A sections of the bar.
This maintains the lowest possible electric potential at the point of
MGB-GWB connection. The connection sequence of P-A-N-I as shown in
Figure 1 is important to assure the overall protection effectiveness.

**3.5** **[Potential of IGZ Equipment]{.underline}:** All equipment in
the IGZ electrically floats at essentially the same potential as the GWB
when a single-point grounding is used. When all equipment is at the same
potential, no damaging voltages appear across sensitive components and
surge currents are eliminated.

**4. MASTER GROUND BAR (MGB)**

**4.1** **[Purpose and General Description]{.underline}:** The MGB is
the hub of the basic central office grounding system. It is a common
point of connection for the P‑surge producers and A‑surge absorbers, and
equipment grounds for both the N‑nonisolated and I‑isolated equipment
areas. The sizing of ground conductors is discussed in Section 8. The
MGB is a copper bar insulated from its support. The MGB may be located
either on a wall near the MDF, or on the cable vault wall. In small
offices the MGB may be located on the MDF as described in 4.2.2 and
7.1.4. The various connections to the MGB should be tagged or stenciled
for identification as described in 8.3.

**4.2** **[Surge Producers (P section of MGB)]{.underline}**: The MGB is
the preferred connection point for surge producers such as MDF protector
ground, entrance ground bar, etc.

**4.2.1** [Cable Entrance Ground Bar (CEGB)]{.underline}: The CEGB is a
copper bar insulated from its support. Cable shields should be bonded
directly to a CEGB in offices with a cable vault. The CEGB is connected
by an insulated conductor sized according to Section 8 using the most
direct route to the MGB.

**4.2.2** [MDF Ground Bar (MDFGB)]{.underline}: The main frame protector
blocks should be bonded directly to the MDFGB. A detailed discussion of
MDF protection is found in Section 7. The MDFGB is the bonding point for
terminating the MDF end of tip cable shields to ground. The MDFGB may be
used as the MGB in very small offices where installation of a
wall-mounted MGB is impractical. With this application, the bar should
be insulated from its support and have sufficient length for the
connection sequence shown[^1] in Figure 1. It is important, that the
integrity of all sections of the bar are preserved for the life of the
ground bar arrangement. The MDFB may be insulated from its support as
required by the manufacturer.

**4.2.3** [Radio and Microwave Equipment Grounds]{.underline}: Connect
all [indoor]{.underline} cabinets of the radio and microwave equipment
directly to the MGB. No connections should be made to the GWB or other
central office ironwork. Where the MDFGB is used as the MGB, these
equipment grounds should be connected to the P section of the bar.
Radio/microwave towers have their own outdoor, dedicated grounding
systems. Surge voltages should be equalized by bonding the dedicated
grounding system to the central office ground field outside the building
for personnel safety and equipment protection. This connection is
discussed in 4.3.2.

**4.2.4** [Standby Power Plant Framework Ground]{.underline}: Unless the
Central Office ground field is within three feet of the generator frame,
a ground rod is recommended. This ground rod should be driven into the
earth as close as possible to the generator frame and where someone
would be expected to be in contact with both the frame and the ground.
This ground rod should be solidly bonded to the generator frame using a
conductor sized according to NEC Article 250-95.

**4.2.4.1** The ground rod should be installed in addition to the
connection to the MGB. Because the generator frame is connected to the
MGB via equipment grounding connections (the generator installer
provides), the frame will be at the electrical potential of the MGB.
This potential could be much different than the potential existing on
the surface of the soil in the immediate vicinity of the generator. (The
generator installer connects the NEC Article 250-50(a) required green
wire equipment grounding conductor from the generator frame to the
electric service neutral which, in turn, is connected to the MGB.) With
the frame connected to the MGB [without a ground rod installed, there is
a possibility for a potential difference (voltage) between the frame and
the soil. With the ground rod installed, the frame-to-soil potential
differences are minimized.]{.underline}

**4.2.4.2** Because one can never be certain of the exact location of
buried facilities and in the interests of being consistent from
installation to installation, it is better to simply standardize on and
always install a ground rod. NEC Article 250-91(c), "Supplementary
Grounding" discusses this permissible, not mandatory, method of
augmenting the grounding. The equipment grounding conductor is detailed
in Article 250-91(b).

**4.3** **[Surge Absorbers (The A section of MGB)]{.underline}:** The
MGB is the preferred connection point for the three primary surge
absorbers. They are the power system multigrounded neutral, the central
office ground field, and the metallic water system. Bonding of the power
neutral and water pipe, on the MGB [does not]{.underline} replace the
requirements of the National Electrical Code for separately bonding the
commercial power service.

**4.3.1** [Multigrounded Neutral (MGN)]{.underline}: The MGN with its
multiple connections to earth throughout the power system normally has a
low impedance to earth ground. Because of its low impedance the MGN is
an excellent surge absorber and the most important ground connected to
the MGB. The MGN may occasionally become a momentary surge producer
because of nearby lightning strikes or power system transients. Refer to
Section 8 for a discussion of ground system conductor sizes. In any
case, the grounding conductor between the MGN and the MGB should be the
same size or larger than the commercial MGN service entrance conductor
to the building. Occasionally a non-MGN power system (e.g., delta or
unigrounded wye system) will be encountered. A bond is still required
between the central office power service grounding electrode and the
MGB. Non-MGN systems do not qualify as primary surge absorbers.
Therefore Non-MGN systems have to be excluded from the calculations of
ground resistance discussed in 4.6.1.

**4.3.2** [Central Office Ground Field]{.underline}: The outdoor portion
of the ground conductors connecting the central office ground field to
the MGB should be buried a minimum of 2.5 feet (0.76 meters) below
finished soil grade and should enter the building through a
[nonmetallic]{.underline} conduit. The conductor should be placed in a
straight line with no splices to reduce the impedance presented to fast
rising surges. See Section 8 for a discussion of grounding conductors.
Lightning rods and/or radio/microwave towers should be connected to the
central office ground field outside the building as described below.

**4.3.2.1** [Lightning Rod Ground]{.underline}: Lightning rod systems
are grounded via a separate dedicated ground field. A bond should be
installed between the central office ground field and the lightning rod
ground field, to minimize inductive noise coupling, reduce the chance of
flashover, and to protect personnel and equipment. A connection point
between the two ground fields should be made accessible to permit
temporary disconnection for earth resistance-to-earth measurements. The
preferred location for this access point is above the central office
ground field near where the attachment to the central office ground
field is made. An easily accessible, permanent handhole closure is
recommended for this connection. All conductors should follow the most
direct route with a minimum of bends. See Figure 2, "Outdoor Grounding
Conductor Connections," and Figure 6, "Central Office Protection
Grounding."

**4.3.2.2** [Radio/Microwave Tower and Fence Ground]{.underline}: A bond
should be made between the central office ground field and the
radio/microwave tower and fence ground for the same reasons discussed
above. All provisions for this grounding should be identical to those
described in 4.3.2.1 and in Appendix C paragraph C.4.2. Where lightning
rod, tower and fence ground systems exist, all systems may be connected
to the central office ground field in the same handhole closure.

**4.3.3** [Central Office Metallic Water System]{.underline}: It is
important to bond to the central office metallic water system, where one
exists, to comply with National Electrical Code (NEC) requirements. The
bond to the metallic water system is also an additional low impedance
connection to earth ground. When no water system is present in the
building, this bonding connection is omitted. If the central office
water system entrance piping includes at least 10 feet (3.0 meters) of
buried metallic pipe in direct contact with earth (NEC Articles 250-80
and 250-81) from either a drilled well or public water system, it will
qualify as a metallic water system. The water system metallic entrance
pipe has to be owned and controlled by the telephone company, if it is
to be used as a primary surge absorber. Ground wire connections should
be made to the main entrance pipe of the water system. When there is a
water meter or insulating joint in the pipe, a bypass bonding wire
should be installed to ensure electrical continuity. Permission from the
owner of the water system is required when the pipe on the street side
of the meter, is not owned by the telephone company or where there is an
insulated coupling at the meter. To comply with Article 250-80 of the
National Electrical Code, an electrician should bond the electric
service grounding system to the water piping as shown in Figure 6,
"Central Office Protection Grounding."

**4.3.4** [Building Structural Ground]{.underline}: A connection should
be made to the building structural ground for earth grounding and
potential equalizing. This ground is not considered to be a primary
surge absorber. A low resistance path to ground is provided by
reinforced concrete that is in direct contact with bare earth, such as
building footings. Structural steel used in some buildings can have
voltage differences from equipment frames installed in the building.
These voltage differences occur when equipment frames rise in voltage
because of current surges through the MGB or when lightning strikes the
structure. During building construction, rebars should be lashed to
steel column anchor bolts at each floor/roof level. Connection to the
steel columns should be made between the nearest accessible point and
the MGB. Ground wire connections should be made directly to the rebar
during construction of new reinforced concrete buildings containing no
steel columns.

**4.4** **[Non-IGZ Grounds (N section of MGB)]{.underline}:** The N
section is primarily a common voltage reference point to which all
non-IGZ equipment frames are connected. The single-point grounding
system is designed to confine all lightning and surge currents to the P
and A sections of the MGB. The connections to the N section prevent
voltage differences between equipment racks, etc., in the non-IGZ area.
Surge currents and shock hazards for personnel in the building are
thereby effectively minimized. All equipment frames, ironwork (not cable
rack ironwork, see 5.5.2), and other exposed metallic surfaces that
could become energized are bonded to the MGB at this point. The N
section is also the connection for equalizing voltages on the positive
(+) central office power bus. This connection between the positive (+)
battery terminal and the MGB is not normally a dc power current carrying
conductor and is provided only for equalizing voltage differences.

