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DOE Handbook
Electrical Safety
- Special Occupancies -


This section covers the specific requirements and information for installing electrical equipment and wiring in explosive and hazardous locations and underground facilities. Classifications of areas or locations with respect to hazardous conditions are discussed. Information is provided on the correct methods and techniques needed for system grounding, lightning protection, and controlling of static electricity.

This section references DOE, NFPA, and Department of Defense (DoD) standards. These standards and manuals should be referenced to ensure safe and reliable installations of electrical equipment and wiring methods in explosive and hazardous locations.


This section references DOE M 440.1-1, DOE Explosives Safety Manual, NFPA 70 (NEC), NFPA 77, NFPA 780, and DoD 6055.9-STD, DoD Ammunition and Explosives Safety Standards. These standards and manuals should be referenced to ensure safe and reliable installations of electrical equipment and wiring methods in explosive and hazardous locations.


Whenever an electrical storm approaches, personnel shall exit any location where a hazard exists from explosives being detonated by lightning. Evacuation may be necessary from locations listed below:

1. All outdoor locations, locations in buildings that do not have lightning protection, and locations within inhabited building distance of the hazard. (When an electrical storm is imminent, work with explosives operations shall not be undertaken.)

2. Locations (with or without lightning protection) where operations use electrostatic-sensitive bulk explosives or electroexplosive devices (EEDs).


The following guidelines shall be used for shutdown of an operation during an electrical storm:

1. Process equipment containing explosives shall be shut down as soon as safety permits.

2. When buildings or bays containing explosives are evacuated, functions that cannot be shut down immediately shall be operated by the minimum number of personnel required for safe shutdown. When the operation has been brought to a safe condition, those remaining shall

3. Automatic emergency power equipment shall be provided if electrical power is critical to an explosives operation during a power shutdown or interruption.


It is DOE policy to install lightning protection on all facilities used for storage, processing, and handling of explosive materials where operations cannot be shut down and personnel


evacuated during electrical storms. Specific operations shall be assessed for the risk of
detonation of explosives by lightning. Such assessment shall consider the need for the
protection factors outlined in Appendix I, NFPA 780. When risk is high, as in operations with
highly sensitive electrostatic materials or components, operations shall be conducted only in
lightning-protected facilities. Approved lightning protection systems shall conform with the
requirements of Appendix I, NFPA 780.

Lightning-protection systems should be visually inspected every 7 months and a report on their
conditions filed at least annually. Any evidence of corrosion, broken wires or connections, or any
other problem that negates the system's usefulness shall be noted and the problem repaired.
Lightning protection systems should be tested electrically every 14 months to ensure testing
during all seasons, or immediately following any repair or modification. The testing shall be
conducted only with instruments designed specifically for earth-ground system testing. The
instruments shall be able to measure 10 ohms ±10% for ground resistance testing and 1 ohm
±10% for bonding testing. Electrical resistance readings shall be recorded.

Inspection records shall contain the most recent electrical test report and any subsequent visual
inspection reports for each building with a lightning-protection system.


Static electricity shall be controlled or eliminated in areas where materials are processed or
handled that are ignitable by static spark discharge. This category includes spark-sensitive
explosives, propellants, and pyrotechnics, as well as solvent vapors and flammable gases.
Approved systems to dissipate static electricity shall conform to the requirements of NFPA 77

Bonding straps shall be used to bridge locations where electrical continuity may be broken by
the presence of oil on bearings, or by paint or rust at any contact point. Permanent equipment in
contact with conductive floors or tabletops is not considered adequately grounded. Static
grounds shall not be made to gas, steam, or air lines; dry-pipe sprinkler systems; or air
terminals of lightning protection systems. Any ground that is adequate for power circuits or
lightning protection is more than adequate for protection against static electricity. TESTING EQUIPMENT GROUNDING SYSTEMS

Grounding systems shall be tested for electrical resistance and continuity when installation is
complete and, in the case of active equipment, at intervals to be locally determined. The
grounding system shall be visually inspected for continuity before it is reactivated if the
equipment has been inactive for more than 1 month. All exposed explosives or hazardous
materials shall be removed before testing. During a test for resistance to ground, all equipment,
except belt-driven machines, shall be considered as a unit. In measuring the total resistance to
ground for belt-driven machinery (to ensure compliance with Section, resistance of the
belt is to be excluded. All conductive parts of equipment shall be grounded so that resistance
does not exceed 25 ohms, unless resistance is not to exceed 10 ohms because of the lightning
protection system. For existing equipment, the rate of static electricity generation shall be
considered before changes are made in grounding systems. The resistance of conductive
rubber hose shall not exceed 250,000 ohms.


Conductive floors and shoes should be used for grounding personnel conducting operations
involving explosives that have an electrostatic sensitivity of 0.1 joule or less. Many flammable
liquids and air mixtures can be ignited by static discharge from a person. In areas where
personnel come close enough to have possible contact with static-sensitive explosives or
vapors, conductive floors shall be installed except where the hazards of dust-air or flammable
vapor-air mixtures are eliminated by adequate housekeeping, dust collection, ventilation, or
solvent-recovery methods. Conductive floors may also be required where operations are
performed involving EEDs that contain a static-sensitive explosive.

Conductive floors are not required throughout a building or room if the hazard remains localized.
In such cases, conductive mats or runners may suffice. These mats or runners shall be subject
to all the specifications and test requirements that apply to conductive floors. Conductive
wristbands may be substituted for conductive mats and footwear at fixed, grounded

Conductive floors shall be made of nonsparking materials such as conductive rubber or
conductive flooring material and shall meet the following requirements:
1. The flooring and its grounding system shall provide for electrical resistance not to exceed
1,000,000 ohms (measured as specified in Section

2. The surface of the installed floor shall be reasonably smooth and free from cracks. The
material shall not slough off, wrinkle, or buckle under operating conditions. Conductive tiles
are not recommended for use in areas where contamination can be caused by explosive
dust. The large number of joints and the tendency of tiles to loosen provide areas where
explosive dust can become lodged and that are not easy to clean with normal cleaning

3. Where conductive floors and shoes are required, resistance between the ground and the
wearer shall not exceed 1,000,000 ohms, which is the total resistance of conductive shoes
on a person plus the resistance of floor to ground. Where conductive floors and shoes are
required, tabletops on which exposed explosives or dust are encountered shall be covered
with a properly grounded conductive material meeting the same requirements as those for

4. Conductive floors shall be compatible with the explosive materials to be processed.

5. Conductive wristbands shall not exceed a resistance of 1,000,000 ohms between the wearer
and ground. This resistance shall be measured with a suitably calibrated ohmmeter.
Wristbands shall be of a design that maintains electrical contact with the wearer when
tension is applied to the ground lead wire or the wristband is placed under strain. CONDUCTIVE FLOOR TEST

Before use, tests shall be conducted on all conductive floors; subsequent tests shall be made at
least semiannually. Test results shall be permanently recorded and a copy filed in a central


location. Instruments used in testing shall be used only when the room is free from exposed
explosives and mixtures of flammable gases.

