- Enclosed Electrical / Electronic Equipment -
9.0 ENCLOSED ELECTRICAL / ELECTRONIC EQUIPMENT|
This section provides guidelines to
1. complement existing electrical codes and recommend industry standards,
2. improve electrical safety in the work environment for personnel within the DOE complex.
3. eliminate the ambiguity and misunderstanding in design, construction and implementation
requirements for electrical/electronic equipment, and
4. assist the AHJ in providing information for acceptance of equipment within the scope of this
This section addresses enclosed electrical/electronic equipment electrical safety guidelines which are not specifically addressed elsewhere in the Electrical Safety Handbook. These types of equipment include: instrumentation and test consoles; enclosed electrical/electronic equipment; other laboratory diagnostic electrical/electronic equipment (stationary or mobile) mounted in or on an enclosure, rack or chassis; and special electrical/electronic equipment facility requirements.
9.3 GROUNDING AND BONDING
Many ground system types exist within electrical equipment. All metal parts of electrical
equipment enclosures and chassis shall be bonded and grounded as per the NEC. The
methods chosen to avoid ground loops and reduce noise shall meet the requirements of the
9.3.1 OBJECTIONAL CURRENT OVER GROUNDING CONDUCTORS
Enclosed Electrical/Electronic equipment has both power and signal conductors entering and
leaving these enclosures. Objectionable currents and noise may be the result of the design or
installation of conductors and equipment and their grounding locations. NEC 250.6 addresses
these objectionable currents and noise (See Section 10.9.2.1).
NEC 250.6 must be used with care because it seems to give blanket authority to do whatever is
necessary to stop objectionable currents from flowing in the grounding system. This is not the
intent. NEC 250.6D specifically indicates that the introduction of noise or data errors in
electronic equipment shall not be considered objectionable currents, as addressed therein.
Therefore, such objectionable currents must be handled in other ways. NEC Section 250.6
principally deals with objectionable currents that can flow over grounding conductors due to
severely unbalanced loads or improper installation practices. NEC 250.96(B) provides
requirements for isolation of grounding circuits to reduce electrical noise (EMI). Because of the
complexity and number of interconnections of most grounding systems, the NEC allows
modifications of the grounding system and connections in order to address such problems.
1) Arrangement to prevent objectionable current. Grounding of electrical systems, circuit
conductors, surge arresters, and conductive noncurrent-carrying materials and equipment
shall be installed and arranged in a manner that will prevent an objectionable current over
the grounding conductors or grounding paths. Use of a single-point grounding system, as
well as meeting the other requirements of NEC Article 250, will usually overcome problems.
2) Alterations to stop objectionable current. If the use of multiple grounding connections results
in an objectionable current, one or more of the following alterations are permitted to be
made, provided that the requirements of NEC 250.4(A)(5)(B)(4), are met. Such permitted
1. Discontinue one or more, but not all, of the grounding connections;
2. Change the locations of the grounding connections;
3. Interrupt the continuity of the conductor or conductive path interconnecting the grounding
4. Take other suitable remedial action satisfactory to the authority having jurisdiction.
3) Temporary currents not classified as objectionable currents. Temporary currents resulting
from accidental conditions, such as ground-fault currents, that occur only while the
grounding conductors are performing their intended protective functions shall not be
classified as objectionable. This does not prohibit changes in the system to correct
excessive current during a fault condition.
4) Limitations to permissible alterations. The intent of NEC 250.6 is not to permit electronic
equipment to be operated on AC systems or branch circuits that are not grounded as
required by NEC Article 250. Currents that introduce noise or data errors in electronic
equipment are not considered to be the objectionable currents addressed in this Section.
Voltage differences and thus objectionable currents may exist because impedances to
ground are not equal throughout a grounding system due to variations of the resistance of
the earth, improper connections, or other problems.
Even though voltage differences allow unwanted currents to flow in the grounding
conductors, and induced noise may travel over this path, it is not to be used as a reason to
disconnect all grounding connections to any system component. At least one grounding
connection must remain.