**4.5** **[IGZ Grounds (I section of MGB)]{.underline}:** The I section
of the MGB normally should have the least voltage variation of any
section along this ground bar. All ground connections to the GWBs are
made in this section.

**4.6** **[Ground Resistance Objective]{.underline}:** Making a
reasonable effort to meet the objective ground resistance is an
important factor in implementing a single-point grounding system.
Installation of a perimeter ground around and outside the building
foundation perimeter is recommended. Other types of ground fields are
acceptable where the ground resistance objective can be met (see 4.6.2).

**4.6.1** The combined central office ground resistance from the three
primary surge absorbers, as defined in 4.3, should be five ohms or less
when measured at the MGB, subject to the limitations of 4.6.2. Where all
three primary surge absorbers are present at a central office, the
five ohm objective should be met [when any two]{.underline} of the
grounds are connected. For central office buildings where only two surge
absorbers are available, the objective for the central office ground
field is five ohms or less. (See Bulletin 1751F‑802 for a discussion of
grounding techniques.)

**4.6.2** Establishment of a low resistance ground field can be
difficult at the location of some rural central office buildings. The
actual measured resistance to remote earth of the ground field provides
a guide for determining if it is practical to attempt achieving the
five ohm objective resistance. Where the measured value of the ground
field alone is between five and 25 ohms, further efforts to reduce the
resistance are not recommended. The work required to reduce the
resistance an additional one or two ohms could be very expensive. When
the actual measured resistance exceeds 25 ohms, additional effort should
be made to reduce the resistance. Earth resistivity measurements should
be completed at various depths and locations around the building before
initiating any reduction effort. Calculation of the approximate
anticipated resistance to earth based on the recorded results of the
measurements should be completed for various ground configuration and
electrode lengths. The results of these calculations will indicate the
probability of attaining the objective ground resistance (see Bulletin
1751F‑802).

**4.6.2.1** Following are some techniques which may reduce the central
office ground field resistance:

**4.6.2.1.1** Attach to building rebar ground.

**4.6.2.1.2** Drive extended or sectional ground rods to a depth of up
to 30 feet (9.1 meters).

**4.6.2.1.3** Establish a second ground field.

**4.6.2.1.4** Install one or more 6 to 10 inch (15 to 25 centimeter)
well casings. The well casing should extend below the water table level.

**4.6.2.2** Application of chemical soil treatment is not recommended.
Chemical treatment is not permanent and has to be periodically renewed.
Where chemical treatment is used, a program should be initiated to
measure the ground resistance at six-month intervals to ensure they are
all still effective.

**4.6.3** [The resistance of the central office ground field should be
determined prior to selecting the specific equipment for
installation]{.underline}. The manufacturers of equipment should be
advised when the five ohm central office ground field objective cannot
be achieved by established methods.

**5. CENTRAL OFFICE GROUND WINDOW BAR (GWB)**

**5.1** **[Equipment Ground Originating inside IGZ]{.underline}:** All
equipment grounds that originate inside the IGZ are terminated on the
GWB which should preferably be physically located inside the IGZ and
insulated from its support. The use of a GWB provided by the equipment
manufacturer as an integral part of the switching equipment is
acceptable. Normally those grounding conductors originating inside the
IGZ that are terminated on the GWB will be placed by the personnel
installing the switching equipment.

**5.2** **[Equipment Located in a Remote Area]{.underline}:** A separate
IGZ should be established with its own GWB where additional electronic
or digital switching equipment is located in a remote area of the same
floor or on another floor of the building.

**5.3** **[Connect Each GWB to the MGB]{.underline}** with a conductor
following the most direct route. This grounding conductor should be
2/0 gauge or coarser copper with a resistance of less than 0.005 ohms
(see Section 8). The use of parallel conductors for redundancy is
acceptable.

**5.4** **[The Conductors Terminating on the GWB]{.underline}** should
be suitably identified as described in Paragraph 8.3.

**5.5** **[The Frame Grounds of Equipment Located inside
IGZ]{.underline}:** The frame grounds of ONLY that switching equipment
and associated electrical equipment located INSIDE the IGZ should be
connected to the GWB. This includes but is not limited to those items
described in the following paragraphs:

**5.5.1** All metal frameworks of the switching systems (e.g., frames,
cabinets, bays, etc.) should be connected to the GWB. The
manufacturer\'s recommendations for establishing these connections
should be followed.

**5.5.2** The cable racks (insulated from MDF ironwork, see 7.1), static
control ground mats, discharge plates, transmission equipment, and
protective grounds of any other IGZ equipment that obtains power from
the main power plant should also be connected to the GWB. [Borrowers
should comply with any special recommendations from the equipment
manufacturer.]{.underline}

**5.5.3** [The manufacturer\'s instructions on isolation of the battery
charger framework ground from the internal positive (+) chassis
connection should be followed.]{.underline}

**5.5.4** The ac conductors including the protective grounding
conductors serving all 120 volt ac electrical convenience receptacles
and all direct wire peripheral equipment, located in the IGZ, should be
sized in accordance with normal \"green wire\" criteria. Each
termination point should be tagged to indicate that the green wire is a
GWB isolated ground wire. The manufacturer\'s recommendation for the
metallic racks within the IGZ will determine how the green wire is
handled in the IGZ. The metallic racks may be insulated from the
concrete floors and reinforcing steel or connected to the green wire,
depending on the manufacturer's recommendation. Routing of the ac
conduit and protective green wire ground in the manner described below
ensures compliance with National Electrical Code requirements.

**5.5.4.1** [Racks insulated from building]{.underline}: The conduit
carrying 120 volt ac conductors into the IGZ should be routed to a
junction box located adjacent to the GWB. The green wire should be
solidly connected to the junction box and a wire connection established
between the junction box and the GWB. Use of metallic or non-metallic
conduit for extending and bonding the ac conductors into the IGZ is at
the option of the manufacturer. Where metallic conduit is used, care
should be taken during installation to assure it is insulated from
foreign grounds (building structural steel and reinforced concrete
members) beyond the GWB. There is no need to install isolated
[orange]{.underline} convenience receptacles with this configuration
since everything beyond the GWB in the IGZ is at GWB ground potential.
Isolated ac ground convenience receptacles may be installed as required
by the manufacturer.

**5.5.4.2** [Racks not insulated from building]{.underline}: The conduit
carrying 120 volt ac conductors into the IGZ should be routed directly
to the metallic racks. Since these racks are at the same ground
potential as the conduit and green wire by being connected to the
reinforced concrete floor, there should be no connection to the GWB.
Isolated ac ground convenience receptacles may be installed as required
by the manufacturer. Equipment in the IGZ should be isolated from the
metallic racks which are not isolated from building grounds.

**5.5.5** Where overhead lighting fixtures located in the IGZ are an
integral part of, or are in electrical contact with, the equipment
frame(s), the associated green protective ground wires should be
connected to the GWB isolated ground wire system. For convenience, they
may also be connected to the GWB where the connections above do not
exist. All fixtures connected to the GWB system need to be isolated from
building structural steel and reinforced concrete members. Green wires
associated with lighting fixtures having no electrical contact with the
equipment frames may be connected in the conventional way to the ac
distribution panel ground.

**5.5.6** The protective grounds for facsimile machines, computer
monitors, test equipment and other ac powered devices located or used
within the IGZ area are normally provided by the green wire leads in the
attached power cords. The green wire pins should not be removed from the
3-wire power cords of such equipment and 2-wire adapters should not be
used.

**5.5.7** Every precaution should be taken to ensure the integrity of
the IGZ. No foreign grounds should be permitted to come into contact
with any equipment within the IGZ except through the GWB, except as
indicated by the equipment manufacturer.

**6.** **ISOLATED GROUND ZONE (IGZ)**

**6.1** **[Introduction]{.underline}**

**6.1.1** If practical, permanent markers should be placed on the floor
to identify the IGZ boundaries. Paint or tape of distinctive color such
as orange should be used.

**6.1.2** Precautions should be taken to ensure that no permanent or
temporary ground connections are permitted to cross the IGZ boundary
except as defined in 5.5.4.2.

**6.2** **[Installation and Bonding of Metal Framework]{.underline}:**
The metal framework associated with digital electronic central office
equipment and associated peripheral equipment should be installed and
bonded according to the manufacturer\'s requirements. Some manufacturers
require the frames be isolated from the floor while others permit
anchoring directly to the floor.

**7. MAIN DISTRIBUTING FRAME (MDF)**

**7.1** **[Special Grounding Considerations]{.underline}** are required
at the MDF to control incoming surges and protect personnel. The design
should provide for this with any of the existing or new MDF protectors
that are available. The MDF is treated as being outside of the IGZ in
all cases. Cable rack, grid or runway metal should be insulated from MDF
ironwork and grounded to the GWB, see 5.5.2.

**7.1.1** MDF protector assemblies may be mounted directly on the
vertical frame ironwork. The assemblies mounted on each vertical frame
ironwork should be interconnected with a #6 copper conductor to provide
a low resistance path for surge currents. Each vertical group of
protector assemblies should be connected to the MDFB with a #6 copper
conductor. Alternative means of connection to the [MDFB which do not
rely on the frame ironwork]{.underline} for conducting surge currents to
ground are acceptable.

**7.1.2** The MDFB should be [insulated]{.underline} from the ironwork
[in all cases where it is used as a MGB]{.underline} (see 4.2.2). The
MDFB may be insulated from its support as required by the CO
manufacturer.

