![](media/image1.wmf){width="2.279861111111111in"
height="0.7430555555555556in"}

April 15, 1999

**Electrical Design Standards for Electronics to be used in Experiment
Apparatus at Fermilab**

These electrical design standards are to assist the system designer
during the early stages of the experiment electronics design. In
addition, they are used by Fermilab appointed reviewers during the
safety inspection of experiment electronics installations. These
standards have been gleaned from experience as well as from several
other sources (see references). The application of these standards early
in the design process can lead to simpler and safer designs. It can also
speed the approval process for the Operational Readiness Clearance for
Experiments which is required prior to the operation of an experiment.

Commercially available crate-based electronics systems (CAMAC, NIM,
FASTBUS, VMEbus, etc.) have been developed in accordance to available
standards (IEEE, etc.) and will not, in general, be reviewed. The
installations of these systems will be inspected and must meet accepted
standards. It is important to note, however, that the non-standard
(custom) use of these crate systems must follow these standards. All
custom systems will be reviewed.

Fermilab encourages collaborators to discuss design issues as early as
possible and throughout the design cycle. We are happy to hear from you
and stand ready to assist you in your design effort. Questions should be
directed to your Experiment's Liaison or the Chair of the Engineering
Standards Committee. Their phone numbers are available on the Fermilab
Web page.

**Overcurrent Protection Devices**

Either fuses or resettable positive temperature coefficient (PTC)
devices may be used for overcurrent protection of conductors and/or
printed circuit boards. Whichever device is used must be sized and
applied per the manufacturers recommendations and must have an
interrupting current rating greater than the maximum fault current
available in the circuit.

Over-current Protection for Printed Circuit Boards

A fuse is required for each voltage that is capable of delivering enough
power to damage the board in the event of a short circuit*.* The fuse
should be located electrically between the power pins and the components
on the card. For multilayer printed circuit cards, pins that distribute
power at the same voltage level should be connected to a common section
of the power plane, separate from the rest of the power plane. The fuse
should then be used to connect this isolated section with the remainder
of the plane. The fuse rating should be high enough to handle the inrush
current at power-up yet small enough to blow before an over-current
situation causes damage to the board or components on the board (beyond
the cause of the over-current condition.) Do not install fuses in
parallel.

Over-voltage Protection for Printed Circuit Boards

A transient-voltage-suppressor (TVS) is required to address the
situation where a power supply goes out of regulation or an accidental,
external connection causes a voltage that is higher than the acceptable
maximum to be presented to the power connections of the printed circuit
board. A TVS must be wired in parallel with the load being protected,
and on the load side of the fuse Each of the various voltage planes
should have its own TVS installed between the voltage plane and ground.
Should the power supply cause a voltage excursion above the TVS's clamp
voltage, the TVS will conduct at the clamp voltage. This will cause a
large current to flow through the fuse, thereby causing the fuse to
blow. After the fuse blows the card is no longer in danger of being
damaged by the over-voltage condition. If the transorb that is used in
the design is selected carefully, by taking into account the fuse value
and operating voltage, the transorb should not be damaged in an
over-voltage incident. The intent is to limit the damage that results
from an over-voltage condition to that of several blown fuses in a
crate. This is certainly preferable to components being destroyed and,
in the worst case, catching fire. However, it also means that provisions
must be made to be able to replace blown fuses in an efficient manner so
as to minimize the down-time of the experiment.

Tests conducted at Fermilab indicate that General Semiconductor
Industries, Inc., ICTE-5 transorb is a good choice for DC voltages of +5
volts and also for -5.2 volts for fuses like the Littlefuse 251000
series up to 10 amps. One must match the capability of the TVS with the
fuse.

Powering Printed Circuit Cards

For multilayer printed circuit boards it is common practice to utilize a
layer of copper foil for power distribution and another layer for ground
return. The connector manufacturer will rate a connector pin for the
maximum current that it can carry. Pins may be connected in parallel
when a greater amount of current is required than one pin can carry.
However, the current carrying capacity of each pin must be derated. This
is to account for the fact that, due to differences in the resistance of
the paths (which include the pins), an unequal amount of current will
flow through the various pins. The pin that is in the lowest resistance
path will carry a greater portion of the total current then a pin in a
path with higher resistance. The total current will be determined by the
load presented by the circuit board. If the system is implemented in a
fashion that requires all pins to carry the maximum rated current then
some pins will carry a greater amount of current than is allowable
because some pins will carry less due to their higher resistance. A rule
of thumb is to use 2/3 capacity, i.e., a single pin rated at 3 amps may
be considered to handle 2 amps when multiple pins are in parallel.
Boards powered in this fashion should not be plugged in "live" because
the first pin making contact must handle all the current.