9.3.2 EQUIPMENT GROUNDING CONDUCTOR
The equipment grounding conductor of a power-supply cord or interconnecting cable should be
size in accordance with NEC 250.122 and the associated NEC Table 250.122. The minimum
size equipment grounding conductor is based on the total rating of the enclosed equipments in
amperes. Note that the minimum size equipment grounding conductor may be smaller than the
size for the current-carrying conductors; i.e., the grounded (neutral) and ungrounded
conductors, which are sized per NEC Article 310.15 – usually following NEC Table 310.16.
9.3.3 ENCLOSURE GROUNDING AND BONDING
Enclosure grounding and bonding should comply with the following requirements: (See Figs. 9-1 thru 9-3)
Note: This drawing represents typical 120/208 Volt, Three Phase Wye, 5 wire, ac power.
1) Have a common grounding or bonding bus (normally a cabinet rail).
2) When the enclosure contains more than one bay, bond all grounding or bonding busses together.
3) All mounted chassis within rack cabinets shall have a grounding or bonding conductor attached to the common grounding or bonding bus when the chassis is not grounded or bonded through the power cord.
4) The grounding or bonding conductor shall be permanent and continuous.
5) Subassemblies mounted in other types of enclosures should be bonded by adequate preparation of the mounting surfaces or by the use of a bonding conductor.
6) To provide protection against grounding or bonding conductor breakage, conductors between the common grounding or bonding bus and moveable chassis should be braided cable or stranded wire.
NOTES: 1. This drawing represents typical 208/120-V, Three Phase Wye, 5 wire,
ac power for Rack #1.
2. Multiple bays must be bonded together even if multiple Power
Distribution Units are installed in separate bays.
All grounding or bonding points should be tight for good continuity, identified by green color,
permanently labeled, and properly prepared by cleaning metal surfaces to bare metal or by the
use of serrated bushings. Anodized aluminum must be cleaned to bare metal.
The resistance across the bonding point should be very low, so that heating stress effects due
to power loss across the bonding point are minimized. If a measurement is required, the
method of measurement is to be determined by the user. The user may determine a maximum
resistance, e.g., 0.1 ohm.
9.3.4 SPECIAL CONSIDERATIONS
Systems feeding power isolation transformers must continue the equipment grounding
conductor to the equipment or the ungrounded equipment must be guarded and labeled.
For two-wire cord connected equipment, an equipment grounding connector should be installed
according to the manufacturer's instructions.
9.4 RACK POWER DISTRIBUTION
The following guidelines will provide the necessary information to correctly install power
distribution equipment within instrumentation racks containing electrical and electronic
9.4.1 GENERAL REQUIREMENTS APPLYING TO ALL AC POWER EQUIPMENT
WITHIN OR ATTACHED TO INSTRUMENT RACKS
Knowledge of the loads that will be connected within a rack cabinet is necessary before starting
design of a rack power distribution system. All components must be sized correctly for the loads
and should provide for expansion.
Equipment enclosures may or may not contain a power distribution unit. A rack power
distribution unit contains a main overcurrent protection device and multiple branch circuits that
are individually protected against overcurrent. Without a power distribution unit, the power wiring
is considered part of one branch circuit.
Branch circuit loading shall meet the requirements of NEC Article 210. (See NEC 210.21
External convenience outlets should be connected to a separate circuit breaker.
Where three-phase, four-wire service is utilized, the loads should be evenly distributed on all
phases and there should be consideration of sizing the neutral conductor for certain loads (such
as computer equipment) due to the presence of harmonic currents. (See NEC 210.4 and
188.8.131.52 OTHER GENERAL EQUIPMENT REQUIREMENTS
Rack power distribution components or assemblies must be listed by an NRTL, or have AHJ
approval (See Section 2.5).
9.4.2 CONDUCTORS AND CABLES SPECIFIC REQUIREMENTS.
Each type of internal wiring for equipment or an accessory shall be acceptable for the particular
application when considered with respect to (1) the current, ambient temperature, voltage, and
other conditions of service to which the wiring can be subjected, and (2) exposure to oil or
The term "cables" refers to groupings of wires typically used for control signals, data, or DC
power. The term "cords" refers to AC power cords.