**7.1.3** Protective \"ground connections\" should be made between the
MDFB and frame ironwork for personnel protection regardless of the type
protector assemblies used. The protective ground leads should be
14 gauge and less than 12 inches (30 centimeters) in length. Paint has
to be thoroughly removed at points of connections to the ironwork. One
connection should be made for every 35 feet (10.7 meters) of frame
length.

**7.1.4** Where the MDFB is used as the MGB in very small offices
(4.2.2), the protective \"ground connections\" (7.1.3) should be
connected in the N section of the bar. The MDF protector ground should
be connected to the P section of the bar.

**7.2** **[Transmission Equipment Termination and
Protection]{.underline}:** Digital carrier equipment and sensitive
electronic pair gain systems should normally be located inside the IGZ.
Some carrier equipment have internal gas tubes for bypassing voltage
surges to ground. Equipment of this type should be located outside the
IGZ. All equipment frames located outside the IGZ should be grounded
through connections at the N section of the MGB. The equipment located
inside the IGZ should be grounded to the GWB.

**7.2.1** Protectors for all carrier equipment are normally located on
the MDF, though exceptions may be made to this rule. The protectors for
some toll carrier entrance cables are mounted in the carrier bays
located in a non-IGZ area.

**7.2.1.1** Shields of intraoffice cable connecting the MDF to carrier
equipment bays should be open at the MDF end and grounded at one point
to the MGB or GWB. This grounding arrangement provides electrostatic
shielding and maintains GWB integrity.

**7.2.1.2** Separation of the transmit and receive sides of the cable
for T-carrier systems should be maintained. This may be accomplished by
using compartmental separation or separate transmit and receive cables
all the way to the MDF protector assembly. Between that point and the
carrier equipment the separation is usually maintained through use of
shielded jumpers, separate shielded transmit and receive cables, or
multipair cables with individually shielded pairs.

**7.3** **[Entrance and Tip Cables]{.underline}:** The most important
characteristics of tip cables, from a protection standpoint, are
resistance to flammability and ease of termination. They should also be
chemically compatible (i.e., should not chemically react) with the
outside plant cables. They should be sized as described in 7.3.3.

**7.3.1** Most RUS accepted polyvinyl chloride (PVC) insulations and
jacket formulations used in telephone cables have adequate flame
resistance. They can, however, be damaged chemically by cable filling
compounds that are in common use. Polypropylene and polyethylene
insulation, polyethylene jackets, and some filling compound types will
promote combustion. Use of filled cables in switchboard rooms should be
avoided because of fire hazard. Because of these considerations,
nonfilled PVC insulated and jacketed cables (or other insulation with
equivalent flame resistance) are preferred for use inside central office
buildings and for terminations on the MDF. For compatibility reasons,
polyethylene grease (low molecular weight polyethylene) and petroleum
jelly (petrolatums) filled cables should not be spliced to conductors
insulated with PVC. PVC jacketed tip cables currently available are not
usually suitable for outdoor use because of their low resistance to
ultraviolet attack and their tendency to become brittle at low
temperatures.

**7.3.2** The recommended procedure, for use with either filled or
nonfilled 24 gauge or smaller gauge polypropylene and polyethylene
insulated outside plant cables, is to use a special 22 gauge
polyethylene insulated PVC covered conductor tip cable with a PVC outer
jacket (ALVYNR), or equivalent, in place of PVC insulated. Only those
cables that are included in the "List of Materials Acceptable for Use on
Telecommunications Systems of RUS Borrowers,\" Informational Publication
344-2, should be used. With this arrangement, if the outside cables are
filled, the outer PVC covering of the tip cable conductors can be
attacked by the filling compound. The PVC covering may crack in the
immediate vicinity of the splice after having been in place for
sometime. Tests have shown that the polyethylene insulation on the wire
beneath the PVC covering will remain intact and retain adequate
dielectric strength. This provides an electrically satisfactory splice
in spite of the loss of the thin PVC outer layer. The portion of the tip
cables run in the office and terminated on the MDF retain their PVC
covering and remain flame resistant.

**7.3.3** If the first sections of the outside plant cables are coarser
than 24 gauge, an additional splice would be needed to install a fuse
link between the tip cables and each outside plant cable coarser than
24 gauge. Fuse links are typically 24 gauge and have a minimum length of
4 feet (1.2 meters) as shown in Figure 3: Entrance and Tip Cable
Arrangements. The additional splices are expensive and undesirable.
Therefore, they should be avoided when possible. One means of avoiding
the extra splice is to use a 24 gauge entrance cable between the office
and riser pole, manhole or pedestal outside the office.

**7.3.4** In the event that neither a cable vault nor a splicing trough
exists, the outside plant cables should be brought into the central
office and spliced to the ALVYNR, or equivalent, tip cables as close as
practicable to the cable entrance. When this design is used, the
entrance of the outside plant cable into the building and the splice
itself should be enclosed in a fireproof box mounted on the inner side
of the building wall as shown in Figure 4: Cable Entrance without Vault.
Where this is not feasible, and a pedestal is to be used for outside
plant cable termination, the following recommendations are made:

**7.3.4.1** When the pedestal closest to the central office is used in
place of a cable vault, the following are recommended:

**7.3.4.1.1** The pedestal should be a BD-7 or larger. The steel
grounding bar in the pedestal should be replaced with a tinned‑copper
grounding bar which will become the Cable Entrance Ground Bar (CEGB).
The CEGB should be bonded to the pedestal housing.

**7.3.4.1.2** The pedestal CEGB should be solidly bonded to an 8 foot
(2.45 m) ground rod driven into the ground near the pedestal entry
access door to be used by personnel. If multi‑door access is expected,
installation of a small grounding ring around the pedestal should be
considered. This ring should consist of 8 foot (2.45 m) ground rods at
each corner of the pedestal connected to one another using at least a
#6 AWG copper conductor.

**7.3.4.1.3** The CEGB should be bonded to the COGF via a 2/0 AWG
insulated copper conductor using the most direct route with few or no
bends.

**7.3.4.1.4** If the COGF is not readily accessible, the CEGB should be
connected to the P Section of the Master Ground Bar (MGB) in the central
office via a copper conductor sized in accordance with paragraph 8. This
conductor should enter the CO via a non‑metallic conduit.

**7.3.4.1.5** The outside plant cable shields should be connected
(solidly bonded) to the CEGB using the proper size grounding straps,
lugs, and bolts. At the pedestal the TIP cable shields should not be
connected to anything and should be taped or otherwise insulated. The
TIP cable shields should be grounded at the Main Distributing
Frame (MDF) Ground Bar inside the CO. TIP cables outside of the CO
should be installed inside weather proof conduit.

**7.4** **[Protection]{.underline}:** Incoming cable pairs terminated on
MDF protector assemblies should be protected with protector modules.
These modules, which connect an arrester between each cable conductor
and ground, effectively limit foreign potentials that will reach the
equipment in the office. The modules should contain white coded carbon
blocks, orange coded gas tube, or other (i.e. solid state) arresters
that are RUS accepted. The arresters breakdown at less than 1000 Volts
under surge conditions.

**7.4.1** Cable pairs associated with carrier, concentrator special
circuits or other systems should be protected with orange coded gas tube
or other protector modules. This equipment is tested to withstand only
the maximum voltage passed by these modules. Past experience with most
electronic equipment has shown there is very little margin above the
test level. Other types of special high voltage gap protection as
recommended by the equipment manufacturer are acceptable.

**7.4.2** There have been reports of electronic equipment failures in
central offices equipped with blue coded arresters. The replacement of
existing blue coded with white coded carbon block arresters is essential
when an existing mainframe is retained for protection of a new
electronic digital switch.

**7.5** **[Current Limitation]{.underline}:** RUS accepted mainframe
protectors are capable of carrying, without hazard, the sustained
current which may result from commercial ac power contacts to outside
plant cable having 22 gauge or finer wire. There are a number of MDF
protectors available on the market which do not have adequate current
carrying capability. It is important that the borrower\'s engineer
ascertain that the MDF protectors delivered by the COE contractor are
actually RUS accepted.

**7.6** **[Heat Coils]{.underline}:** Since heat coils proved to be
\"high maintenance\" items compared to fuse links, fuse links are
preferred for meeting National Electrical Code objectives in the Central
Office. Heat coils should not be used with carrier frequency pairs
because of high frequency attenuation. Because, the addition of heat
coils increased the cost of telephone system with virtually no
protection benefits, their use by RUS borrowers has been mostly
eliminated.

**8. GROUNDING CONDUCTOR SIZING, ROUTING AND TERMINATING**

**8.1** **[Sizing of Protective Grounding Conductors]{.underline}**: The
point of reference for sizing all protective grounding conductors except
green wire conductors and dc power conductors is the MGB. To determine
the appropriate conductor size first establish the actual conductor
routing distance between the two points of connection via the desired
route (i.e., between the MGB and CEGB). Next refer to Figure 6 to
determine the resistance objective between the two points. Finally, from
Figure 7: Maximum Conductor Length to Meet the Grounding Conductor
Resistance Objectives, find the wire size with a maximum footage for the
desired resistance objective equal to or greater than the wire distance
between the two points. Use of Figure 7 or calculated resistance values
are permissible in lieu of measurement. Although minimum conductor
[impedance]{.underline} is the ultimate goal in installing a grounding
conductor, for purposes of ease in sizing, dc resistance of the wire is
used. The resulting conductors for the dc resistances shown on Figure 6
will result in larger conductors than would otherwise be used for normal
power wiring. These larger size conductors are intended to handle
expected rapidly rising surges prevalent in central offices. The general
guidelines in the following paragraphs are also recommended.