Using multiple pins in parallel to supply power to a multilayer printed
circuit board requires special considerations for fusing. One approach
separates the voltage layer into two parts, one of which is connected to
the power pins on the connector and the other is connected to the
components. The two parts of the voltage plane are connected by a fuse.
See Figure 1. (Note that a transient-voltage-suppressor is not shown.)

![](media/image2.wmf)

**Figure 1.**

An alternative is to divide the power layer into separate sections with
each section supplied by its own pin. Since no current sharing takes
place one does not need to derate the current carrying capacity.
However, each section would need to be separately fused and protected
against over-voltage with individual transient-voltage-suppressors.

Special consideration is required for making soldered connections
between the power/ground layers and pins. The term "thermal" refers to
the portion of the foil layer of a power/ground plane where the
connection is made to a pin. Heat that is applied to a pin which is in
contact with the copper layer will be drawn away by the relatively large
thermal mass of the layer. As a means to thermally (but not
electrically) isolate the hole from the rest of the layer some of the
copper outside the perimeter of the hole is etched away in the
fabrication process leaving a few radial paths of copper to make the
electrical connection from the pin to the foil layer. If pins are
connected to the power plane using thermals then the thermal must be
able to carry the required current without excessive local heating. Note
that if heating is taking place at the connector between the pin and the
board while the board is operating then there is probably an excessive
voltage drop.

Ground Returns

To keep the inductance in a signal return path down to a reasonable
value it is required that the designer ensure that an adequate number of
pins are reserved for return signals. When many digital signals are
changing simultaneously on the data or address lines on a circuit card a
great deal of instantaneous current will flow through the ground pins.
(Another occurrence that causes a great deal of current to flow is the
refresh cycles on a board that is heavily populated with dynamic RAM
memory.) Pins and traces act as inductors in parallel. With few pins,
the inductance and current can be high enough to cause a significant
shift in the voltage potential of the ground pins. As more pins are used
for the ground returns the magnitude of the voltage shift is reduced.
Wider traces will help reduce the inductance and decrease noise. A
calculation should be done to determine the magnitude of voltage shift
that can be expected.

Wire Size for Power Distribution

Copper is the only conductor to be used. Table 1 tabulates the minimum
wire size required for a given available maximum current (ampacity).
This table has been derived from several sources and the ampacities are
conservatively specified taking into account: bundled conductors,
limited air flow and 30°C (86°F) ambient. Insulation material must be
rated for 90°C (194°F) minimum. The use of this table will result in a
conservative design for the typical environments seen in experimental
areas. It reduce design difficulties if used early.

![](media/image3.wmf)

**Table 1.**

Higher temperatures or other unusual conditions may require larger wire
size and/or different insulation material. Special conditions that may
require conductors outside of those specified in Table 1 must be
reviewed by Fermilab Division Safety personnel.

Table 2 is included since copper bus is often used for detector power
distribution. Table 2 clearly shows that for any given cross-section
there is a substantial temperature rise for higher currents.
Consequently, the one square inch per 1000 amperes is the maximum
recommended design parameter (see item "d" in the General Considerations
section below). The temperature rise must be taken into account when
designing the power distribution bus.

![](media/image4.wmf)

**Table 2.**

Ribbon Cable for Power Distribution

Ribbon cables can be a special problem when they are used to carry power
supply currents. They are usually small gauge (28 AWG) and generally do
not have 90°C insulation as required in these standards. For example, in
many experiments, ribbon cables are used to provide power to printed
circuit boards as well as carry signals, e.g., multiwire proportional
chamber systems that use banks of Nanometric N277-C amplifiers. It is
not unusual that the ribbon cables carry several amperes of current. A
short circuit in any one printed circuit board could cause excessive
current flow in the ribbon cable with the potential of cable
overheating, insulation failure, and fire. To minimize this possibility,
the following apply:

> 1\. The length of all 28 AWG ribbon cables used for power distribution
> in excess of one (1) ampere shall be less than 15 feet long.
>
> 2\. The maximum current through any single conductor in the ribbon
> cable shall be limited to 2.4 amperes.
>
> 3\. In applications where [several]{.underline} (2 to 6) conductors in
> one ribbon cable are used in parallel to carry currents in excess of 3
> amperes total, a maximum of 2.2 amperes per conductor is permitted.
> The limits of Table 1 must be applied for greater than 6 conductors.
>
> 4\. Each parallel wire is to be bonded solidly together at the power
> supply and load connections so as to prevent one of the parallel wires
> from carrying more than its share of the load current.
>
> 5\. In addition, misalignment of plugged in printed circuit cards
> could lead to excessive conductor current. Therefore, mechanisms to
> prevent such misalignment shall be employed.