The basic insulation on each wire shall be rated for at least the maximum voltage to which the
wire is connected, and for at least the temperature it attains. Additionally, the insulation should
be rated for the maximum voltage of nearby conductors and wire to which each wire may be
exposed. Insulating tubing, sleeving, and tape shall be rated for at least the maximum voltage
against which it insulates, and for at least the temperature it attains. Power and signal wires
should be routed separately within a chassis.
Wires shall be routed away from sharp edges, screw threads, burrs, moving parts, etc. Holes
through which wires are routed shall have smooth, well-rounded surfaces, or shall have a
bushing. Clamps for guides used for routing or wiring shall have smooth, well-rounded edges.
Pressures exerted by such clamps should not cause cold-flow or otherwise deform the basic
184.108.40.206 FLEXIBLE CABLES
Flexible cables may be used:
1. Where flexible cables and attachment plugs are furnished by the manufacturer as part of the
equipment to be mounted in the rack.
2. For connection of stationary equipment to facilitate their frequent interchange.
3. To prevent the transmission of mechanical vibration.
4. Where the fastening means and mechanical connections are specifically designed to permit
ready removal for maintenance and repair.
5. For data processing cables approved as part of the data processing system.
6. For temporary wiring.
Where breaking or loosening of a circuit connection would render an electric shock or could
result in a fire, such connection shall be made mechanically secure. Mechanical security of
connections may be provided by crimped, closed ring or flanged lug, or a wrapping that forms at
least an open U or by cable clamps, or by cable lacing, insulating tubing, or similar means.
220.127.116.11 STRAIN RELIEF
Wiring, cords, or cables shall be provided with strain relief as required to prevent damage.
Additional insulation may be required when the construction of the strain relief may damage the
insulation. The use of type NM (Romex) cable clamps on flexible cords and cables is not
permitted. Use listed or labeled clamps. The use of any metal clamp or other means that may
cause undue stress on the cables within or external to instrument racks is not allowed. Cord and
cable support for AC power cable or other heavy duty or large diameter cables must distribute
the load over a large area of the outer covering of the cable.
18.104.22.168 SEPARATION OF VOLTAGES
Insulated conductors of different circuits shall be separated or segregated from uninsulated live
parts connected to different circuits unless provided with insulation suitable for the highest
Segregation of insulated conductors may be accomplished by clamping, routing, or equivalent
means that provide permanent separation from insulated or uninsulated live parts of a different
Loose strands of stranded internal wiring, connected to a wire-binding screw, shall be prevented
from contacting other uninsulated live parts not always of the same potential and from
contacting noncurrent-carrying metal parts. This may be accomplished by use of pressure
terminal connectors, soldering lugs, crimped eyelets, or soldering all strands of the wire
22.214.171.124 OTHER CONCERNS
Conductors shall not be bundled together in such a way that the temperature rating of the
conductors is exceeded. Bundled conductors may require derating of their ampacities. For
example, see NEC 310.15(B)(2) and Table 310.15(B)(2)(a)
Flexible cord should be listed or labeled and used only in continuous lengths without splice or
tap when initially installed.
Repairs are permitted if the completed splice retains the insulation, outer sheath properties, and
usage characteristics of the cord being spliced. In most instances, the entire length of flexible
cord should be replaced, in order to assure integrity of the insulation and usage characteristics.
9.4.3 POWER SWITCHES AND INTERLOCK DEVICES SPECIFIC REQUIREMENTS
For all electrical/electronic enclosures utilizing power switches or interlocks, the following should
1. Interlocks should be utilized where exposed voltages (50 volts or greater) are present in
equipment and access to the exposed live parts is not controlled (See Section 9.6.4).
2. Ensure all line-side unprotected contacts are guarded on interlocking contactors or other
3. Be suitable for the conditions, use, and location.
4. Circuit breakers used for the equipment power switch will be rated for switching under load.
5. Provide provisions for lockout/tagout requirements.
9.5 CHASSIS POWER DISTRIBUTION
Manufacturers are responsible for determining the safety of such chassis and/or enclosures and
for providing documentation showing how that determination was made. Listed equipment
should be selected by design when available. Unlisted commercial equipment and in-house
fabricated equipment shall be approved by the local AHJ.