**8.1.1** The finest recommended conductor size is 6 gauge, except for
the 14 gauge protective grounds at the MDF described in 7.1.3.

**8.1.2** The conductor between the MGB and GWB should always be
2/0 gauge or coarser. The suggested size provided in this paragraph
pertains to protective grounding conductors only, [not to dc power
conductors]{.underline}. The maximum resistance of this conductor should
be less than 0.005 ohms.

**8.1.3** The conductor between the MGB and the neutral ground bar in
the ac service entrance panel board should always be 2/0 or coarser. The
maximum resistance of this conductor should be less than 0.005 ohms.

**8.1.4** The maximum conductor resistance from the MGB to the initial
point of connection with all surge producers should be less than
0.01 ohms.

**8.1.5** The maximum conductor resistance from the MGB to the point of
connection with all surge absorbers should be less than 0.01 ohms,
except as described in 8.1.3.

**8.1.6** The maximum conductor resistance from the MGB to the point of
connection with all equipment grounds should be less than 0.01 ohms.

**8.1.7** Where an intermediate ground bar (IGB) or connection is
provided, the 0.01 ohm objective should be divided on either side of the
IGB or ground connection.

**8.2** **[The Planning and Installation of the Wiring]{.underline}** is
critical to the provision of an effective grounding system. Care should
be taken to minimize induction that may appear in grounding system
wiring. Recommended guidelines for installation of grounding system
conductors include:

**8.2.1** Grounding conductors should be insulated to permit integrity
testing. Conductors should also be free of splices. If splices have to
be made, only compression connectors or exothermic welding should be
used.

**8.2.2** Grounding conductors should be routed in a manner that will
avoid sharp or right angle bends. Routes should follow the most direct
path with gradual bends to minimize the inductive reactances that tend
to impede surge currents and reduce the overall effectiveness of the
grounding system.

**8.2.3** Grounding conductors except the green wires and dc power
conductors should not be routed closely parallel to other conductors in
the office so as to minimize induction of surges into equipment wiring.
These conductors should not be routed through cable racks or troughs, or
within confines of any iron work.

**8.2.4** Grounding conductors should only be placed in nonmetallic
conduit. If a grounding conductor has to be routed through metallic
conduit both ends of the conduit need to be bonded to the grounding
conductor. In addition, grounding conductors should not be encircled
with metal clamps as such clamps could create high inductive reactance
that will impede the flow of surge current along the conductor.

**8.2.5** Wire-to-wire and wire-to-ground rod connections should be made
only with compression connectors or exothermic weld connections. Solder
joints should not be used for any central office system grounding
connection.

**8.2.6** Wire-to-bonding-bar (busbar) connections should be made with
lugs that have a compression connector or exothermic weld connection.
The lugs should have bolt-on provisions for the busbar connections using
copper bolts and nuts. Periodically, some of the busbar connections may
be removed for test purposes.

Two hole, bolted connections are recommended for terminating grounding
conductors on the MGB, MDFB, GWB.

**8.3** **[Stenciling and Tagging]{.underline}:** It is desirable that
the following stenciling and tagging be provided for simplification of
maintenance and testing:

**8.3.1** Permanent adhesive cable labels or suitable plastic tags
should be provided on ground wire leads at all busbars to identify the
origin of each conductor.

**8.3.2** The location for each grounding conductor should be identified
on each ground bar by permanent adhesive label or stenciling.

**8.3.3** The designated P, A, N and I segments of the MGB should be
clearly identified.

**8.3.4** Permanent identification tags should be placed on lightning,
CO and radio/microwave ground leads at their accessible points of
connection to the central office ground field outside the CO building.

**9.** **POWER SERVICE PROTECTION**

**9.1** **[The Minimum Protection for AC Power Serving Central Office
Buildings]{.underline}** should consist of a suitable arrester in the
electric power secondary circuit. The borrower is responsible for
determining that the characteristics of the secondary power arrester
coordinate with the dielectric strength and surge current carrying
ability of all items of ac powered equipment in the central office.
These items would include heating, air conditioning equipment, etc. This
normally means a secondary power arrester having a surge breakdown not
exceeding 1800 volts peak, and a valve device to prevent power follow
current.

**9.2** **[Power Arrester Protecting Kilowatt-hour Meter]{.underline}:**
In some instances a secondary power arrester may be provided by the
power company to protect its kilowatt-hour meter at the building service
entrance. These devices may not be suitable for protecting central
office equipment because they are usually designed to coordinate only
with the dielectric strength of kilowatt-hour meters (usually 9 to
10 kV). This voltage is normally too high for telephone equipment
powered by commercial AC lines.

**9.3** **[Protecting the AC Power Service Entering a Central
Office]{.underline}:** The use of a secondary arrester to protect the ac
power service entering a central office building is strongly
recommended. Some secondary arresters have a rapid response and
coordinate readily with normal ac powered equipment. The arresters may
be mounted either at the weather head or at the load center. Others have
poorer characteristics and have to be mounted at the weather head, with
at least 20 feet (6.1 meters) of steel conduit separating the arrester
from the load center to assure proper operation.

**9.3.1** If, after the installation of a secondary arrester, power
failures are still experienced from surges on the ac bus, a
supplementary protector; as shown in Figure 5: Typical Installation of
Secondary Arrester and Branch Circuit Power Service Protector, should be
applied to the affected branch circuit. Recommended supplementary
protection consists of a maximum duty gas tube in series with
self-restoring circuit breakers or an impedance, to prevent the tube
from holding over after the surge has passed.

**10. RADIO OR MICROWAVE INSTALLATIONS**

**10.1** **[Radio or Microwave Towers Which Are Located on or in Close
Proximity to CO Buildings]{.underline}** require special protective
considerations. Their height and conductivity increases the probability
of a direct lightning strike.

**10.2** **[Protection of the Tower and Associated
Equipment]{.underline}:** Details for the protection of the tower and
associated equipment are covered in Appendix B of this bulletin and in
TE&CM 825 (proposed conversion to Bulletin 1751F‑825).

**10.3** **[Bonding Tower Ground and Area Fences to CO Grounding
Systems]{.underline}:** It is important for protection of the central
office equipment that the tower grounding system be bonded to the CO
grounding system. Fences and gates should be grounded as detailed in
Appendix C paragraph C.4.2. These connections should be made outside the
building as described in 4.3.2. Thus a direct strike to the tower or
fence should be diverted to the grounding system rather than enter the
office.

**11. ELECTROSTATIC & ELECTROMAGNETIC FIELD EFFECTS**

**11.1** **[Static Electricity]{.underline}** is the accumulation of
stationary electrical charge on a body or conducting medium created by
physical motion such as drawing a comb through hair. Even circulating
air currents can cause a charge buildup, especially during periods of
low humidity. The electrostatic charge is discharged by grounding the
charge storing medium.

**11.2** **[FET, MOS, and CMOS]{.underline}:** Many circuit packs used
in electronic, digital switching equipment contain active devices such
as field effect transistors (FET), metal oxide semiconductors (MOS), and
complementary metal oxide semiconductors (CMOS). These static-sensitive
components can be permanently damaged when voltages higher than their
breakdown point are applied to them. The human body can develop and
store a charge of up to 40,000 volts by walking across a nonconductive
floor during periods of low humidity. Because of this possible build up
of static charge, special provisions should be applied to prevent
circuit component damage from this potential hazard when handling
printed circuit cards designated by the supplier to be sensitive to
static discharge.

**11.3** **[The Accumulation of Electrostatic Discharge by a Human
Body]{.underline}** may be reduced in a confined area such as a central
office by increasing the relative humidity. Body electrostatic
accumulation, at 60% relative humidity, is minimal. Even at this
excessive humidity level there is no guarantee the electrostatic build
up is eliminated. Further, the humidity may also cause equipment
contamination, corrosion, or leakage path problems on the printed
circuit cards and associated components.

**11.4** **[Electrostatic Conditions That Produce Equipment
Problems]{.underline}:** There are two kinds of electrostatic conditions
that produce equipment problems; direct arcs into the electronic
equipment, and radiated energy that reaches circuits through electric
and magnetic field coupling. Discharged electrostatic energy can create
a localized voltage (electric) field and current (magnetic) field in
adjacent circuit cards. Both types of fields can cause permanent
equipment damage and/or logic circuit errors.

**12. GENERAL ENVIRONMENTAL AND HANDLING REQUIREMENTS FOR ELECTROSTATIC­
SENSITIVE EQUIPMENT**

**12.1** **[Proper Environmental and Handling
Considerations]{.underline}** for electrostatic sensitive equipment are
essential to prevent component damage and service systems downtime. The
general procedures recommended in 12.2 and 12.3 will reduce the
probability of equipment damage.

**12.2** **[Environmental Conditions]{.underline}:** The following
environmental conditions should be maintained where possible:

**12.2.1** Appropriate relative humidity levels should be maintained
since static charges accumulate more readily under very dry climatic
conditions. Refer to the equipment manufacturer\'s relative humidity
recommendations.

**12.2.2** Adequate air and dust filters should be installed in air
ducts.

**12.3** **[Precautions for Maintenance Procedure]{.underline}:** The
following precautions should be observed when performing building and
equipment maintenance procedures:

**12.3.1** Grounding straps should be worn when handling printed circuit
cards designated by the manufacturer as being susceptible to damage.
Refer to the equipment manufacturer\'s procedures relating to this
subject.

**12.3.2** Grounded conductive floor tiles or mats should be installed,
where required. The manufacturer\'s recommendations should be followed
for installation connection to ground, and maintenance of the floor to
preserve conductivity.