Maximum floor resistance shall be measured with a suitably calibrated insulation resistance
tester that operates on a normal open-circuit output voltage of 500 V dc and a short-circuit
current of 2.5 mA with an effective internal resistance of approximately 200,000 ohms. Minimum
floor resistance shall also be measured with a suitably calibrated ohmmeter.

Each electrode shall weigh 2.3 kg and shall have a dry, flat, circular contact area 6½ cm in
diameter, which shall comprise a surface of aluminum or tinfoil 1.3 to 2.5 mm thick, backed by a
layer of rubber 0.6 to 0.65 cm thick, and measuring between 40 and 60 durometer hardness as
determined with a Shore Type A durometer.

The floor shall be clean and dry. Only electrode jelly shall be used to establish a good contact.
(Brushless shaving soap and saline solution shall not be used.)

The resistance of the floor shall be more than 5,000 ohms in areas with 110-V service, 10,000
ohms in areas with 220-V service, and less than 1,000,000 ohms in all areas, as measured
between a permanent ground connection and an electrode placed at any point on the floor and
also as measured between two electrodes placed 3 ft apart at any points on the floor.

Measurements shall be made at five or more locations in each room. If the resistance changes
appreciably during a measurement, the value observed after the voltage has been applied for
about 5 sec shall be considered the measured value. (See Figure 5-1.)

RF Cafe - Testing shoes on wearer

Figure 5-1. Testing shoes on wearer. HUMIDIFICATION

Humidification to prevent accumulations and subsequent discharges of static electricity is
usually effective if the relative humidity is above 60 percent. However, certain materials such as
metallic powders and some pyrotechnic mixtures cannot be exposed to air with 60 percent
Figure 5-1. Testing shoes on wearer.


relative humidity because of the possibility of their spontaneous ignition. Where this technique is
used to prevent accumulations of static electricity, a daily check of the humidity levels will be
performed before work starts. GROUND-FAULT CIRCUIT INTERRUPTER

GFCI protection shall be provided in static-grounded areas where personnel are using handheld,
portable, ac-powered electrical equipment operating at 120 V.


Electrical equipment and wiring in locations containing explosives shall comply with relevant
provisions of the NEC and DOE regulations, plus the requirements in this section. PERMANENT EQUIPMENT AND WIRING

The NEC and this section are minimum requirements for DOE facilities containing explosives.
Though the NEC does not specifically address explosives, Article 500, Hazardous (Classified)
Locations, does establish requirements for the design and installation of electrical equipment
and wiring in locations containing combustible dusts and flammable liquids, vapors, or gases
that in general are comparably hazardous. All permanent electrical equipment and wiring in
work areas containing explosives hazards shall conform to the standards of the NEC Hazardous
Locations Class II or Class I and II (dual rated). For Class II installations, provisions should be
made for easy conversion to Class I. HAZARDOUS LOCATIONS

NEC definitions of and requirements for hazardous locations Class I and Class II are modified
as follows for application to DOE explosives facilities:

1. Areas containing explosive dusts or explosives which may, through handling or processing,
produce dust capable of being dispersed in the atmosphere shall be regarded as Class II
Division 1 hazardous locations.

2. Areas that contain exposed explosives but where no dust hazard exists shall be regarded as
Class 11 Division 2 hazardous locations.

3. Suitable National Electrical Manufacturers Association (NEMA)-rated enclosures shall be
provided in those locations where water/ explosives mixtures may contact electrical
equipment and wiring.

4. Areas where explosives are processed and sublimation may occur or where flammable
gases or vapor may be present in quantities sufficient to produce explosive or ignitable
mixtures shall be regarded as Class I Division 1 and Class II Division 1 hazardous locations.

5. To ensure a location is assigned to the proper hazardous location class and division, it is
necessary to know the properties of the explosives involved there, including, at a minimum,
sensitivity to heat and spark and thermal stability. If the properties of an explosive area are
such that Class II Group G equipment provides inadequate surface temperature limits,
special protection shall be provided or the equipment excluded from the hazardous location.
This equipment shall not have a surface temperature exceeding the lowest onset of the


exotherm of the explosive as determined by the differential thermal analysis test or the
differential scanning calorimetry test. When NEC Class I or II equipment is not available, the
substitute equipment shall be purged or sealed to prevent explosives contamination, shall
be determined intrinsically safe by facility management, or shall be administratively
controlled. If the equipment is purged, it shall be monitored for flow.

6. Areas that contain explosives that are not defined as hazardous locations (areas containing
no dust, vapor, gas hazards, or exposed explosives; for example, storage magazines), shall
be evaluated and documented by facility management to ensure that electrical ignition
sources are minimized or shall be regarded as NEC Class II.

7. Procedures shall be established by each DOE facility to control the use and modification of
electrical equipment in explosives areas and ensure that uniform standards are adhered to
throughout the facility. ELECTRICAL SUPPLY SYSTEMS

There may be multiple hazards where explosives facilities are located near electrical supply
lines. To protect against these hazards, the NESC (ANSI/IEEE C2) and the following
requirements apply to all new construction or major modification and should be considered for
existing facilities:

1. Electric lines serving explosive facilities shall be installed underground from a point not less
than 50 feet from such facilities. This also applies to communications and instrumentation
lines and security alarm systems.

2. Electric service lines required to be close to an explosives facility shall be no closer to that
facility than the length of the lines between the poles or towers supporting the lines, unless
an effective means is provided to ensure that broken, energized lines cannot come into
contact with and present a hazard to the facility or its appurtenances.

3. Unmanned electrical substations shall be no closer to explosives facilities than public traffic
route distances.

4. Electric transmission lines (carrying 69 kV or more) and the tower or poles supporting them
shall be located not closer to explosives than:

a. Inhabited-building distance if the line in question is part of a system serving a large, offsite

b. Public traffic route distance if loss of the line shall not create serious social or economic

c. Underground utility separation distance criteria found in Table 5-1.


Table 5-1. Quantity-distance separation for protection of underground service installations

RF Cafe - Quantity-distance separation for protection of underground service installations

a If the planned building is designed to contain the
effects of an explosion, the formula D (distance) =
3.0 w1/3 (w=weight) can be used to determine
separation distances for less than 20,000 lb. BUILDING SERVICE ENTRANCE

The electrical service entrance for explosives facilities shall be provided with:

1. An intermediate, metal-oxide surge lightning arrester on the primary side of the transformer.

2. Surge arresters and surge capacitors on the supply side of the main service disconnect.

3. Interconnected grounding between the lightning arrester, surge arrester, surge capacitors,
service entrance ground, and building ground.


Certain provisions shall be complied with before tests are performed. Qualified personnel shall
be used to determine the time and procedure of the test.


In setting up a test at a firing site, all preparatory work shall be completed before explosives are
received. Such work shall include the following items:

1. Checking all firing site safety devices at regular intervals. Such safety devices include
warning lights, door and gate firing circuit interlocks, emergency firing circuit cutoff switches,
and grounding devices (including those that are remote from the firing bunker).