Remote Sensing for Power Supplies

In detector electronics systems, the "bulk" power supply is often
located some distance from the electronics. Remote sensing is often used
to minimize the deleterious effect that long power leads have on voltage
regulation at the load. In most cases the sense wires are small in
gauge, at least as long as the power leads and are connected from the
load connection to the sense inputs on the power supply. If connected
this way, and a short circuit inadvertently develops anywhere along
their length, these wires will likely fail. Several methods are
available to solve this problem, e.g., fuses or current limiting
resistors can be installed at the load side sense connection as shown
schematically in Figure 2. Sizing the sense wire to carry the total load
current is a possible solution, but is not appropriate in most cases.
Note that for most power supplies, shorted sense inputs results in
increased output voltage and higher output current.

![](media/image5.wmf)

**Figure 2.**

General Considerations for Power Distribution

> a\. Splices should be kept to a minimum and preferably not used at
> all.
>
> b\. All connectors, lugs, terminals, etc., must be of copper. Tinned
> copper is preferable. Stainless steel or silicon-bronze nuts and bolts
> are recommended for joining copper flags or lugs. Copper or brass
> bolts should not be used for power connections where greater than 50
> amperes is possible.
>
> c\. All wires should have strain relief at the connection points. Stud
> and lug connections are a special problem and should be restrained to
> prevent accidental rotation and a loose connection. Belleville washers
> can be of value with these connections.
>
> d\. All current distribution bus shall be copper and have a cross
> section of at least one square inch per 1000 amperes (see Table 2).
>
> e\. All bolted connections shall have a contacting bus surface area of
> at least one square inch per 1000 amperes.
>
> f\. Bolted connections must have a contact pressure of 2000 pounds per
> square inch.
>
> g\. Soldered connections should not be used for connections carrying
> greater than 50 amperes. Silver brazing is acceptable.
>
> h\. The operating temperature of copper bus can easily reach 100°C
> (212°F). The bus operating temperature must be compatible with the
> insulation rating of the connected load cables. Note that any fuses
> used should be mounted away from a bus that operates at elevated
> temperature. Elevated fuse temperatures will cause premature fuse
> failure.

Connections From the Power Supply to the Backplane

Any connector that is used in power distribution must be rated to carry
the maximum current that may be presented either from the power supply
itself or, if a fuse is located between the connector and the power
supply, from the fuse. Additionally, the design of the connector should
be such that it would require a significant amount of effort to
accidentally short two of its conductors together. Note that when power
connections are secured with a bolt or screw, the connection must be
designed so that surfaces other than the bolt or screw can carry the
rated current.

Copper bus bar used to distribute power along the backplane should be
fabricated with a cross section that is large enough to ensure that the
current density is kept to under 1000 amps per square inch. Copper bus
bar should be clean and flat to minimize the resistance of the
connection.

When connecting a power supply to multiple crates all cables must be
protected from the effects of a short. One way is to size the wire to
safely carry the maximum current the power supply can output. When using
wire that cannot safely carry the power supply's maximum current, a fuse
is required to prevent the wire from overheating. An example is shown in
Figure 3.

![](media/image6.wmf)

Figure 3.

A drawback to the approach shown in Figure 3 is that a short circuit in
either of the crates causes all three crates to loose power when the
fuse blows. An alternative approach, shown in Figure 4, addresses this
issue by utilizing three fuses. The important point is that each wire in
the power distribution system is protected from the possibility of
overheating due to current flow which exceeds its capacity. Drawings of
this type, which will include the power supply capacity, wire sizes, and
fuse ratings, will be required to be provided for safety reviews.

![](media/image7.wmf)

**Figure 4.**

Power Dissipation of Printed Circuit Boards

As a "rule of thumb," the power density of a circuit card should be
limited to a maximum of 1/2 watt per square inch. Areas of the board may
be allowed to have a greater power density but arrangements should be
made to ensure that the heat is dissipated effectively and without
inducing hot spots on other parts of the board or on adjacent boards.