**12.3.3** Printed circuit cards should not be touched or handled by
their components or connector pins.

**12.3.4** The repair or modification of circuit cards should not be
attempted in the local office. Units should be returned to the
manufacturer for repair if tests have shown that particular cards are
defective. An adequate stock of spares should be maintained in proper
storage containers (See RUS Form 522, Bulletin 1753E‑001, Part 1,
Paragraph 24.6)

**12.3.5** Conductive printed circuit card containers should be used as
recommended by the equipment manufacturer.

**12.3.6** Cards should be installed according to manufacturer's
instructions, especially where an Enable/Disable feature is provided and
the manufacturer recommends that no card should be inserted or removed
until the Enable/Disable switch is in the [disable position]{.underline}
and/or the card slot connection is disabled by software command.

**12.3.7** Only the grounded conventional or isolated ac ground
convenience outlets [located in the IGZ]{.underline} may be used for
operating tools, test equipment and custodial equipment inside the IGZ.
Refer to the equipment manufacturer\'s instructions regarding the use of
ac tools or test equipment in the equipment area.

**12.3.8** Steel wool, steel wool pads or dry untreated cloths or mops
for floor maintenance should not be used.

**12.3.9** Defective fluorescent lighting components should be replaced.
These include defective starters, flickering fluorescent tubes, or noisy
ballast transformers. Failure to replace these items may introduce noise
into power supply lines and systems.

**12.4** **[Precautions for Operating Motor Driven
Devices]{.underline}:** The following precautions should be observed
when operating motor driven devices in the central office building:

**12.4.1** All cleaning equipment and motor driven tools should be in
good working order.

**12.4.2** Motor driven devices should all have grounded 3-conductor
cords to bleed-off static charges or brush-noise generated radio
frequency transients.

**12.4.3** Motors that are not an integral part of the manufacturers
equipment should not be started, operated or stopped inside the IGZ.

**12.4.4** Equipment should be removed from service when adding or
removing wire-wrap connections. Where this is not possible, manual or
pneumatic wire-wrapping tools with insulated bits should be used.

**12.4.5** Tools with Silicon Controlled Rectifier (SCR) motor speed
controls should not be used. The SCR can cause transients in the power
supply line and generate magnetic fields.

**12.5** **[Precautions for Memory Devices]{.underline}:** The following
precautions should be observed for magnetic tapes, hard drive, floppy
discs, and other memory devices:

**12.5.1** Motor driven equipment should not be located adjacent to tape
transports or memory devices. An extra long hose should be used when
vacuuming with the cleaner itself located several feet outside of the
IGZ. A centralized vacuum cleaner system should be considered at least
for new CO buildings.

**12.5.2** Magnetic apparatus such as recording tapes and tape
transports should not be exposed to the magnetic fields produced by
electric motors.

**12.5.3** Magnetic tapes should be stored in radio frequency tight high
mu ferrous metal cabinets to avoid information loss.

**13. DISCHARGE PLATES**

**13.1** **[For Protection of Static Sensitive Equipment]{.underline}**,
all personnel should fully discharge any static charge on their body
before touching or handling any part of the service system. This is
especially important in common control areas. Central office personnel
when working in the switching area should touch the nearest discharge
plate before touching any part of the service system.

**13.2** **[Installation of Electrostatic Discharge
Plates]{.underline}** should be considered where they have not been
provided by the equipment manufacturer. They should not be installed
until the manufacturer has been consulted for recommendations on
locations and ground connections. The shape and method of attaching the
plates should be accomplished in a manner that will not create any
hazard to personnel or limit access to the equipment. Personnel
discharge plates should be located, where practical, at intervals within
an arms length of any maintenance location.

**13.3** **[Supplemental Discharge Plates]{.underline}** may also be
provided by:

**13.3.1** Hinged metallic doors when they are grounded with a 14 gauge
conductor to the building structural steel or the MGB. Conductive paint
should be applied to the doors and metallic door knobs should be left
bare.

**13.3.2** Light switches and ac power outlets with metallic
plates/covers which are electrically connected to the grounded green
wire inside the electrical box.

**13.4** **[Warning Signs]{.underline}:** Appropriate warning signs
should be posted on all equipment area entry doors and inside the CO
where they can be easily seen without creating a safety hazard. The
signs should be worded to warn personnel of the electrostatic sensitive
area and the need for discharging body static before handling equipment.

Figure 1: Master Ground Bar

![](media/image1.emf)

Figure 2: Outdoor Grounding Conductor Connections

![](media/image2.emf)

Figure 3: Entrance and Tip Cable Arrangements

![](media/image3.emf)

Figure 4: Cable Entrance without Vault

![](media/image4.emf)

Figure 5: Typical Power Service Protector Installation

![](media/image5.emf)

Figure 6: Central Office Protection Grounding

![](media/image6.emf)

Figure 7: Maximum Conductor Length to Meet the Grounding Conductor
Resistance Objectives

qq**APPENDIX A**

**VOLTAGE FROM SURGE CURRENTS**

**A1. GENERAL**

**A1.1** **[Purpose]{.underline}:** This appendix discusses the voltage
effects on grounding conductors from self inductance in the presence of
high surge currents with fast rise times. The discussion is designed to
provide a better understanding of the basis for some of the general
rules relating to routing of grounding conductors in central office
buildings.

**A1.2** **[Self-Inductance of Conductors]{.underline}:** Every
conductor has self inductance which provides an impedance to lightning
and other surges. A significant voltage difference will occur between
the ends of a grounding conductor during the period a surge current is
flowing. This potential difference should not appear across sensitive
electronic equipment. In addition, points in the overall grounding
system, between which the potential can appear, should not be located so
personnel can touch both simultaneously.

**A2. SELF-INDUCTANCE**

**A2.1** **[Equation of Self-Inductance]{.underline}:** The
self-inductance ($L_{g}$) of a solid, round, non-magnetic and straight
ground wire in air or plastic conduit may be approximated with:

$L_{g} = 0\text{.}2l\left( \ln\frac{\text{155}l}{d} \right)$ (A1)

Where: $L_{g}$ = Self-inductance, microhenries (µH)

$l$ = Wire length, meters

$d$ = Wire diameter, centimeters

**A2.1.1** All grounding connections in a typical small rural central
office can probably be made using only #6 (0.162 inches,
0.411 centimeters in diameter) and 2/0 (0.365 inches, 0.927 centimeter
in diameter) conductors. Lengths of 30 feet (9.1 meters) might be
required for some connections. From equation (A1) the self-inductance
for 30 feet (9.1 meters) of #6 wire is 14.9 µH and with 210 feet
(63.7 meters) is 129 µH.

**A2.2** The self-inductance ($L_{g}$) of a ground wire in steel conduit
where ends of conduit are not bonded to wire is given as:

$L_{g} = 0\text{.}2l\left( \ln\frac{\text{155}l}{d} + \text{1200}\ln\frac{d_{1}}{d_{2}} \right)$
(A2)

Where: $\text{1200}$ = estimated permeability of iron relative to air

$d_{1}$ = outside diameter of conduit in centimeters

$d_{2}$ = inside diameter of conduit in centimeters

**A2.2.1** The self-induction of the 30 feet (9.1 meters) lengths of #6
and 2/0 wire encased in unbonded rigid steel conduit with an outer
diameter of 1.315 inches (3.340 centimeters) and inner diameter of
1.049 inches (2.664 centimeters) may now be determined from equation
(A2). The self inductance of the #6 wire is 511 µH and 2/0 wire is
509 µH.

**A2.2.2** A grounding conductor 30 feet (9.1 meters) long would not
likely be placed in steel conduit. A more common use of conduit is for
carrying the conductor through a wall via a one foot (0.3 meters)
length. A one-foot (0.3 meters) length of #6 wire through a one-foot
(0.3 meters) unbonded rigid steel conduit will have a self inductance of
16.8 µH and 2/0 will have 16.8 µH. The self inductance of one-foot
(0.3 meters) of wire in steel conduit is higher than for 30 feet
(9.1 meters) of bare wire in air.

**A3. VOLTAGE LEVEL FROM SELF-INDUCTANCE**

**A3.1** The calculation of the momentary voltage that will develop
across a length of conductor using the conductor self-inductance is
possible. This voltage is given by the differential relationship:

$e = L\frac{\text{di}}{\text{dt}}$ (A3)

Where: $e$ = voltage (volts)\
$L$ = inductance (Henries)\
$\text{di}$ = change in current (Amperes)\
$\text{dt}$ = change in time (seconds)

**A3.1.1** Assuming a moderate surge of 2000 peak amperes with a rise
time of 10 microseconds through the 30 feet (9.1 meters) bare wire
described in A2.1.1, from equation (A3), the voltage developed across
the wire could be:

#6 = 2980 volts

2/0 = 2680 volts

**A3.1.2** If this wire is placed in unbonded 30 feet (9.1 meters) rigid
conduit as described in A2.2.1 the voltage developed would be:

#6 = 102,200 volts

2/0 = 101,800 volts

**A3.1.3** Study of the example described in A2.2.2 where a one foot
(0.3 meters) length of unbonded rigid steel conduit is used to pass the
grounding conductor through a wall is more practical. From equation (A3)
the voltage developed across the one foot (0.3 meters) conductor length
would be 3360 volts for #6 and 3350 volts for 2/0 wire. The voltage
developed across one foot (0.3 meters) of wire in conduit is 13 percent
higher than for 30 feet (9.1 meters) of bare #6 wire and 25 percent
higher than for 30 feet (9.1 meters) of 2/0 wire.