2. Completing all firing pad and shot stand setup work that requires power tools or other
potential spark-producing devices. The firing pad shall be cleared of all unnecessary gear.
Special precautions and procedures shall be developed and implemented if power tools or
other spark-producing devices are needed after the explosive has been received at the firing

3. If a special structure is required, as much work as possible shall be accomplished on it,
including assembly of all materials.

4. When possible, all diagnostic equipment shall be set up and checked, and dry runs shall be

Whenever pin switches and other noninitiating circuits are to be checked (such as for charging
current or leakage) and are in contact with or close to explosives, the check shall be performed
remotely. Other noninitiating electrical circuits include strain gauges, pressure transducers, and
thermocouples, which may be affixed to or close to the explosives within an assembly. If a
continuity-only (resistance) check is desired, this may be accomplished as a contact operation
with an electrical instrument approved for use with the particular explosive device. When lowfiring
current actuators are involved, it may be advisable to conduct these tests remotely. LIGHTNING STORMS

All operations in open test-firing areas shall be discontinued during lightning storms when
explosives are present. Completion of a test after receipt of a lightning alert should be allowed
only if test preparation has progressed to the extent that discontinuance of testing would
represent a greater personnel risk than would completion of testing. LOW-ENERGY ELECTROEXPLOSIVE DEVICES

When using hot-wire or low-energy EEDs for a test firing, the following requirements shall be

1. Establishment of procedures to ensure that RF, FM, and TV transmitters having
sufficient output energy to initiate an EED at the test site are either restricted to a safe
distance from the site or not operated. Tables 5-2, 5-3, and 5-4 specify minimum safe
distances for the various types of transmitters at several output power levels.

2. Blasting caps and other low-firing current igniters or detonators shall be kept separate
from explosives at all times, except during actual test charge assembly and setup.


Table 5-2. Minimum safe distances between radio frequency (RF) transmitters and electric blasting operations.

RF Cafe - Minimum safe distances between radio frequency (RF) transmitters and electric blasting operations

b Present maximum power of U.S. broadcast transmitters in commercial AM broadcast frequency
range (0.535 to 1.605 MHz).
c Present maximum for international broadcast.

Table 5-3. Minimum safe distances between TV and FM broadcasting transmitters and electric blasting operations.



RF Cafe - Minimum safe distances between TV and FM broadcasting transmitters and electric blasting operations

b Present maximum power, channels 2 to 6 and FM.
c Present maximum power, channels 7 to 13.
d Present maximum power, channels 14 to 83.


Table 5-4. Minimum safe distances between mobile RF transmitters and electric blasting operations.

RF Cafe - Minimum safe distances between mobile RF transmitters and electric blasting operations

c Citizens band radio (walkie-talkie), 26.96 to 27.23 MHz and cellular telephones, 3 watts power, 845
MHz; minimum safe distance; 5 ft.
d Maximum power for 2-way mobile units in VHF, 15.08- to 161.6-MHz range, and for 2-way mobile and
fixed station units in UHF, 450- to 460-MHz range.
e Maximum power for major VHF 2-way mobile and fixed-station units in 35- to 44-MHz range.
f Maximum power for 2-way fixed-station units in VHF, 150.8- to 161.6-MHz range.
g Maximum power for amateur radio mobile units.
h Maximum power for some base stations in 42- to 44-MHz band, 1.6- to 1.8- MHz band.

3. The entire wiring system of the explosive charge and of any low-firing-current initiators shall be
kept insulated at all times from every possible source of extraneous current. Shunts shall be left
on all low-energy initiators or lead wires until actual connections are to be made. Connections
shall be taped or otherwise insulated.

4. Test unit low-firing-current actuators or detonators shall be clearly marked. No contact
operations involving electrical testing shall be permitted on this type of unit unless an electric
meter for the specific application is used.


Each DOE explosives testing facility shall use standard audible signals to warn personnel of any
impending firing in a test area. Signals shall be established by each facility and approved by facility
management. FIRING LEADS

All detonator lead wires shall be electrically insulated. Firing leads or cables of low-energy
detonators for explosive assemblies shall be kept properly shorted during setup on the firing

Testing instruments shall meet certain criteria and be certified and labeled for the types of
testing they are permitted to perform. CLASSIFICATION

Testing instruments shall be assigned to categories on the basis of electrical characteristics that
affect their safe use with explosives systems. Specifically, instrument categories shall be
established so that testing instruments in each category can be safely applied to one or more of
the following classes of explosives systems:

1. Low-energy or hot-wire initiators (blasting caps, actuators, squibs, etc.)

2. High-energy initiators (exploding bridgewires, slappers, etc.)

3. Noninitiating electrical circuits.

Testing instruments that do not meet the safety criteria may be used on an explosives system
only if the activity is considered a remote operation and adequate personnel shielding or
separation distance is provided. CERTIFICATION

Each DOE facility using electrical testing instruments on explosives systems shall establish a
formal system for reviewing and certifying those instruments. Procedures for marking
instruments to show their approved use and restrictions on their use shall also be established,
so that every testing instrument is prominently labeled with its approved use and with a warning
if there is a restriction on its use.

Inspection and calibration of certified instruments shall be required at prescribed intervals or
whenever the instrument is opened for servicing or repair.

Records of all certified testing instruments shall be maintained by each DOE facility using
electrical instruments to test explosives systems. These records shall include type,
manufacturer, model, electrical specifications, wiring diagrams, and failure mode analyses. The
Explosives Safety Committee chairperson shall be notified in writing by DOE facilities when they
approve new electrical testing instruments for use with initiating systems. The chairperson shall
disseminate this information to all committee members.


Instruments used with electrical initiating circuits connected to electro-explosive devices may be
further categorized for use with either low-energy initiators or high-energy initiators. All testing
instruments used for this purpose shall be current-limited. Before being used on initiating
circuits, every instrument wiring diagram and internal circuitry design shall be analyzed,
examined, and certified for the following:

1. The output current through a resistance equivalent to that of the minimum resistance
initiator of the class shall not exceed 1 percent and shall not exceed 10 percent of the nofire
rating for the most sensitive initiator of the class. The current-limiting features of the
testing instrument shall be internal to the instrument and shall not depend on the circuit load

2. The internal circuitry shall ensure isolation features that require, at a minimum, two
independent failure modes before the specified output current can be exceeded.

3. A comprehensive (point-to-point, if possible) wiring check shall be made to ensure that the
wiring corresponds to the diagram and that all components are functioning properly and


Testing instruments in this category are used with electric circuits connected to instruments
such as strain gauges, pin switches, pressure transducers, thermocouples, and electrical
components that are affixed to or within an assembly with explosives. These instruments shall
meet the following requirements:

1. Each use of the testing instrument shall be analyzed to ensure that there is no credible
scenario where the normal test energy from the testing instrument can ignite explosive
charges or initiators in the test. This testing shall be consistent with Section

2. Where a testing instrument is used to make measurements on sensors directly applied to
explosives (e.g., bonded strain gauges or pin switches), the testing instrument shall be
certified and controlled.