Cooling of Electronics Chassis

The use of natural convection to cool electronics is desirable. For
greater heat loads, forced air cooling utilizing fans is the next
choice. When the power density of the electronics is greater than what
can reasonably cooled with fans, then water-cooled heat sinks or
air/water heat exchangers are utilized. However, water cooling increases
the complexity of an installation and adds another source of problems
during the operation of the experiment. The design should be such that
the cooling fluid, be it air, water, or some other, should not vary more
than 10°C over the area of the board. This is to keep the loss in noise
immunity in the electronics, which is caused by difference in
temperature, to a reasonable amount.

The temperature of the cooling water can become a problem. If the
temperature of the water is below the dew point of the ambient air then
water will condense and drip onto the electronics. The dew point is
likely to be highest during the hot summer months, precisely when one
needs the most cooling performance from the system. A goal is to have
the electronics operate with the coolant between 30° and 40°C. This is
adequate to prevent condensation in almost all cases and minimizes the
loss of noise margin due to large temperature differences between
components with signal connections on the circuit boards being cooled.

Whichever system is chosen, good air flow is necessary in order to
ensure that the cooling air passes over the electronic components that
need to be cooled. When empty slots on the front of the crate are not
covered by blank panels, the cooling air encounters a low impedance at
the opening and a large percentage is forced out the front rather than
up past the components on the upper portion of the cards. Install blank
front panels on the front of empty card slots to alleviate this loss of
cooling effectiveness.

For systems that require yet higher cooling performance one must
consider that, even with blank front panels in place, the empty slots
will present a lower impedance to the air flow than slots which have
cards present. A greater volume of air will flow in the empty slots
causing less air to flow in the occupied slots. An effective, though
somewhat costly, solution is to populate the empty slots with dummy
cards that present the same impedance to the flow of cooling air as the
printed circuit cards do. This approach will not be required in most
installations.

Rack Protection

When there is a large investment in the cost of the equipment that will
be in a rack and when the equipment is custom made (as opposed to
off-the-shelf commercial electronics), a device to detect faults in the
system and shut the system down when a fault occurs should be
considered. A rough rule of thumb is that when custom electronics can be
protected for 10% or less of its cost, then a rack protection system
should be considered. Parameters that can be monitored include:
temperature, smoke, voltage, cooling air flow, cooling water flow,
condensation/water leakage, and current. Sophisticated rack protection
systems have been fabricated for approximately \$2500 per rack. Systems
that have a reduced set of parameters to monitor may cost less. Note
that rack protection systems which monitor the voltage on high-current
conductors must have the monitoring points protected from the hazard of
an over-current situation caused by a short circuit on the monitoring
wire.

I/O Connector Location

An issue that often entails tradeoffs is the method of connecting I/O
cables to the cards in a chassis. If the location of the connectors is
the front panel of the cards then it is easy to get to the connector on
a card in a sparsely populated chassis. However, if there are 26 cards
in a chassis, with each having one or more cables running to it, the
front of the chassis can become a jumble of cables. This creates a
routing problem for positioning the cables themselves and additional
problems with having enough room to remove the cables to a particular
card, draw the card out of the chassis, replace the card, and then
reconnect the cables.

If the location of the I/O connections is the back of the circuit board
then provisions have to be made to make the connections. Some of the
standard bus backplanes used at Fermilab do have provisions for this.
Examples include VMEbus and FASTBUS which have, as part of their
specifications, user-defined pins on the backplane which are not bussed
but go straight through the backplane to a connector mounted on the back
side of the backplane to which an I/O connector can be mated. This moves
the challenge of cable routing to the back of the chassis and leaves the
front open to easily remove and replace circuit boards since the I/O
connector is actually mated to the back of the backplane and not to the
circuit board being replaced. A combination of I/O connectors on the
front and back can be employed when no other option is as attractive.

For systems that have large number cables which are densely packed
together, the routing and physical support of the cables becomes a
design challenge wherein the best solution is not always obvious. The
location of densely packed cables impacts the flow of cooling air.
Consideration for cable support and routing must be given very early in
the system design cycle because of the resulting implications for
printed circuit board layout and chassis configurations.

It is also possible to fabricate a custom backplane that utilizes the
user-defined pins and electrically connects them from one card slot to
another for the use of the boards in a specific system. Though the pins
defined by the backplane specification remain unchanged, one must keep
in mind that some commercial cards expect that the user-defined pins are
available for their use and this could cause a conflict with the
utilization of cards that were not designed specifically for the custom
backplane.