9.5.1 AC POWER DISTRIBUTION
126.96.36.199 CHASSIS BONDING AND GROUNDING
Metal chassis shall be effectively bonded to a main grounding point in the rack cabinet where
necessary to assure electrical continuity and shall have the capacity to conduct safely any fault
current likely to be imposed on it. (NEC 250.96)
In a chassis with ac service connected to it, the grounding terminal of its receptacle shall be
internally bonded to the chassis frame. (NEC 250.146)
If solder is used, the connection of the equipment grounding conductor shall not depend on
solder alone. (NEC 250.8)
The leakage current of cord connected equipment should be very low.
188.8.131.52 CONNECTIONS, CONNECTORS, AND COUPLINGS
Input/output ac power connections to the chassis shall comply with NEC requirements.
The exposed, noncurrent-carrying, metal parts of panel mount connectors operating at 50 volts
or greater shall be bonded to the chassis.
Plugs and sockets for connecting any AC power source shall be NRTL-listed for the application.
(Ref. ISA-S82.01-1992, Section 6.10.3.a)
AC power plugs and sockets shall not be used for purposes other than the connection of AC
Connectors operating at 50 V or greater shall be listed, rated or recommended for their intended
Any connector used to provide power at 50 V or greater shall not allow personnel to make
inadvertent contact with the power source.
If plug pins of cord-connected equipment receive a charge from an internal capacitor, the pins
shall not be capable of rendering an electric shock or electric burn in the normal or the single
fault condition 5 seconds after disconnection of the supply. Plug-in type connectors intended to
be connected and disconnected by hand shall be designed so that the grounding conductor
connection makes first and breaks last with respect to the other connections. [NEC 250.124(A)].
The following applies for all AC power connectors within or external to electrical/electronic
1. There should be no exposed current-carrying parts except the prongs, blades, or pins.
2. The connector shall prohibit mating of different voltage or current rating than that for the
3. All connectors must be protected against overcurrent in accordance with their rated
ampacity. (NEC 240.5)
4. Connectors must be NRTL-listed for the application.
5. Use of MS, PT, or other non-approved connectors is not permitted except when justified to
and approved by the AHJ.
If conditions require the use of a non-NRTL listed or labeled connector, such as an "MS"
(military standard pin and socket type) or "PT" (similar to "MS" but smaller) type, for input/output
ac power, a warning label should be affixed next to the connector stating: "WARNING - POWER
MUST BE REMOVED BEFORE CONNECTING/DISCONNECTING."
184.108.40.206 TERMINALS/LIVE PARTS
All terminals/live parts with a potential of 50 volts or greater shall be guarded to protect from
accidental contact or bringing conductive objects in contact with them (NEC 110.27). Consult
ANSI/ISA-S82.01-1988, Table 9-1 for spacing information regarding live parts.
All energized switching and control parts shall be enclosed in effectively grounded metal
enclosures and shall be secured so that only authorized and qualified persons can have access.
9.5.2 DC POWER DISTRIBUTION
Guidelines for dc power distribution include:
1. The metal chassis or cabinet should not be used as a return path.
2. High-current analog or switching do power supplies should use separate return paths from
3. All of the guidelines pertaining to ac power such as grounding, proper wire size, high
voltage, etc. should apply to do circuits as well.
An accessible terminal charged by an internal capacitor should be below 50 volts within 5
seconds after interruption of the supply.
As with ac power, avoid contacting dc parts when working on a live chassis. The use of the
appropriate class gloves should be considered when performing this type of work.
9.6 PROTECTIVE DEVICES FOR ENCLOSED ELECTRICAL/ELECTRONIC EQUIPMENT
This section deals with the various protective devices commonly found in electrical/electronic
equipment not discussed elsewhere.
9.6.1 SURGE ARRESTERS
The more common types of surge arresters used with electronic equipment are the metal oxide
varistor (MOV), avalanche diodes, and spark gap arresters. The type and electrical rating of the
surge arrester is generally determined by the requirements of the circuit being protected, and by
the amplitude and duration of the expected surge. (See ANSI/IEEE C62.11-1987.)