3. Testing instruments shall be prominently marked with restrictions on their use. Many of
these testing instruments do not meet the requirements for use with initiating systems and
shall be marked to prevent their use on this type of circuit.


Explosives are hazardous by themselves, but around electricity they become even more
dangerous: an arc, spark, or hot surface can easily touch off an explosion. Therefore, the
electrical installation shall contain these ignition sources or house them in an area well
separated from the explosives storage area.

The electrical installation shall prevent accidental ignition of flammable liquids, vapors, and
dusts in the atmosphere. In addition, because portable electrical equipment is often used


outdoors or in corrosive atmospheres, its material and finish should be such that maintenance
costs and shutdowns are minimized. (See Figure 5-2.)

RF Cafe - Arcs and sparks are sources of ignition that produce enough heat to cause an explosion if the air and gas mixture is between the lower and upper flammable limits of the liquid involved

Figure 5-2. Arcs and sparks are sources of ignition that produce enough heat to cause an explosion if the air and gas mixture is between the lower and upper flammable limits of the liquid involved.


When flammable gases or combustible dusts are mixed in the proper proportion with air, a
source of energy is all that is needed to touch off an explosion. One prime source of energy is
electricity. During normal operation, equipment such as switches, circuit breakers, motor
starters, pushbutton stations or plugs, and receptacles can produce arcs or sparks when
contacts are opened and closed, which can easily cause ignition. Other energy hazards are
devices that produce heat, such as lighting fixtures and motors. Surface temperatures of these
devices may exceed the safe limits of many flammable atmospheres. Finally, many parts of the
electrical system can become potential sources of ignition in the event of insulation failure.
Included in this category are wiring (particularly splices), transformers, impedance coils,
solenoids, and other low-temperature devices without make-or-break contacts.

Nonelectrical sources such as sparks from metal can also easily cause ignition: a hammer, file,
or other tool dropped on masonry or on a nonferrous surface could be a hazard unless it is
made of nonsparking material. For this reason, portable electrical equipment is usually made
from aluminum or other material that will not produce sparks if it is dropped.

Figure 5-2. Arcs and sparks are sources of ignition that produce enough heat to cause
an explosion if the air and gas mixture is between the lower and upper flammable
limits of the liquid involved.



The following three basic conditions are necessary for a fire or explosion to occur:

1. A flammable liquid, vapor, or combustible dust is present in sufficient quantity.

2. A flammable liquid, vapor, or combustible dust mixes with air or oxygen in the proportion
required to produce an explosive mixture.

3. A source of energy is applied to the explosive mixture.

In applying these principles, the quantity of the flammable liquid or vapor that may be liberated
and its physical characteristics are taken into account. Also, vapors from flammable liquids have
a natural tendency to disperse into the atmosphere and rapidly become diluted to
concentrations below the lower explosion limit, particularly when there is natural or mechanical
ventilation. Finally, the possibility that the gas concentration may be above the upper explosion
limit does not ensure any degree of safety since the concentration first passes through the
explosive range to reach the upper explosion limit.


Each area that contains gases or dusts that are considered hazardous shall be carefully
evaluated to make certain that the correct electrical equipment is selected. Many hazardous
atmospheres are Class I Group D or Class II Group G. However, certain areas may involve
other groups, particularly Class I Groups B and C. Conformity with the NEC requires the use of
fittings and enclosures approved for the specific hazardous gas or dust involved. The
determination of the area classification wiring and equipment selection for Class I, II, and III
areas should be made by a person cognizant of the requirements. The determination of the area
classification, wiring, and equipment selection for Class I, Zone 0, 1, and 2 areas shall be under
the supervision of a qualified registered professional engineer.


The use of intrinsically safe equipment is primarily limited to process control instrumentation
because these electrical systems lend themselves to the low energy requirements. The
installation rules are covered in Article 504 of the NEC. The definition of intrinsically safe
equipment and wiring is: "Equipment and wiring that are incapable of releasing sufficient
electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous
atmospheric mixture in its most easily ignited concentration." UL and Factory Mutual list several
devices in this category. The equipment and its associated wiring shall be installed so they are
positively separated from the nonintrinsically safe circuits. Induced voltages could defeat the
concept of intrinsically safe circuits.


In Class I Division 1 and 2 locations, conventional relays, contactors, and switches that have
arcing contacts shall be enclosed in explosion-proof housings, except for those few cases
where general-purpose enclosures are permitted by the NEC. By definition, enclosures for these
locations must prevent the ignition of an explosive gas or vapor that may surround it. In other
words, an explosion inside the enclosure shall not start a larger explosion outside. Adequate
strength is one requirement for such an enclosure. For an explosion-proof enclosure, a safety


factor of 4 is used. That is, the enclosure shall withstand a hydrostatic pressure test of four
times the maximum pressure from an explosion within it.

In addition to being strong, the enclosure shall be flame-tight. This term does not imply that the
enclosure is hermetically sealed but rather that the joints cool the hot gases resulting from an
internal explosion so that by the time they reach the outside hazardous atmosphere, they are
too cool to affect ignition. The strains and stresses caused by internal explosive pressures are
illustrated in Figure 5-3 (dotted lines indicate the shape that a rectangular enclosure strives to
attain under these conditions). Openings in an enclosure strive to maintain the shape of the
enclosure. Openings in an explosion-proof enclosure can be threaded-joint type (Figure 5-4) or
flat-joint type (Figure 5-5).

RF Cafe - The right mixture of air and gases in an enclosure can cause an explosion that creates internal pressures that can rupture the enclosure if not released properly

Figure 5-3. The right mixture of air and gases in an enclosure can cause an explosion
that creates internal pressures that can rupture the enclosure if not released properly.

RF Cafe - Threaded joints can be used as an escape path to cool the hot gases as they pass through the threads to the outside of the enclosure

Figure 5-4. Threaded joints can be used as an escape path to cool the hot gases as
they pass through the threads to the outside of the enclosure.


RF Cafe - Flat (ground) joints can be used as an escape path to cool the hot gases as they pass through the flat (ground) joint.

Figure 5-5. Flat (ground) joints can be used as an escape path to cool the hot gases as they pass through the flat (ground) joint.

In Class II locations, the enclosure shall keep dust out of the interior and operate at a safe
surface temperature. Because there will be no internal explosions, the enclosure may have
thinner wall sections. The construction of these enclosures is known as dust-ignition-proof.