Auxiliary Cards

In this context, an auxiliary card is a printed circuit board that
mounts on the backside of a backplane, at 90 degrees to the backplane,
and is electrically connected to a card installed on the front side of
the backplane.

Cooling can become a problem if the heat generated by the electronics on
the auxiliary card is greater than what can be cooled by convection with
air at the temperature in the rack. Fans and heat exchangers are
commonly available for the cards mounted on the front side of the
backplane but it is difficult to find cooling devices for the back side
which, in most cases, is left open.

Providing power to the auxiliary cards can also be a challenge. Some of
the backplane specifications do not have power available on the
connector to which an auxiliary card will be attached. When the power
pins are available, there may not be enough current capacity available
from the pins to supply both the auxiliary card and its corresponding
card on the front of the crate. Designing and fabricating a custom
backplane and using the user-defined pins to bus power to the auxiliary
cards in one solution, albeit an expensive one. However, one drawback to
this approach is that any time one would like to power up the card for
testing or other purposes, the card must be in a chassis that is
equipped with the custom version of the backplane as opposed to the more
commonly available standard version. This can become a very significant
handicap when there are more people troubleshooting parts of the system
then there are fully-configured test stands available.

Yet another option for powering an auxiliary card is a power connector
mounted on the card and a dedicated cable bringing power from the power
supply. This solution requires that the power cable be protected against
over-current situations with a fuse and also requires careful routing of
the power cable.

Keying Unique Cards and Slots

When a variety of cards of the same form factor but differing
functionality are used in the same backplane chassis, consider whether
damage can result if one of the cards is powered up in a slot that is
configured for a different card. If damage could result, then a keying
system should be implemented for slots and cards to avoid inadvertently
inserting a card in the wrong slot.

Design for Test

The procedure for troubleshooting electronic systems is greatly enhanced
by incorporating several features on printed circuit cards. Front panel
LEDs that indicate the status of various module functions help to
identify which card in a crate, or which subcircuit on a card, is not
working properly. A circuit that is sometimes referred to as a
"heartbeat" for modules that include a microprocessor utilizes a
one-shot mono-stable multivibrator and an LED. The one-shot may be
triggered by a software/firmware routine that executes periodically. The
implication is that the processor is not running if the heartbeat LED is
not illuminated.

Status registers that can be read out via computer can be utilized to
troubleshoot individual boards as well as system problems.

Test points comprise nodes in the circuit that are wired to front panel
connectors to which probes from test equipment can be attached in order
to quickly determine the status a portion of the circuitry.

Documentation

It is essential to have well-written documentation for the electronics
system in order to ensure that the system can be reviewed, that it can
be maintained, and that others can be trained to operate it in the
shortest amount of time. Drawings that show the power distribution from
power supplies to the crates will be required for safety reviews.
Additional documentation should include accurate schematic diagrams,
as-built drawings, and diagrams of interconnecting wiring. An operating
manual should be written that describes how the system is to function.
Well-documented source code files for programmable logic device (PLD)
integrated circuits should be available.

Documentation that clearly describes the procedures for verifying the
functionality of the circuit boards, and the system as a whole, should
be written.

References

Fermilab ES&H (Environment, Safety & Health) Manual section 5041, *Low
Power Electrical and Electronics Safety*

Fermilab ES&H Manual section 5045, *High Voltage Coaxial Connectors*

Fermilab ES&H Manual section 5046, *Low Voltage, High Current Power
Distribution Systems*

*MECL System Design Handbook, Fourth Edition*. Compiled by the Computer
Applications Engineering Department of Motorola Semiconductor Products
Inc. Author: William R. Blood, Jr.

This is an excellent reference. Though it is oriented to Motorola's
emitter coupled logic, the content has taken on even greater relevance
as other logic families have approached ECL in speed and power
dissipation.

Investigation of the Fire in Experiment E-687 at Fermi National
Accelerator Laboratory, October 3, 1987. Report from Fermilab Internal
Investigation Committee.

Alpha Wire Corporation Full-Line Catalog W-10, © 1987

National Electrical Code - 1993

Fermilab internal memo, TM #1675, 2100.000, "Ampacity Tests of 28 AWG
Ribbon Cables," Erika Drennan, Research Division, E/E Department, April
1990

Overcurrent Protection in Electronics Systems, SLAC Committee Report SM
205, June 9, 1989