Metal oxide varistors and avalanche diodes are voltage-dependent devices whose impedance
changes from a near-open circuit to a highly conductive level when subjected to transient
voltages above their rated voltages. An MOV is considered "sacrificial" in that a portion of its
material is literally burned off each time such a surge is encountered. The response time of an
MOV is limited to approximately 500 picoseconds while avalanche diodes can respond in
approximately 50 picoseconds. Lead lengths can greatly increase the response times of these
devices. The normal failure mode of both devices is a short circuit although sustained voltages
well beyond the rating of the MOV can cause the device to rupture and result in an open circuit.
When used at a point on a circuit, a surge arrester should be connected between each
ungrounded conductor and ground.
For power line applications, MOV manufacturers recommend a varistor be used with a fuse that
limits the current below the level that MOV package damage could occur. In general, circuit
breakers are not recommended for this application since circuit breaker tripping is too slow to
prevent excessive fault energy.
Consult the manufacturer's application data sheets for more information.
Fuses are temperature-sensitive, current-sensing elements that are generally used as short
circuit protective devices in individual electrical chassis. The fusing characteristic, or opening
time versus current, must be within the safe time/temperature characteristic of the device being
Designers must carefully consider the load requirements in the fuse selection process,
particularly when high surge currents may be encountered during initial turn-on. Operating
time/current characteristics of the various types available can usually be found in fuse
manufacturers catalogs. A fuse's interrupting current capacity must also be considered when
connected to a power distribution system having a significant fault current capacity.
The voltage rating on a fuse shall be equal to or greater than the device's operating voltage.
In general, cartridge fuses should have a disconnecting means on the supply side, (NEC
240.40), and shall not be connected in parallel unless factory assembled and listed as a unit
9.6.3 CIRCUIT BREAKERS
A chassis or cabinet shall not employ circuit breakers as “on/off” switches unless rated for the
application by the manufacturer.
9.6.4 POWER INTERLOCK DEVICES
Cabinets and equipment having potentially dangerous currents and/or voltages present should
have a means of controlling access, or a power interlock device designed to interrupt the power
to the cabinet. Provisions should also be made to discharge any stored energy, such as in
capacitors or inductors, to less than 50 volts within 5 seconds when the safety interlock is
opened. Interlocks may not be used as a substitute for lockout/tagout. [29 CFR 1910.333(c)].
9.7 DISCONNECTING MEANS
All enclosed electrical/electronic equipment shall be provided with a means for disconnecting it
from each external or internal operating energy source. This disconnecting means shall
disconnect all current carrying conductors.
Interlock systems are not a recommended disconnecting means for cabinets and equipment
having potentially dangerous currents and/or voltages present. (See Section 9.6.4)
Permanently connected equipment and multi-phase equipment should employ a listed switch or
circuit breaker as means for disconnection.
All cord-connected equipment should have one of the following as a disconnecting device:
1. A switch or circuit breaker,
2. Plug that can be disconnected without the use of a tool, or
3. A separable plug, without a locking device, to mate with a socket-outlet in the building
Where equipment is connected to the source of supply by flexible cords having either an
attachment or appliance plug, the attachment or appliance plug receptacle may serve as the
disconnect (NEC 422.33).
Where a switch is not part of a motor, motor circuit or controller, the disconnecting means
should be within 50 feet and in sight of the operator and marked as the disconnection device for
Where a disconnecting means is not part of the equipment, the disconnecting means should be
near the equipment, within easy reach of the operator during normal operation of the equipment,
and marked as the disconnection device for the equipment.
If a disconnecting device is part of the equipment, locate it as close as practical to the input
9.7.2 EMERGENCY SHUTDOWN
The emergency shutdown switch should be within arm's reach of the operator, be easily
identifiable, deenergize all power to all equipment associated with the system, be separate from
the routine on/ off switch, and be located to protect the employee from moving parts. However,
the emergency shutdown switch should not disconnect auxiliary circuits necessary for safety
(such as cooling).