Purging/pressurization systems permit the safe operation of electrical equipment under
conditions of hazard for which approved equipment may not be commercially available. For
instance, most switchgear units and many large motors do not come in designs listed for Class I
Groups A and B. Whether cast-metal enclosures or sheet-metal enclosures with pressurization
should be used for hazardous locations is mainly a question of economics, if both types are
available. As a typical example, if an installation had many electronic instruments that could be
enclosed in a single sheet-metal enclosure, the installation lends itself to the
purging/pressurization system. However, if the electronic instruments require installation in
separate enclosures, use of the cast metal in hazardous-location housing would almost
invariably prove more economical. Pressurized enclosures require:

1. A source of clean air or inert gas

2. A compressor to maintain the required pressure on the system

3. Pressure control valves to prevent the power from being applied before the enclosures have
been purged and to deenergize the system should pressure fall below a safe value.
Figure 5-5. Flat (ground) joints can be used as an escape path to cool the hot gases as
they pass through the flat (ground) joint.


In addition, door-interlock switches are required to prevent access to the equipment while the
circuits are energized. All of these accessories can add up to a considerable expenditure. For a
detailed description of purging/pressurizing systems see NFPA 496, Purged and Pressurized
Enclosures for Electrical Equipment in Hazardous Classified Locations.


Hazardous areas and locations are classified by group, class, and division. These classifications
are determined by the atmospheric mixtures of various gases, vapors, dust, and other materials
present. The intensity of the explosion that can occur depends on concentrations, temperatures,
and many other factors that are listed in NFPA codes.

Hazardous locations must be well understood by anyone designing, installing, working on, or
inspecting electrical equipment and wiring in such areas. Such locations carry a threat of
flammable or combustible gases, vapors, or dusts being present some or all of the time.
Information in this section will assist in classifying areas or locations with respect to hazardous
conditions, whether from atmospheric concentrations of hazardous gases, vapors, and deposits,
or from accumulations of readily ignitable materials.

This section covers the requirements for electrical equipment and wiring in locations that are
classified according to the properties of the flammable vapors, liquids, or gases or combustible
dusts that may be present and the likelihood that a flammable or combustible concentration is
present. The hazardous (classified) locations are assigned the following designations:

1. Class I Division 1

2. Class I Division 2

3. Class II Division 1

4. Class II Division 2.

5. Class I, Zone 0, Zone 1, Zone 2

Class III fibers and flyings are not covered in this section:

5.3.1 CLASS I

Class I locations are identified in the NEC as those in which flammable gases or vapors are or
may be present in the air in amounts sufficient to create explosive or ignitable mixtures. Gases
or vapors may be continuously or intermittently present. However, if a gas or vapor is present,
there is a potential that a flammable mixture will be present.

From an engineering standpoint, greater precautions are needed if a particular set of conditions
is likely to occur (e.g., the presence of a flammable mixture within the explosive range) than if it
is unlikely. This is the reason for dividing hazardous locations into two divisions. DIVISION 1

NEC 500.5 defines Class I Division 1 hazardous locations as those in which:


1. Ignitable concentrations of flammable gases, liquids, or vapors can exist under normal
operating conditions;

2. Ignitable concentrations of such gases or vapors may exist frequently because of repair or
maintenance operations or because of leakage; or

3. Breakdown or faulty operation of equipment or processes might release ignitable
concentrations of flammable gases, liquids, or vapors and might also cause simultaneous
failure of electrical equipment.

Note: In each case, ignitable concentrations are mentioned. This means concentrations
between the lower and upper flammable or explosion limits (see Section 5.3.5 and Table 5-5).
The fine-print note to NEC 500.5(B)(1) describes a number of areas and occupancies normally
classified as Class I Division 1 locations.


Table 5-5. Class I Division 1 and Class I Division 2 summary of selected hazardous atmospheres

RF Cafe - Class I Division 1 and Class I Division 2 summary of selected hazardous atmospheres


Table 5-5. Class I Division 1 and Class I Division 2 summary of selected hazardous atmospheres (continued)

RF Cafe - Class I Division 1 and Class I Division 2 summary of selected hazardous atmospheres (continued)


NEC Article 100 defines a flammable liquid as one that has a flashpoint below 38°C (100°F) or
one whose temperature is raised above its flashpoint. Flashpoint is the lowest temperature to
which a combustible or flammable liquid may be heated before sufficient vapors are driven off
and the liquid will flash when brought into contact with a flame, arc, spark, or another ignition
source. (See Section 1-3 of NFPA 497M for more details.) DIVISION 2

NEC 500.5(B)(2) defines Class I Division 2 locations as those:

1. In which flammable liquids or gases are handled, processed, or used, but where such
materials are normally confined in closed containers or closed systems from which they can
escape only in case of accidental rupture or breakdown of such containers or systems or in
case of abnormal equipment operation.

2. In which gases or vapors are normally prevented, by positive mechanical ventilation, from
forming ignitable concentrations and which might become hazardous through failure or
abnormal operation of the ventilating equipment

3. That are adjacent to a Class I Division 1 location and to which ignitable concentrations of
gases or vapors might occasionally be transmitted unless such transmittal is prevented by
adequate positive-pressure ventilation from a source of clean air, and effective safeguards
against ventilation failure are provided.

The fine-print note #2 to NEC 500.5 describes a number of areas and occupancies normally
classified as Class I Division 2 locations. For example, piping systems without valves, meters,
and devices do not usually cause a hazardous condition, even though they carry flammable
liquids, because they are considered a contained system. Therefore, the surrounding area can
be classified as a Class I Division 2 location.

5.3.2 CLASS II

A Class II location is defined in NEC 500 as an area where combustible dust presents a fire or
explosion hazard. Class II locations are divided into two divisions based on the normal presence
or absence of dust. CLASS II DIVISION 1

A Class 11 Division 1 location is one:

1. In which combustible dust is in the air under normal operating conditions in quantities
sufficient to produce explosive or ignitable mixtures;

2. Where mechanical failure or abnormal operation of machinery or equipment might cause
such explosive or ignitable mixtures to be produced and might also provide a source of
ignition through simultaneous failure of electrical equipment, operation of protective
devices, or other causes; or

3. In which combustible dusts of an electrically conductive nature may be present in
hazardous quantities. (See Table 5-6.)


Table 5-6. Summary of typical combustible dust hazardous atmospheres.
Class Division Group Temperature, atmosphere Covered Measured Limiting value

RF Cafe - Summary of typical combustible dust hazardous atmospheres

a Chart from Crouse-Hinds ECM Code Digest, 1990.
b NEMA Enclosures Type 9 shall be used for Class 11 Groups E, F, or G.


A Class II Division 2 location is one where:

1. Combustible dust is not normally in the air in quantities sufficient to produce explosive or
ignitable mixtures;

2. Dust accumulations are normally insufficient to interfere with the normal operation of
electrical equipment or other apparatus, but where combustible dust may be suspended in
the air as a result of infrequent malfunctioning of handling or processing equipment; and

3. Combustible dust accumulations on, in, or in the vicinity of the electrical equipment may be
sufficient to interfere with the safe dissipation of heat from electrical equipment or may be
ignitable by abnormal operation or failure of electrical equipment. (See Table 5-6.)