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**qqAPPENDIX B**

**PROTECTION FOR ANTENNA INSTALLATIONS**

**B1.** **GENERAL**

**B1.1** **[This appendix]{.underline}** provides guidelines for
designing and constructing protection systems for antenna installations.
It is not meant to suggest development of systems that are unreasonably
expensive or otherwise impractical. Wire and ground rod sizes are
presented as minima and do not preclude the use of larger sizes.

**B1.2** **[Susceptibility to Lightning Stokes]{.underline}:** Antennas
and supporting structures for microwave, mobile radio and paging systems
are generally susceptible to lightning strokes because of their height
and high conductivity. Rural conditions contribute even more to this
susceptibility because of generally poorer grounding conditions and the
lack of shielding by tall buildings. Digital central office equipment is
especially vulnerable since even low voltages can severely damage
components of a digital switch.

**B2.** **Protection Grounding and Bonding**

**B2.1** **[Bonding of all-metal elements]{.underline}:** Large currents
caused by near or direct lightning hits to towers and associated
structures can cause hazardous voltage differences between metal parts.
These voltage differences result from current flowing through the
resistance and inductance of these metal parts and grounding conductors.
Bonding is essential to prevent voltage differences. All-metal elements
of the building structure, such as steel reinforcing rods in concrete,
metal sheathing, metal roof supports and trusses, and metal piping and
conduit systems should be bonded to the station ground. Although it is
desirable that the central office ground have low resistance, it is more
important that all metal parts be bonded together to keep them at the
same voltage. Thus, sensitive equipment such as digital switches can
remain at the same potential relative to the rest of the office, even
though there may be a rise in the overall ground potential.

**B2.2** **[Placing Grounding Conductors in Metal
Conduit]{.underline}:** Avoid placing grounding conductors in metal
conduit, if possible. However, where grounding or bonding conductors are
run in metal conduit to avoid mechanical damage, the conductor should be
bonded to the conduit at both ends to decrease inductance and prevent
arcing.

**B2.3** **[Buried Connections]{.underline}** should be welded to assure
a permanent low resistance.

**B2.4** **[Ground Rods]{.underline}** should always be driven straight
into the ground to achieve maximum depth and lowest resistance to
ground.

**B3.** **TOWER INSTALLATIONS**

**B3.1** **[Design Objective]{.underline}:** The design objective of the
tower ground is to produce a network that matches or comes close to
matching the ground resistance of the central office. This match should
prevent a large current from flowing into the central office by giving
it a low impedance path to ground at the tower. Refer to Bulletin
1751F‑802 and to the manufacturer\'s instructions for additional
information. Every effort should be made to obtain the design objective.

**B3.2** **[Self-Supporting Tower Installations]{.underline}:** A
typical self-supporting tower installation is shown in Figure B1:
Self-Supporting Tower Installation. As a minimum, these structures
should have a ground rod 5/8 inch x 8 foot (1.6 centimeters x
2.4 meters) driven at the base of each footing and bonded to the tower
leg with a #6 copper conductor. This conductor should gently slope
downward with no sharp bends (as shown in Figure B2: Bonding Details).
However, ground rods should be placed close to the tower base and the
conductor dressed in a manner that will avoid accidental damage to the
grounding system or injury to workers. All of these ground rods should
be bonded together with a #6 bare copper conductor buried at least
two feet underground. Reinforcing bars in the concrete base should be
bonded to the ground rods as well as to the tower structure. Where these
bars touch each other, they should be fused together to prevent arcing.
Waveguide and coaxial cable should be bonded in accordance with 6.3 and
6.4. It is also necessary to bond the antenna supporting structure to
the Central Office ground (see 3.4).

**B3.3** **[Guyed Tower Installations]{.underline}:** A guyed tower
installation is shown in Figure B3: Guyed Tower Installation. Guyed
towers should be protected by a ground ring at least 10 feet (3 meters)
across. This ring should be made of #6 bare copper conductor buried at
least two feet underground and bonded to at least four 5/8 inch x 8 foot
(1.6 centimeter x 2.4 meters) ground rods. The structure itself should
be bonded to the ground ring by two separate connections of #6 copper
conductor. If the base is concrete, the reinforcing bars should be
bonded to the structure and to the ground ring. Waveguide and coaxial
cable should be bonded in accordance with 6.3 and 6.4. The ground ring
should also be bonded to the central office ground. A ground rod should
be driven at each guy anchor and bonded to the guy wire with a #6 or
larger copper conductor. Multiple guy wires on the same anchor should
all be bonded together and to the ground rod with a #6 conductor.

**B3.4** **[Bonding the Tower Ground to the Central Office]{.underline}
[Ground]{.underline}:** Although the tower ground should absorb most of
the lightning energy, it should be bonded to the Central Office ground
to prevent a voltage difference from developing between the two. This
connection should be made outside the central office building in a
ground well as shown in Figure 24 of 1751F-802. Refer to Figure 6 and
Table C-1 of this bulletin for proper conductor size.

**B4.** **POLE-MOUNTED ANTENNAS**

**B4.1** **[A pole-mounted installation]{.underline}** is shown in
Figure B4: Pole Mounted Installation. Grounding systems for protection
of pole mounted antennas need to protect both the pole and the equipment
connected to the antenna. Poles that have antennas extending above the
top of the pole are in a cone of protection formed by the antenna. The
antenna acts as a lightning rod and needs to be appropriately grounded
(see 6.2). Microwave, and other installations in which the top of the
pole is exposed, need a lightning rod mounted on top of the pole
extending at least 1 foot (30 cm) above the top of the pole. A down lead
of #6 copper conductor should be used to connect the lightning rod or
antenna to the ground ring at the base of the pole. The ground ring
should consist of at least 3 ground rods separated from each other by at
least 10 feet (3 meters), and bonded together with #6 bare copper
conductor buried at least two feet underground. The ground ring should
be bonded to the Central Office ground field external to the Central
Office building. This connection should be made to a ground rod inside a
ground well as shown in Figure 24 in 1751F‑802. Refer to Figure 6 and
Table C-1 of this bulletin for proper conductor size.

**B5.** **ANTENNA TOWERS MOUNTED ON TOP OF BUILDINGS**

**B5.1** **[Grounding and Bonding of Antenna Towers]{.underline}**:
Antenna towers on top of buildings should have a ground ring of #6
copper conductor bonding the legs at their bases. #2 copper conductor
down leads should be bonded to each tower leg and brought down the
outside of the building to ground rods except on structural steel
buildings (See Figure B5: Building Mounted Installation). These ground
rods should also be bonded together with #6 bare copper conductor buried
between them. This ground ring should be bonded to the central office
ground in a ground well as shown in Figure 24 in Bulletin 1751F‑802. A
reinforced concrete building should have its antenna structure bonded to
the reinforcing bars in the concrete. These bars should in turn be
bonded to the ground ring. Reinforcing bars should be fused together
wherever they touch to prevent arcing.

**B5.1.1** Structural steel buildings should have each leg of the tower
bonded to the structural steel with #2 copper conductor. The structural
steel should be bonded to the ground ring with #2 copper conductor at
each corner of the building. If these bonds are made, the structural
steel usually provides an adequate ground itself without use of the #2
copper conductor ground leads, down the side of the building.

**B5.1.2** Structural steel or reinforcing bars should be integrated
into the building ground and bonded to the water system and power system
neutral.

**B6.** **ANTENNAS AND CONNECTING COAXIAL TRANSMISSION LINES AND
WAVEGUIDES**

**B6.1** **[Antennas and connecting transmission lines]{.underline}**
need to be suitably protected from lightning without introducing
significant attenuation to the radio signal. Ideally, lightning current
should flow through other conductors such as metal towers and grounded
down leads. However, since this is not always the case, bonding
procedures have to be employed to protect equipment connected to the
transmission line.

**B6.2** **[Antennas]{.underline}** should have a good ground path to
adequately dissipate lightning currents. Ground plane antennas, such as
folded monopole antennas, can be directly bonded to the tower or ground
down lead. Microwave horns, dishes, and reflectors should also have a
direct path to ground via a grounding kit connected to the waveguide at
a point close to the antenna (see 6.3). Coaxial antennas should be
protected by a star gap arrester. A star gap arrester is a serrated
washer connected directly to the center conductor of the antenna.
Lightning surges effectively see a short circuit across the gap to
ground through the outer conductor while the transmission path is left
relatively unaffected. Other types of air gap arresters may also be
used. One additional method employs a shorted quarter wavelength stub
connected to the coaxial cable at the base of the tower or pole. The
shorted end of the stub is connected to ground giving lightning a good
ground path. Since the shorted stub is a quarter wavelength, it should
not affect the radio signal.

**B6.3** **[Waveguides]{.underline}:** Using a grounding kit, waveguides
should be bonded to the tower or ground down lead at least at the top
and bottom of the antenna structure. Grounding kits are usually
available from the manufacturer. The braided conductor of the grounding
kit should be connected in a downward direction from the waveguide to
the tower since lightning will generally not flow in an upward direction
from the waveguide to the tower. A properly installed ground kit is
shown in Figure B6: Tower Grounding Kit Installation. Waveguides should
be bonded to the tower at points of support to prevent arcing. Ice
shields or other supporting structures running between the tower and
central office should he bonded to both the tower and central office
ground field. A grounding kit should be used to bond waveguides to the
central office and tower ground field just prior to the waveguide\'s
entrance to the building (as shown in Figure B7: Entrance Grounding at
Central Office).