9.7.3 SPECIAL CONSIDERATIONS
The disconnecting means should interrupt the source voltage for secondary or remote controlled
equipment, such as those using thyristor controls. Do not rely on disconnecting the control
9.8 MARKING AND LABELING REQUIREMENTS
9.8.1 GENERAL MARKING REQUIREMENTS
For all chassis and rack cabinets (electrical, computer, power distribution, etc.), the
manufacturer's name, trademark, or other descriptive marking of the organization responsible
for the product should be identified.
Other markings for power requirements are:
2. Maximum rated current in amperes
6. Duty cycle
7. Other ratings as specified in the NEC (NEC 110.21)
9.8.2 HAZARD MARKING REQUIREMENTS
All enclosures containing exposed energized circuits over 600 volts nominal should be marked
"Danger High Voltage Keep Out" with a label that is permanent. These areas shall be
accessible to authorized personnel only. The label shall be placed in a noticeable location on
the access panel to the enclosure. Mark all other hazards that are associated with the
9.8.3 OTHER REQUIREMENTS
All equipment markings shall be of sufficient durability to withstand the environment involved
and should be large enough to read.
To obtain the correct chassis load requirements for marking and labeling, monitor individual
chassis while under load. Many chassis have components that are not energized except under
A normal current draw may be a few amperes, but when the chassis is sourcing current to a
load, the current draw may be much higher. Individual loads, internal and external, may be
tabulated and added to determine the chassis current labeling requirements.
For rack cabinets with power distribution units, the outside of the rack cabinet should be labeled
with the input parameters of the power distribution system installed within it.
For rack cabinets without power distribution units the outside of the rack cabinet should be
labeled with the total current on the combined systems installed within it.
9.9 WORKING CLEARANCES
Clear working space and headroom shall meet the NEC requirements (see Figs. 9-4 and 9-5).
The clear working space and passageways to this space should not be used for storage. At
least one entrance of sufficient area shall be provided to give access to working space above
electrical equipment. For example, 24 inches may be sufficient in depth and 30 inches in width
with 6 ½ foot height
Fig. 9-4. Top View of Equipment Layout in a Room (Drawing is not to scale)
While maintenance, repair or calibration are being performed, personnel should identify clear
working spaces via suitable means such as "Danger" or "Caution" barrier tape, or barricades to
keep other personnel from entering the clear working spaces.
Fig. 9-5. Side View of Equipment Layout in a Room (Drawing is not to scale)
9.10 CABLE/UTILITY MANAGEMENT SYSTEM
9.10.1 USAGE WITH ENCLOSED ELECTRICAL/ELECTRONIC EQUIPMENT
In certain locations cable supports and/or enclosures are installed for dedicated usage with
enclosed electrical/electronic equipment. For these situations it is acceptable for these
cable/utility management systems to be utilized for the required purposes of the equipment. This
may include a bundle of cables, hoses, and tubing that is required to be run from the equipment
console to the unit under test. In these situations the use of a cable/utility management system
is considered to be a part of custom-made equipment consisting of enclosed electrical/electronic
equipment, cabling, cable/utility management system, and unit under test with associated
equipment (See Figure 9-6).
In cable/utility management systems where cables other than those of the equipment exist, the
decision should be documented that any risk posed by the situation is acceptable for the
operation to be performed and to the functions of the existing cables.
An assessment of any hazards identified with the equipment and the operation with which it is
involved should be performed to assure safe operation of components in the cable/utility
management system. Where any cable/utility runs include hazardous fluids or pressurized
gases, the utilization of these utilities with the cables involved must be determined to be safe.
Metallic cable/utility management systems that support electrical conductors shall be grounded
or bonded to the equipment. Grounding integrity should be checked by inspection by a qualified
worker for all components with exposed metal parts. This inspection should be documented.