5.3.3 GROUPS

Until publication of the 1937 edition of the NEC, Class I hazardous locations were not
subdivided; a flammable gas or vapor was classified as presenting a single degree of hazard. It
was recognized, however, that the degrees of hazard varied with the substance and that
equipment suitable for use where gasoline was handled was not necessarily suitable for use
where hydrogen or acetylene was handled.

The difficulty of manufacturing equipment and enclosures for use in hydrogen atmospheres was
also recognized, as was the expense of the equipment. It was not logical from an engineering
standpoint, for example, to require in gasoline stations use of explosion-proof equipment that
was also suitable for use in hydrogen atmospheres. Not only would this unnecessarily increase
the cost of the electrical installation in one of the most common types of hazardous locations,
but it would also make some types of equipment unavailable. Even today, there are no listed
motors or generators suitable for use in Group A or B atmospheres.


Ignition temperature of a substance, whether solid, liquid, or gaseous, is the minimum
temperature required to initiate or cause self-sustained combustion independently of the heating
or heated element.

Ignition temperatures observed under one set of conditions may be changed substantially by a
change of conditions. For this reason, ignition temperatures should be viewed only as
approximations: Ignition temperatures under one set of conditions may be changed substantially
by a change of conditions. Some of the variables known to affect ignition temperatures are
percentage composition of the vapor or gas-air mixture; shape and size of the space where the
ignition occurs; rate and duration of heating; kind and temperature of the ignition source,
catalytic or other effect of materials that may be present; and oxygen concentration. Another
variable is the many differences in methods and conditions of testing ignition temperature (size
and shape of containers, method of heating, and ignition source).


As mentioned in Section, in the case of gases or vapors that form flammable mixtures
with oxygen, there is a minimum concentration of gas or vapor in air or oxygen below which


propagation of flame cannot occur on contact with a source of ignition. There is also a maximum
concentration of vapor or gas in air above which propagation of flame cannot occur. These
boundary-line mixtures of vapor or gas with air, which if ignited will just propagate flame, are
known as the lower and upper flammable or explosion limits and are usually expressed in terms
of percentage by volume of gas or vapor in air.

In popular terms, a mixture below the lower flammable limit is too lean to burn or explode and a
mixture above the upper flammable limit is too rich to burn or explode.


The flashpoint of a flammable liquid is the lowest temperature at which the liquid gives off
sufficient vapor to form, with the air near its surface or within the vessel used, an ignitable
mixture. An ignitable mixture is a mixture that is within the flammable range (between upper and
lower explosive limits) that is capable of propagating flame away from the source of ignition
when ignited. Some evaporation takes place below the flashpoint but not in sufficient quantities
to form an ignitable mixture. This term applies mostly to flammable and combustible liquids,
although there are certain solids, such as camphor and naphthalene, that slowly evaporate or
volatilize at ordinary room temperature or liquids, such as benzene, that freeze at relatively high
temperatures and, therefore, have flashpoints while in the solid state.


A wide variety of explosion-proof, ignition-proof electrical equipment is available for Class I, II,
and III areas. Selection of such equipment shall fully comply with current NFPA requirements.
Excellent references of manufacturers' electrical equipment available and used in hazardous
areas is the Crouse-Hinds ECM Code Digest, or the Appleton NEC Code Review which are
based on the current NEC.


Seals are to be provided in conduit and cable systems to minimize the passage of gases or
vapors from one portion of the system to another. The seals also keep an explosion from being
transmitted and ignition from traveling between sections of the system. SEALS

The following are uses and requirements for seals:

1. They restrict the passage of gases, vapors, or flames from one portion of the electrical
installation to another at atmospheric pressure and normal ambient temperatures.

2. They limit explosions to the sealed-off enclosure and prevent precompression or pressurepiling
in conduit systems.

3. While it is not a code requirement, many engineers consider it good practice to divide long
conduit runs into sections by inserting seals not more than 50 to 100 feet apart, depending
on the conduit size, to minimize the effects of pressure-piling. Sealing fittings are required.


4. At each entrance to an enclosure housing with an arcing or sparking device when used in
Class I Division 1 and 2 hazardous locations, seals must be as close as practicable to and in
no case more than 18 in. from such enclosures.

5. At each 2-inch or larger entrance to an enclosure or fitting housing terminals, splices, or taps
when used in Class I Division 1 hazardous locations, seals must be as close as practicable
to and in no case more than 18 inches from such enclosures.

6. Seals must be located in conduit systems when the conduit leaves the Class I Division 1 or
2 hazardous locations.

7. Seals must be located in cable systems when the cables either do not have a gastight or
vapor-tight continuous sheath or are capable of transmitting gases or vapors through the
cable core when these cables leave the Class I Division 1 or Division 2 hazardous locations.
NEC 502.5 requires the use of seals in Class II locations under certain conditions. Any
approved sealing fittings can be used to meet this requirement. DRAINS

In humid atmospheres or in wet locations where it is likely that water can enter the interiors of
enclosures or raceways, the raceways should be inclined so that water will not collect in
enclosures or on seals but will be led to low points where it may pass out through drains.
Frequently the arrangement of raceway runs makes this method impractical if not impossible. In
such instances, drain sealing fittings shall be used. These fittings prevent accumulations of
water above the seal.

In locations usually considered dry, surprising amounts of water frequently collect in conduit
systems. No conduit system is airtight; therefore, it may breathe. Alternate increases and
decreases in temperature and barometric pressure because of weather changes or the nature
of the process carried on in the location where the conduit is installed will cause breathing.
Outside air is drawn into the conduit system when it breathes in. If this air carries sufficient
moisture, it will be condensed within the system when the temperature decreases and chills the
air. With internal conditions being unfavorable to evaporation, the resultant water accumulation
will remain and be added to by repetitions of the breathing cycle. In view of this likelihood, it is
good practice to ensure against such water accumulations and probable subsequent insulation
failures by installing drain sealing fittings with drain covers or inspection covers even though
conditions prevailing at the time of planning or installing may not indicate the need. SELECTION OF SEALS AND DRAINS

Different types of seals and drains are made to be used for vertical or horizontal installations
and are to be used only for the purpose for which they were designed. Care shall be taken when
selecting and installing such fittings. PRIMARY CONSIDERATIONS

The following primary considerations should be used when selecting seals and drains:

1. Select the proper sealing fitting for the hazardous vapor involved (i.e., Class I Groups A, B,
C, or D).


2. Select a sealing fitting for the proper use in respect to mounting position. This is particularly
critical when the conduit runs between hazardous and nonhazardous areas. Improper
positioning of a seal may permit hazardous gases or vapors to enter the system beyond the
seal and to escape into another portion of the hazardous area or into a nonhazardous area.
Some seals are designed to be mounted in any position; others are restricted to horizontal
or vertical mounting.

3. Install the seals on the proper side of the partition or wall as recommended by the

4. Only trained personnel should install seals in strict compliance with the instruction sheets
furnished with the seals and sealing compound. Precautionary notes should be included on
installation diagrams to stress the importance of following manufacturer's instruction.