**B6.4** **[Coaxial Cable]{.underline}:** The outer conductor of the
coaxial cable should be bonded to the tower or ground down lead at least
at the top and bottom of the antenna structure. Non-insulated coaxial
cable should also be bonded at intermediate points of support to prevent
arcing. Added protection can be provided by means of a ground entry
plate connected to the building (see Figure B7). All transmission lines
should be bonded to the plate with grounding kits. The braided end of
the grounding kit should run in a downward direction. The plate itself
should be bonded to the central office and ground field by a #6 copper
conductor. If convenient, this connection should also be to a ground rod
in a ground well as shown in Figure 28 in Bulletin 1751F‑802.

**B7.** **PROTECTION OF RADIO EQUIPMENT**

**B7.1** **[Radio equipment]{.underline}** should be integrated into the
central office grounding scheme to divert lightning currents from it and
the Central Office switch. Transmitters, receivers, and multiplexers
should be mounted on grounded metal racks bonded to the master ground
bar by a conductor or bus with a resistance no greater than 0.01 ohms
(refer to Figure 6).

Figure B1: Self-Supporting Tower

![](media/image9.emf)

Figure B2: Bonding Details

![](media/image10.emf)

Figure B3: Guyed Tower Installation

![](media/image11.emf)

Figure B4: Pole Mounted Installation

![](media/image12.emf)

Figure B5: Building Mounted Installation

![](media/image13.emf)

Figure B6: Tower Ground Kit Installation

![](media/image14.emf)

Figure B7: Entrance Grounding at Central Office

![](media/image15.emf)

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**qqAPPENDIX C**

**ANALYSIS OF CENTRAL OFFICE GROUNDING SYSTEMS**

**C1. GENERAL**

**C1.1** **[Evaluating a Central Office Grounding System]{.underline}:**
This Appendix discusses a technique for analyzing a central office
grounding system. The method presented will provide information as to
where there may be missing elements in a grounding system. Use of this
technique is recommended for analyzing newly installed grounding
systems. Further, annual analysis of all grounding systems is
recommended so that any problems which may have arisen during the year
may be detected and corrected.

**C1.2** **[Ammeter for Grounding System Analysis]{.underline}:** The
procedure uses a small clamp-on probe together with a portable ammeter.
The ammeter should have a useful range from milliamperes to at least
twenty amperes. There are several types available which are capable of
completing these tests.

**C2. PROCEDURE**

**C2.1** **[Start of Procedure]{.underline}:** Start the analysis at the
Master Ground Bar (MGB). This is the hub of the central office grounding
system (the single point ground).

**C2.1.1** First, prepare a sketch of the MGB with all of the
conductors, labeling each with the name of the ground bar to which it is
connected at its other end.

**C2.1.2** Second, place the clamp or probe around each conductor and
read the ac current. Then, record the current at the appropriate
conductor on the sketch. Repeat until the current associated with each
conductor has been recorded on the sketch. The completed sketch at this
location will be similar to the one shown in Figure C1: Master Ground
Bar Currents.

**C2.2** **[Cable Entrance Ground Bar]{.underline}:** Proceed next to
the Cable Entrance Ground Bar (CEGB) and prepare a sketch similar to the
one prepared for the MGB. Then complete the measuring and recording of
the conductor currents in the same manner as with the MGB. A sample CEGB
sketch is shown in Figure C2: CEGB Currents.

**C2.3** **[Other Ground Bars]{.underline}:** Complete the same
operations for all of the remaining ground bars in the office: MDF
Ground Bar (MDFB), Ground Window Bar (GWB), and any Intermediate Ground
Bars (IGB).

**C2.4** **[Checking the Conductors]{.underline}:** During preparation
of the sketches at each location, check to determine if all of the
conductors are connected to the ground bar in accordance with the
guidelines of this bulletin. The most critical conductors are those to
the Surge Producers (interior radio equipment, cable shields, and cable
pairs), and the Surge Absorbers (power neutral, Central Office ground
field, and water system).

**C3.** **ANALYSIS OF THE CONDUCTOR CURRENTS**

**C3.1** **[Predicting Current Levels]{.underline}:** Prediction of the
precise current level that will be flowing along each grounding
conductor is not possible. These levels are determined by the amount of
voltage induced in the outside telecommunications facilities from
paralleling power systems. Therefore, only a comparative analysis of the
recorded current values can be made. Table C‑1 lists the typical range
of currents for various grounding conductors.

**C3.2** **[Current Levels of Conductors Connected to Surge
Absorbers]{.underline}:** The most important factor in the analysis is
the comparison of current levels on conductors connected to the Surge
Absorbers (power neutral, water system, and Central Office ground
field). The maximum current should be flowing on the ac bypass conductor
(the conductor between the MGB and the neutral ground bar in the power
system entrance panel.) Where there is a metallic water system, the next
highest current should be flowing in the conductor between the MGB and
the water system. There should be less current flowing on the conductor
to the central office ground field than on the other two conductors.

**C3.2.1** Systems installed in areas where there are no metallic water
systems that can be integrated into the system should have the maximum
current flowing to the power system neutral via the ac bypass conductor.
The current flowing to the central office ground field should be smaller
than that flowing to the power system neutral as it would be in systems
where all three Surge Absorbers are present.

**C3.2.2** Some systems will be installed in areas where not only are
there no metallic water systems but, sometimes the power systems are
operating with either unigrounded neutrals or in a delta configuration.
In these areas, the only ground available for the central office will be
the central office ground field. The level of induced power influence in
these areas will normally be quite low and the measured current to the
ground field will usually be in the same range as that found at all
offices.

**C3.3** **[Current Flowing to Power System Neutral]{.underline}:** The
range of current that will be typically found flowing to the power
system neutral will be in the range of one to twenty amperes. The actual
level will fluctuate within this range depending on the level of power
influence.

**C3.3.1** Current flowing toward the water system will be lower than
that flowing in the ac bypass conductor. There are several factors that
control the level of current in the conductor to the water system. The
major factor is how extensive the metallic water system is. When the
metallic portion of the system is only ten to fifteen feet (3 to
4.6 meters) in length, the current will typically be in the range of 100
to 1000 milliamperes in offices that are served by power systems that do
not have a grounded neutral. Currents in this range found in offices
served by grounded-neutral power systems usually indicate that one of
two grounding conductors are missing or open circuited. Either the ac
bypass conductor or the conductor between the water system and the
neutral ground bar in the power entrance panel (required by the National
Electrical Code) is missing or open circuited. This should be corrected
before proceeding further. When the office is served by a grounded
neutral power system or when the metallic water system covers an
extensive area, the current in the conductor to the water system will be
in the range of one to five amperes or more.

**C3.3.2** The current measured on the conductor to the central office
ground field will be significantly lower than that flowing to the other
Surge Absorbers in all cases. This current will typically be in the
range of 100 to 500 milliamperes.

**C3.4** **[The ac Current from Power System Induction in the Outside
Plant Flowing into the Central Office]{.underline}** on the cable
shields will be in the range of one to twenty amperes. This is
controlled by the level of power influence and will fluctuate over a
wide range during the day as power system load demand changes. This
current will be measured on the conductor between the CEGB and the MGB.
Where the conductor from the central office ground field enters the
office in the cable vault and is connected to the CEGB, the total
current to the MGB will be less than current that flows directly to the
ground field from the cable shields. Because of the normal current
fluctuations, the currents assumed to be flowing toward and away from
each ground bar will not always add precisely. Differences as high as
fifteen percent are normal. When the difference is higher than this,
remeasure the currents.

**C3.4.1** The induced current flowing into the office on the cable
pairs will normally flow to ground via the battery feeds on the line
cards. This current returns to the MGB via the conductor to the positive
battery terminal. Its magnitude is typically in the range of 100 to
500 milliamperes. Should a significantly higher current be measured, the
ac bypass conductor is usually missing or open-circuited. Some offices
have line circuits with a high impedance to ground (10,000 ohms and
over). The current between the positive battery terminal and the MGB in
these offices will be negligible.

**C3.4.2** Some central offices are collocated with CATV systems. A
significant current will flow into the offices on the outer shields of
the coaxial cable associated with these systems. These cable shields
should be integrated into the central office grounding system via the
CEGB. The current from CATV systems can exceed that which is associated
with the outside plant of the

telecommunications system. This current will typically range from one to
twenty, or more, amperes.

**C3.5** **[Current on Other Grounding Conductors]{.underline}:** The
range of current measured on all other grounding conductors associated
with the grounding system will vary from a trace to several
hundred milliamperes. These currents are normal and relate to how
closely the equipment in the bays is associated with the outside plant.

**C3.6** **[Current on Conductor to the Ground Window
Bar]{.underline}:** There will always be some current measured on the
conductor to the Ground Window Bar (GWB). This current will usually be
in the range of 50 to 200 milliamperes. When, a higher current is found,
the search should proceed beyond the GWB to determine why the higher
current is present and whether such current is detrimental to the
specific equipment installed in the office.

**C4.** **VISUAL INSPECTION**

**C4.1** **[General]{.underline}:** A visual inspection should be
completed during measuring. Each grounding conductor should be traced
from the ground bar to its far end. There are several items which should
be closely checked during this stage of the operation. These are
outlined in the following paragraphs:

**C4.1.1** **[Conductor]{.underline}:** The conductor should be
continuous, end to end, with no splices, or intermediate terminations.
Also, the conductor should be sized for the distance between the two
ends, as shown in Figure 7 of this bulletin.

**C4.1.2** **[Sharp Bends]{.underline}:** There should be no sharp bends
along the entire length of the conductor. Sharp bends increase the surge
impedance of the conductor reducing the grounding system efficiency.

**C4.1.3** **[Grounding Conductors]{.underline}** should not pass
through any metallic conduit or pipe as this will also increase the
surge impedance of the grounding conductor. Should an existing grounding
conductor be found passing through a metallic pipe, solidly strap it to
the pipe at each end. This will eliminate the adverse condition and
provide a low impedance path for surge currents.