Where cable/utility management systems are installed exclusively for electrical/electronic
equipment usage and where these trays are metallic and not grounded or bonded, approved
documentation shall exist stating the reason for not grounding or bonding the system (See
Equipment cable/utility runs installed in cable/utility management systems should be visually
inspected periodically. These inspections should be performed at the time of installation and any
interval specified in the equipment documentation. Any inspection should, as a minimum,
1. A visual check for the integrity of cable jackets and visible shields;
2. A check for the integrity of all utility hoses by looking and listening for leaks;
3. A visual check on all securing devices used to hold the bundle on the tray to assure the
bundle is positioned properly and no damage has occurred;
4. A visual inspection on all bends for signs of pinching, cutting, exceeding minimum cable
bending radius, or other damage; and
5. Documentation of all results of any inspection.
Supports shall be provided to prevent stress and physical damage to cables where they enter or
exit cable/utility management systems.
9.11 ELECTRICAL SAFETY REQUIREMENTS FOR TESTER FACILITIES
The following is not intended to encompass all of the electrical design requirements which must
be considered in planning electrical systems for facilities intended to accommodate testers. The
information provided should, however, provide a guide to understanding for personnel who
would be tasked with specifying facility electrical safety necessary to the testing environment.
Provisions for an adequate number of receptacle outlets and associated branch circuits to
accommodate cord and plug connected equipment, testers, etc., in a facility must also be
considered in specifying the electrical requirements.
For equipment that cannot tolerate power interruption, consideration should be given to the use
of a continuously operating or standby uninterruptible power supply (UPS) or a generator.
9.11.1 AMPACITY OF FACILITY WIRING AND DISTRIBUTION EQUIPMENT
Consideration must be given to accommodating the anticipated load demand which may occur
as a result of power supplied to the various possible combinations of electrical equipment
connected to a particular branch circuit (See Section 9.4).
9.11.2 FACILITY GROUNDING AT TEMPORARY OR REMOTE SITES
Proper grounding is considered crucial to providing the safest possible electrical installation,
from the standpoint of maximizing the safety of facility occupants and minimizing property
damage and loss.
Designs for equipment to be used at temporary or remote sites must take into consideration the
same grounding issues which may not be accommodated in the same manner as for permanent
facility power wiring (See Section 9.3 and NEC Article 527).
9.11.3 FACILITY LIGHTNING PROTECTION
Lightning protection is required for facilities which will house enclosed electrical/electronic
equipment while such equipment is involved with radioactive, explosive, and similarly hazardous
materials or for facilities that are considered valuable or house valuable contents.
9.11.4 SURGE PROTECTION
In addition to facility lightning protection, the effects of surges resulting from lightning strikes to
power distribution systems may be lessened by the use of lightning arrestors and suppressors
installed at strategic points in the supply system to the facility. An assessment is necessary,
addressing the consequences of lightning-induced surges, in order to determine the degree to
which protection should be provided.
For additional information see Section 9.6.1.
9.12 ENCLOSED POWER ELECTRONICS
Power electronics equipment is equipment that uses electronic components and subsystems to
control significant amounts of electrical energy. Examples of power electronics systems include:
1. Power supplies and modulators for laser systems;
2. Accelerators, magnets, x-ray systems, and other research equipment;
3. Radio and radar transmitters;
4. Variable speed motor drives; and
5. Induction heating systems.
All applicable portions of this section should be addressed due to the hazards involved with this
type of equipment.
Power electronics equipment should be constructed in all-metal enclosures for containment of
fire, high energy, and electromagnetic radiation hazards.
The enclosures should support the housed equipment, provide strength to brace conductors
against short circuit forces, and protect housed equipment against physical damage.
It is usually easier to provide barriers to protect the electronics enclosure from collision and
missile hazards rather than strengthening the enclosure itself.
9.12.2 COMPONENT CLEARANCES
Enclosures must provide adequate clearance from energized parts. The required clearances
depend on the shape of the conductor, the surface characteristics of the conductor and
enclosure, the voltage characteristics, environmental conditions, and creepage. The breakdown
strength along the surface of supporting insulators may require larger clearances than
breakdown in air.
All power electronics enclosures shall provide adequate room for access to parts and
subsystems for expected maintenance and modification. Consideration should be given to
handling provisions for heavy parts and subsystems, access to test points and calibration
adjustments, and work clearances for safe access to enclosure interiors.