5. The NEC prohibits splices or taps in sealing fittings.

6. Sealing fittings are listed by UL for use in Class I hazardous locations with sealing
compound only. This compound, when properly mixed and poured, hardens into a dense,
strong mass, which is insoluble in water, is not attacked by chemicals, and is not softened
by heat. It will withstand with ample safety factor the pressure of exploding trapped gases or

7. Conductors sealed in the compound may be approved thermoplastic or rubber insulated
type. Both may or may not be lead covered (the lead need not be removed).
Caution: Sealing compounds are not insulating compounds; therefore, they shall not be used as

Sealing fittings meet the requirements of NEC when properly installed.

A certain style of sealing fittings are for use with vertical or nearly vertical conduit in sizes from
½ inch through 1 inch. Other styles are available in sizes 1/2 through 6 in. for use in vertical or
horizontal conduits. In horizontal runs, these are limited to face up openings. Sizes from 1¼
through 6 inches have extra-large work openings and separate filling holes so that fiber dams
are easy to make. Overall diameter of sizes 1¼ through 6 inches is scarcely greater than that of
unions of corresponding sizes, permitting close conduit spacing. Other style seals are for use
with conduit running at any angle, from vertical through horizontal.


Manufacturers produce NEC code digests for selection of seals and drains and provide, by
class and division, catalog data and installation diagrams for their use in electrical power and
lighting systems in hazardous areas. The manufacturers' NEC code digests should be in
compliance with current NFPA/NEC requirements. The two that are most used are as follows:

1. Crouse-Hinds ECM Code Digest

2. Appleton's NEC Code Review.


(PER NEMA 250)

Type 7 and 10 enclosures, when properly installed and maintained, are designed to contain an
internal explosion without causing an external hazard. Type 8 enclosures are designed to
prevent combustion through the use of oil-immersed equipment. Type 9 enclosures are
designed to prevent the ignition of combustible dust.

As mentioned earlier, hazardous locations (other than in mines) are classified according to the
flammability or combustibility of the materials that may be present and also according to the
likelihood that a flammable or combustible concentration is present. For definitions and
classifications, see the NEC, Article 500, and NFPA 497M, Classification of Gases, Vapors and
Dust for Electrical Equipment in Hazardous Classified Locations. Descriptions and tests in this
standards publication cover equipment that is suitable for installation in locations classified as
Division 1 or 2. In Division 2 locations, other types of protections and enclosures for
nonhazardous locations may be installed if the equipment does not constitute a source of
ignition under normal operating conditions. See the specific sections of Articles 501 through 503
of the NEC.

Intrinsically safe equipment (not capable of releasing sufficient electrical or thermal energy
under normal or abnormal conditions to cause ignition of specific hazardous atmospheres) may
be installed in any type of enclosure otherwise suitable for the environmental conditions
expected. See ANSI/UL 913, Intrinsically Safe Apparatus and Associated Apparatus for Use in
Class I, 11, III, Division I, Hazardous (Classified) Locations for detailed requirements.
Purged and pressurized equipment should be installed in enclosures suitable for nonhazardous
locations. Hazards may be reduced or eliminated by adequate positive pressure ventilation from
a source of clean air in conjunction with effective safeguards against ventilation failure. See
NFPA 496, Purged and Pressurized Enclosures for Electrical Equipment in Hazardous
Locations for detailed requirements.


Type 7 enclosures are designed for indoor use in locations classified as Class I Groups A, B, C,
or D as defined in the NEC.


Type 7 enclosures shall be capable of withstanding the pressures resulting from an internal
explosion of specified gases and containing such an explosion sufficiently that an explosive gas-air
mixture in the atmosphere surrounding the enclosure will not be ignited. Enclosed heat-generating
devices shall not cause external surfaces to reach temperatures capable of igniting
explosive gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion,
hydrostatic, and temperature design tests.



When completely and properly installed, Type 7 enclosures:

1. Provide to a hazardous gas environment a degree of protection from an internal explosion or
from operation of internal equipment

2. Do not develop external surface temperatures that exceed prescribed limits for the specific
gas corresponding to the atmospheres for which the enclosure is intended when internal
heat-simulating equipment is operated at rated load

3. Withstand a series of internal explosion design tests:

a. That determine the maximum pressure effects of the gas mixture

b. That determine propagation effects of the gas mixtures.

4. Withstand, without rupture or permanent distortion, an internal hydrostatic design test based
on the maximum internal pressure obtained during explosion tests and on a specified safety

5. Are marked with the appropriate class and groups for which they have been qualified.


Type 8 enclosures are designed for indoor or outdoor use in locations classified as Class I
Groups A, B, C, or D as defined in the NEC.


Type 8 enclosures and enclosed devices are arranged such that all arcing contacts,
connections, and any parts that could cause arcing are immersed in oil. Arcing is confined under
the oil such that it will not ignite an explosive mixture of the specified gases in internal spaces
above the oil or in the atmosphere surrounding the enclosure. Enclosed heat-generating
devices shall not cause external surfaces to reach temperatures capable of igniting explosive
gas-air mixtures in the surrounding atmosphere. Enclosures shall meet operation and
temperature-design tests. Enclosures intended for outdoor use shall also meet the rain test (See
#4 in Section 5.8.2).


When completely and properly installed, Type 8 enclosures:

1. Provide, by oil immersion, a degree of protection to a hazardous gas environment from
operation of internal equipment

2. Do not develop surface temperatures that exceed prescribed limits for the specific gas
corresponding to the atmospheres for which the enclosure is intended when internal
equipment is at rated load


3. Withstand a series of operation design tests with oil levels arbitrarily reduced and
with flammable gas-air mixtures introduced above the oil

4. When intended for installation outdoors, exclude water when subjected to a water spray
design test simulating a beating rain

5. Are marked with the appropriate class and groups for which they have been qualified


Type 9 enclosures are designed for indoor use in locations classified as Class II Groups E or G,
as defined in the NEC.


Type 9 enclosures shall prevent the entrance of dust. Enclosed heat-generating devices shall
not cause external surfaces to reach temperatures capable of igniting or discoloring dust on the
enclosure or igniting dust-air mixtures in the surrounding atmosphere. Enclosures shall meet
dust-penetration and temperature-design tests and prevent aging of gaskets (if used).


When completely and properly installed, Type 9 enclosures:

1. Provide a degree of protection to a hazardous dust environment from operation of internal

2. Do not develop surface temperatures that exceed prescribed limits for the group
corresponding to the atmospheres for which the enclosure is intended when internal
equipment is operated at rated load

3. Withstand a series of operation design tests while exposed to a circulating dust-air mixture
to verify that dust does not enter the enclosure and that operation of devices does not cause
ignition of surrounding atmosphere

4. Are marked with the appropriate class and groups for which they have been qualified


Underground facilities consist of electrical equipment and wiring installed in underground
locations. Working conditions underground can present to electrical workers hazards different
from those presented above ground. This section aids in dealing with such problems.
Electrical work in support of construction of mines, shafts, and underground utilities shall be
performed by qualified workers who must meet the requirements in Section 2.8, 30 CFR 75.153
and 77.103. Only those workers shall install equipment and conductors within the construction

Note: DOE does not engage in "mining" as mining is the extraction of minerals for profit.
However, the codes related to mining (30 CFR 57, 75, and 77) should be followed, where
applicable, along with the OSHA regulations set forth in 29 CFR 1910 and 1926.