**C4.1.4** **[Copper Ground Bars]{.underline}** in the grounding system
should be mounted on insulators except for the MDFB and as noted in
paragraph 7.3.4.1.2. The MDFB may be mounted directly on the metal
frame. The protector strips on the MDF should be connected together with
#6 solid copper wire, rather than relying on the steel frame to conduct
surge currents to ground.

**C4.2** **[The Area outside the Building]{.underline}** should also be
inspected to determine that the entire grounding system has been
properly integrated. The following paragraphs list those items which
should be studied during the outside inspection:

**C4.2.1** Where a fence exists near or adjacent to a central office or
similar type telecommunications facility, the following grounding
provisions of a chain link post, chain link fence and, where equipped,
fence barbed wire should be installed at each corner fence post, at
least one fence post between corner fence posts and at each post
supporting a gate, supporting a gate latch unit or a security provision
providing a movable fence opening:

At a point within 6 inches (15 centimeters) laterally away from a fence
post or the vertical projection of the fence post, install a 6 AWG solid
bare copper grounding conductor by:

**C4.2.1.1** Bonding one end of the conductor to the upper most strand
of barbed wire, if barbed wire is not installed go to C4.2.1.3;

**C4.2.1.2** Interweaving the other conductor end vertically down
through the remaining lower strands of barbed wire;

**C4.2.1.3** Bonding the conductor to the upper most horizontal rail of
the chain link fence at the point on the conductor where the conductor
is adjacent to the upper most fence rail when the conductor is pulled
down tightly;

**C4.2.1.4** Interweaving the free conductor end down through the chain
link fence mesh;

**C4.2.1.5** Bonding the conductor to the lower most fence rail of the
fence at the point on the conductor where the conductor is adjacent to
the upper most fence rail when the conductor is pulled down tightly;

**C4.2.1.6** Bonding the conductor end to an 8 foot (2.4 meters) ground
rod driven into the soil approximately two feet (60 centimeters)
horizontally away from the plane of the chain link fence, vertically in
line with the conductor feed, and on the outside of the fence. The top
of the ground rod should be one foot (30.5 centimeters below the soil
surface; and

**C4.2.1.7** Bonding at least one of the installed fence ground rods to
the central office or remote access node ground field with at least a 6
AWG bare copper wire. Bonding to the ground field should be completed
somewhere outside of the building, preferably in a handhold.

**C4.2.1.8** The bonding and grounding described in C4.2.1.1 through
C4.2.1.7 should be bare copper bonding jumper. All wire to metallic post
or ground rod bonding should include the options of brazing, welding,
mechanical and high compression connections, and ground clamps listed
for the purpose by a testing laboratory acceptable to the authority
having jurisdiction. (Reference UL#96 and IEEE 837 documents.) Any tower
within a fenced area shall be grounded as noted in C4.2.2.

**C4.2.2** Radio towers located adjacent to the central office building
should have a dedicated ground field as if they were located in an
isolated area (Refer to Appendix B). This ground field should be bonded
to the central office ground field outside of the building.

**C4.2.3** Air conditioning or heating systems that are mounted on
platforms, outside and adjacent to the building should also be provided
with a separate dedicated ground field. This ground field should be
bonded to the central office ground field outside of the building.
Further, air ducts from the system that enter the building should have
fiber insulating sections in them just before they enter the building.
The internal duct systems should be bonded into the building steel.

**C4.2.4** Connections to the central office ground field, outside of
the building, should be mounted in hand holes or ground wells. Refer to
Bulletin 1751F-802.

**C5. COMMON PROBLEMS**

**C5.1** **[Introduction]{.underline}:** Several common problems listed
below have been found during visual inspections of central office
grounding systems. They are included in this Appendix to alert the
person reviewing the grounding system to some specific items of
importance.

**C5.2** **[Routing of Surge Producers and Surge
Absorbers]{.underline}:** The routing of surge-carrying grounding
conductors (Surge Producers and Surge Absorbers) in cable racks and
troughs parallel to signal carrying-conductors that enter the isolated
zone is a rather frequently observed condition that can result in line
circuit damage during periods of surge activity. Since the signaling
conductors in this area are not shielded, the coupling is from the E,
the voltage field, rather than the H, the magnetic field. The location
of the signaling conductors is on the central office side of the main
frame protectors and thus they are extremely vulnerable to surge damage.

**C5.3** **[Conductor Size]{.underline}** should not change at the end
of a grounding conductor routing. This will reduce the efficiency of the
grounding conductor\'s ability to carry surge currents to ground. This
condition is most frequently found in the ac bypass conductor near the
point where the conductor enters the power system entrance panel, as
illustrated in Figure C3: Undesirable Reduction in Size of Grounding
Conductor. The ac bypass conductor should be continuous from end to end
with no change in conduit or size.

**C5.4** **[Butt Splices]{.underline}:** At some locations not only has
the conductor size been reduced but an unacceptable splice is used, as
shown in Figure C4: Undesirable Butt-Splice in Grounding Conductor.
Surge currents will not follow a sharp turn, which presents a very high
impedance to fast rising surge currents. Instead, the surge currents
will seek and find another path to ground. This is likely to result in
damage to the sensitive electronic equipment. Therefore, butt splices
should never be used in grounding connections.

**C5.5** **[Routing the ac Bypass Conductor to a Branch Power
Panel]{.underline}:** Sometimes, the ac bypass conductor is found routed
to a branch power panel rather than back to the power entrance panel.
This condition can also lead to some unexpected problems. First, the
size of the conductor between the branch and entrance panels is apt to
be smaller than the recommended size. Second, the conductor between the
two panels will be located in metallic conduit and the conductor and
conduit will not be solidly bonded at each end. This will cause a high
voltage differential to be present during periods of surge current.
Last, the conductor between the two panels will be carrying load current
from electrical service beyond the branch panel. Part of this current is
likely to flow back through the MGB. Transients which can be transmitted
into the switching equipment can result in damage to electronic
components. These transients are caused by switching in the power loads.
The ac bypass conductor should be routed in the prescribed manner for
maximum protection of the switching equipment.

**C5.6** **[The protector strips at the MDF]{.underline}** should be
bonded together with a #6 solid copper conductor but inspection may find
that the string of strips has not been bonded to the MDFB. This
arrangement produces a high impedance to surge currents and will force
the surge into the line circuit.

**C5.7** **[Secondary Power Protection]{.underline}:** Many offices are
found lacking secondary power protection. Secondary power protection is
a very important aspect of Central Office protection. There is
justification for using such protection regardless of the lightning
activity of the area since even in areas of moderate to light activity
there will be an occasional thunderstorm and power system transients
(switching, etc.) take place in all areas. The cost of the secondary
protection is small compared to the cost of repairing damage.

[TABLE C‑1]{.underline}

TYPICAL GROUNDING CONDUCTOR CURRENT RANGES

A. Office with CO ground field, MGN and metallic water system.

B. Office with CO ground field and MGN.

C. Office with CO ground field and metallic water system (delta or
uni­grounded wye connected power)

D. Office with CO ground field (delta or unigrounded wye power system)

Office Type: [A B C D\_\_\_\_\_\_\_\_]{.underline}

Conductor

[Between MGB and \...]{.underline}\
Power Entrance Panel Board 1-20A 1-20A 10-500mA 10-500mA\
Water System 1-5A 1-5A .1-1A\
CO Ground Field 100-500mA 100-500mA 100-500mA 100-500mA\
CEGB 1-20A 1-20A .2-1A 100-800mA\
Positive Battery 100-500mA 100-500mA 10-200mA 10-200mA\
GWB 0-100mA 0-100mA 0-100mA 0-100mA\
MDFB 100-300mA 100-300mA 50-100mA 50-100mA

CXR, or other

Equipment Bays 100-300mA 100-300mA 50-100mA 50-100mA

**C5.8** **[The Conductor from the Central Office Ground Field Connected
to the CEGB]{.underline}:** Sometimes, the conductor from the central
office ground field is brought into the cable vault and connected to the
CEGB. This configuration can be used where the conductor installed
between the CEGB and the MGB is large enough to make one bar an
extension of the other. This is accomplished when the conductor is at
least 750 MCM. Smaller conductors may cause problems and should not be
used.

**C5.9** **[Dividing the MGB into Two Bars]{.underline}:** Some offices
have an arrangement where the MGB is divided into two bars. The Surge
Absorbers and Surge Producers are all connected to one bar and all other
grounding conductors are connected to the other. The conductor between
these two ground bars is sometimes undersized. This arrangement is
acceptable if the conductor is large enough to make one bar an extension
of the other. Installation of a 4/0, or larger, conductor is
recommended.

**C5.10** **[Avoiding Bends]{.underline}:** The bends placed in the
conductors to improve the appearance increase the impedance to flow of
surge currents and reduce the efficiency of the overall grounding
system. The conductors should be connected to the ground bars and routed
in the most direct manner possible with, ideally, no bends in the
conductors. Where bends are necessary, they should be gentle so as to
add minimum impedance to the flow of surge currents.

Figure C1: Master Grounding Bar Currents

![](media/image16.emf)

Figure C2: CEGB Currents

![](media/image17.emf)

Figure C3: Undesirable Size Reduction in Grounding Conductor

![](media/image18.emf)

Figure C4: Undesirable Butt-Splice

![](media/image19.emf)

[^1]: For presentation purpose the MGB shown in Figure 6 is square
    shaped to neatly arrange the multitude of connections involved. In
    reality the MDF is a straight slab of copper not squared shaped.