Safe work on high-voltage equipment requires installation of manual grounding devices on
exposed high-voltage conductors. Enclosure size shall provide adequate room to safely apply
and remove grounding devices, and permit grounding devices to remain in place without
interfering with expected work. (See Section 10.10.1.2)
Enclosures shall be sized to allow cables to be installed and routed without infringing on
required clearances from high-voltage conductors.
Subassemblies, circuits, and related equipment should be segregated to the extent possible to
minimize the possibility of a fault in one device damaging another.
Power electronics systems can involve fast pulses, high frequencies and high currents and it is
common for the voltage difference between ground in one circuit and ground in another circuit to
differ substantially. This difference can be hundreds or thousands of volts. Wire and cable shall
be insulated to withstand these potentials. Surge arrester and capacitor protection maybe used
to control these potentials. DC circuits connected to coils, solenoid valves and other inductive
components should be tested for induced voltages and appropriate protection for circuits should
Test points needed for adjustment and diagnosis should be installed on the front panel or other
appropriate location of power electronic systems to facilitate their use without exposure hazard
to employees in the area.
Currents generated only during fault conditions or those introducing noise or data errors shall
not be considered objectionable currents. However, Bonding and grounding may be altered to
reduce the noise or data errors, in accordance with provisions of NEC 250.96(B). Conductors,
bus bars, and internal wiring should be insulated in the event objects are dropped into the
Automatic discharge devices are not a substitute for grounding devices used for personnel
protection. Grounding points shall be located in the system and physically arranged to permit
the attachment of adequate grounding devices for the protection of personnel working on the
These grounding points shall be capable of carrying the short-circuit current to which they may
be subjected and applied using methods appropriate for the voltages or currents involved.
9.13 NON-IONIZING RADIATION
9.13.1 ELECTROMAGNETIC RADIATION
Human exposure to electromagnetic (EM) radiation at certain power-density levels can be
hazardous. The hazards are generally regarded to be associated with the heating of biological
tissue, which occurs when EM radiation is absorbed by a body. This heating is essentially
similar to the cooking process in a microwave oven. Use caution where EM sources are being
used with the shielding altered or removed.
When working with EM radiation, it is recommended that the emitted radiation levels be
estimated by equations and measured by radiation hazard monitors.
EM radiation-safe levels have been established by the Institute of Electrical and Electronics
Engineers and are documented in the IEEE standard - C95.1-1999. Also, see Section 10.8.4.
Exposure to hazardous levels of EM radiation can be lessened by maintaining as much distance
as possible from the source. Power density is reduced by a factor the square of the distance
from the source (e.g., a factor of 4 for 2 times the distance).
9.13.2 ELECTROMAGNETIC RADIATION THREAT TO ELECTROEXPLOSIVE DEVICES
Designers of enclosed electrical/electronic equipment must consider the possible effects on
nearby EED of electromagnetic radiation (EMR); i.e., radio frequency (RF) energy, emitted by
Energy induced into an EED by the electromagnetic field resulting from such emissions may be
adequate to cause the device to initiate detonation.
Factors which should be taken into account in assessing concerns for possible EMR emissions
1. Wiring, shielding, and sensitivity
3. Frequency of the emissions causing coupling of electrical energy
4. Power density
5. Type of emission modulation
Possible measures to mitigate the threat of EMR emissions include:
1. Enclosure and signal line shielding and grounding to prevent leakage of EMR from the
2. Designed-in physical separation or barrier that would ensure that the power density of the
electromagnetic field is inadequate to cause detonation of an EED at the closest possible
distance to the emission source within the equipment.
3. Filter, or provide ferrite beads for, signal lines from the equipment which may conduct EMR
emissions into EED circuitry or secondarily radiate EMR in the proximity of an EED thus
causing a threat of detonation.
4. Ensure that the minimal power necessary is used to operate circuitry capable of producing
5. Label the equipment capable of emitting EMR to indicate the minimum separation distance
to be maintained between the equipment and an EED or EEDs.
6. Use a safety factor in design for EMR reduction; e.g., only 1/10 of the energy that would
initiate an EED is allowed.