Once construction of the underground facilities is completed, all wiring used for construction
activities shall be removed and permanent wiring installed in accordance with 29 CFR 1910,
Subpart S, and the NEC as applicable. When the work is not covered by these codes as
referenced, the applicable paragraphs of 30 CFR 57, 75, and 77 shall prevail.

Electrical equipment and conductors must be used in a manner that prevents shocks and burns
to people. Should electrical equipment and conductors present a hazard to people because of
improper installation, maintenance, misuse, or damage, the equipment and conductors must be
tagged out or locked out as a hazard until fixed. All electrical equipment and conductors shall be
chosen and situated in environments conducive to their design and intended use or as tested by
an NRTL for the purpose intended.

The voltage of bare conductors, other than trolley conductors, that are accessible to contact by
people shall not exceed 50 V. Electrical equipment and conductors, other than trailing cables,
shall be protected against overloads and short circuits by fuses or automatic interrupting
devices used in accordance with 29 CFR 1910.304.

Adequate clearance between equipment and bare overhead conductors must be maintained in
accordance with 29 CFR 1910.303. Conductors not being used to supply power to electrical
equipment shall be deenergized and removed from their power supply or have their power
supply locked out and tagged out in accordance with 29 CFR 1910.147 and 29 CFR 1910.333.
All exposed ends shall be insulated.

Access doors and cover plates shall be closed at all times, except for installation, testing, and
repair. Visible signs warning of danger shall be posted at all substations, switch centers, and
control centers to warn people against entry unless they have been authorized to enter and
perform duties in these locations.


Before any work is performed on electrical equipment or circuits, the power source or sources
shall be deenergized unless power is a required part of the work procedure. Lockout procedures
in 29 CFR 1910.147 and 29 CFR 1910.333 shall be followed. In addition, the following rules
apply for energized work:

1. Power-cable plugs and receptacles for circuits greater than 150 V potential to ground shall
not be connected or disconnected under load unless they are of the load-break type.
Energized power cables in excess of 150 V potential to ground shall be handled in
accordance with 29 CFR 1910.331. Care shall be taken to prevent damage or shock and
burn from the energized cable.

2. Proper tools shall be used to remove or install fuses to protect people from shock or burns.

3. All safety-related electrical work practices covered by the provisions in 29 CFR 1910.331
through .335 shall be followed.

4. Exposed electric connections or resistor grids not protected by location shall be insulated
unless impractical. In this case, guarding shall be installed to prevent accidental contact by
people or equipment.


5. Communication conductors shall be installed in accordance with 30 CFR 57.12010 and

6. Lights and lamps shall be properly guarded if they pose a hazard and shall be kept away
from combustible material.


All electric circuits shall have a grounding system. The system shall protect people from injuries
or fatal shock on inadvertent contact. The system shall limit the voltage on all electrical
equipment with noncurrent-carrying metallic parts. Grounding of ac and do equipment shall be
in accordance with 29 CFR 1910.304(f).

Equipment grounding conductors shall comply with the standards expressed in 29 CFR

All installations, modifications, or repairs pertaining to grounding systems shall be followed by a
continuity test to ensure the integrity of the systems. The frequency and requirements of he
review shall conform to 30 CFR 57.12028.


Cables and insulated conductors shall be protected against physical damage, adverse
environmental conditions, and failure of adjacent mechanical equipment.

Cables and insulated conductors shall not be supported from or be in contact with pipelines.
Sufficient clearance between pipelines and cables is required to prevent shock hazards when
maintenance activities are being performed. A minimum clearance of 10 ft above floor level shall
be maintained for all overhead cables/conductors overhead not protected against physical
damage as set forth in NFPA 70E.

Electric conductors shall be of a size and current carrying capacity to ensure that a rise in
ambient temperature does not exceed the rating of the insulation and conductors. The
capacities of electric conductors supplying electrical equipment shall be in accordance with the
tables set forth in the NEC Article 310. In the case of medium- or high-voltage cable, the
manufacturer's ratings shall not be exceeded.

Splices, terminations, and repairs of electric conductors and power cables shall be permitted
and shall conform to the requirements expressed in NFPA 70E.

Surge arresters and lightning protection are required for underground facilities and shall
conform to the requirements found in 30 CFR 57.12069 and 75.521. Lightning arresters shall be
inspected for damage at least annually or after each electrical storm.

Power cables and insulated conductors in shafts and bore holes shall be supported. Support
structures and guy wires and supports for cables and conductors shall conform with the
requirements expressed in 30 CFR 57.12083.



Trailing cables used in electrical systems of mines shall meet requirements expressed in 30
CFR 57.12038, 30 CFR 75 Subpart G, and 30 CFR 77, Subpart G.

Each trailing cable of portable and mobile equipment shall have short-circuit and ground-fault
protection for each ungrounded conductor. Protective devices shall safely interrupt all
ungrounded conductors under fault conditions. Requirements for over current protection of each
ungrounded conductor shall be those expressed in 30 CFR 57.12003, 30 CFR 75 Subpart G,
and 30 CFR 77, Subpart G.

Trailing cables shall be attached to equipment so that strain on electrical connections does not
occur and damage to cable jacket and internal conductor insulation is prevented. Portable
distribution boxes can be used and shall meet the requirements in 30 CFR 57.12006 and
57.12007. Trailing cables and power conductors shall be protected against physical damage
from mobile equipment by using bridges, trenches, or suspension from the mine roof.
Disconnecting devices for trailing cables shall be equipped with means for attaching a padlock
for LO/TO purposes per 30 CFR 57.12016, 57.12017, 75.511, and 77.501.


Trolley wires and exposed trolley-feeder wires shall be installed and maintained in accordance
to the requirements in 30 CFR 57.12050, 57.12086, and 30 CFR 75, Subpart K.

Trolley wires and trolley-feeder wires shall be protected against over current in accordance to
the requirements of 30 CFR 57.12001 and 75.1001.

Track serving as the trolley circuit return shall be bonded or welded according to the
requirements of 30 CFR 57.12042 and 75, Subpart K. Energized trolley wires and exposed
trolley-feeder wires shall be guarded in places where accidental contact with them is possible.
This includes areas where supplies are stored, loaded, or unloaded.


Contents | Introduction | General Requirements | Electrical Preventative Maintenance | Grounding | Special Occupancies
Requirements for Specific Equipment | Work in Excess of 600 Volts | Temporary Wiring | Enclosed Electrical / Electronic Equipment
Research & Development | Electrical Safety During Excavations | References |
DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. Available on the Department of Energy Technical Standards Program Web site at
http://tis.eh.doe.gov/techstds     DOE-HDBK-1092-2004