- Special Occupancies -
5.0 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).
5.1.2 SHUTDOWN OF OPERATIONS
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
5.1.3 LIGHTNING PROTECTION
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
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,
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
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.
records shall contain the most recent electrical test report and any subsequent visual
inspection reports for each building with a lightning-protection
5.1.4 STATIC ELECTRICITY
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
and IEEE 142.
22.214.171.124 BONDING AND GROUNDING EQUIPMENT
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.
126.96.36.199 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
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,
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 188.8.131.52), 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
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.
184.108.40.206 CONDUCTIVE FLOORS, SHOES, MATS, AND WRISTBANDS
Conductive floors and shoes should be used for grounding personnel conducting
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
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
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
all the specifications and test requirements that apply to conductive floors. Conductive
wristbands may be substituted for conductive mats
and footwear at fixed, grounded
workstations or outdoor location.
220.127.116.11 SPECIFICATIONS FOR CONDUCTIVE FLOORS AND WRISTBANDS
Conductive floors shall be made of nonsparking materials such as conductive rubber or
conductive flooring material and shall meet the
1. The flooring and its grounding system shall provide for electrical resistance not to exceed
(measured as specified in Section 18.104.22.168).
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
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.
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
22.214.171.124 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
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
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
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
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.
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.)
5-1. Testing shoes on wearer.
Humidification to prevent accumulations and subsequent discharges of static electricity is
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.
126.96.36.199 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.
5.1.5 ELECTRICAL EQUIPMENT AND WIRING
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.
188.8.131.52 PERMANENT EQUIPMENT
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
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
Locations Class II or Class I and II (dual rated). For Class II installations, provisions should be
made for easy conversion
to Class I.
184.108.40.206 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
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,
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,
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
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.
220.127.116.11 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
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
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
a If the planned building is designed to contain the
effects of an explosion, the formula D (distance) =
(w=weight) can be used to determine
separation distances for less than 20,000 lb.
18.104.22.168 BUILDING SERVICE ENTRANCE
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.
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.
22.214.171.124 TEST SETUP
In setting up a test at a firing site, all preparatory work shall be completed before explosives are
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
126.96.36.199 PIN SWITCHES
AND OTHER NONINITIATING CIRCUITS
Whenever pin switches and other noninitiating circuits are to be checked (such as for charging
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
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.
188.8.131.52 LIGHTNING STORMS
All operations in open test-firing areas shall be discontinued during lightning storms when
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.
184.108.40.206 LOW-ENERGY ELECTROEXPLOSIVE
When using hot-wire or low-energy EEDs for a test firing, the following requirements shall be
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
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.
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.
b Present maximum power, channels 2 to 6 and FM.
c Present maximum power, channels 7 to 13.
Present maximum power, channels 14 to 83.
Table 5-4. 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
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
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
220.127.116.11 WARNING SIGNALS
Each DOE explosives testing facility shall use standard audible signals to warn personnel of any
firing in a test area. Signals shall be established by each facility and approved by facility
18.104.22.168 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
22.214.171.124 ELECTRICAL TESTING INSTRUMENTS FOR USE WITH EXPLOSIVES SYSTEMS
Testing instruments shall meet certain criteria and be certified and labeled for the types of
testing they are permitted to perform.
Testing instruments shall be assigned to categories on the basis of electrical characteristics that
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.
Each DOE facility using
electrical testing instruments on explosives systems shall establish a
formal system for reviewing and certifying those instruments. Procedures
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.
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
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.
126.96.36.199.3 ELECTRICAL TESTING INSTRUMENTS FOR USE WITH INITIATING
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.
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
188.8.131.52.4 ELECTRICAL TESTING INSTRUMENTS FOR USE WITH NONINITIATING
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
meet the following requirements:
1. Each use of the testing instrument shall be analyzed to ensure that there is
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 184.108.40.206.3.
2. Where a testing instrument is used to make measurements on sensors directly
explosives (e.g., bonded strain gauges or pin switches), the testing instrument shall be
certified and controlled.
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.
5.2 PREVENTION OF EXTERNAL IGNITION
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
separated from the explosives storage area.
The electrical installation shall prevent accidental ignition of flammable liquids,
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.)
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.
5.2.1 SOURCES OF IGNITION
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
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,
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
limits of the liquid involved.
5.2.2 COMBUSTION PRINCIPLES
The following three basic conditions are necessary for a fire or explosion to occur:
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
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
explosive range to reach the upper explosion limit.
5.2.3 EVALUATION OF HAZARDOUS AREAS
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.
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.
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.
5.2.4 INTRINSICALLY SAFE EQUIPMENT
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
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
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
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
flat-joint type (Figure 5-5).
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.
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
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.
5.2.6 PURGING/PRESSURIZATION SYSTEMS
Purging/pressurization systems permit the safe operation of electrical equipment under
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
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
system. However, if the electronic instruments require installation in
separate enclosures, use of the cast metal in hazardous-location housing
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
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
Enclosures for Electrical Equipment in Hazardous Classified Locations.
5.3 HAZARDOUS LOCATIONS
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,
many other factors that are listed in NFPA codes.
Hazardous locations must be well understood by anyone designing, installing, working
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
conditions, whether from atmospheric concentrations of hazardous gases, vapors, and deposits,
or from accumulations of readily
This section covers the requirements for electrical equipment and wiring in locations that are
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
2. Class I Division 2
3. Class II Division 1
4. Class II Division 2.
5. Class I, Zone 0, Zone 1, Zone
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.
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
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.
220.127.116.11 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
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
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
Table 5-5. 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
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.)
18.104.22.168 DIVISION 2
NEC 500.5(B)(2) defines Class I Division 2 locations as those:
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
abnormal equipment operation.
2. In which gases or vapors are normally prevented, by positive mechanical ventilation, from
ignitable concentrations and which might become hazardous through failure or
abnormal operation of the ventilating equipment
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
ventilation failure are provided.
The fine-print note #2 to NEC 500.5 describes a number of areas and occupancies normally
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
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.
22.214.171.124 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
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
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.
126.96.36.199 CLASS II DIVISION 2
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
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
by abnormal operation or failure of electrical equipment. (See Table 5-6.)
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.
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.
5.3.4 IGNITION TEMPERATURE
Ignition temperature of a substance, whether solid, liquid, or gaseous, is the
temperature required to initiate or cause self-sustained combustion independently of the heating
or heated element.
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,
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).
5.3.5 FLAMMABLE (EXPLOSION)
As mentioned in Section 188.8.131.52, 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
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
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
temperatures and, therefore, have flashpoints while in the solid state.
5.4 ELECTRICAL EQUIPMENT FOR CLASS I, II, AND III AREAS
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
areas is the Crouse-Hinds ECM Code Digest, or the Appleton NEC Code Review which are
based on the current NEC.
SEALS AND DRAINS
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.
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
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.
approved sealing fittings can be used to meet this requirement.
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
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
water above the seal.
In locations usually considered dry, surprising amounts of water frequently collect in conduit
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.
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
conditions prevailing at the time of planning or installing may not indicate the need.
184.108.40.206 SELECTION OF SEALS AND
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.
220.127.116.11.1 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
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
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
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
18.104.22.168.2 TYPES OF SEALING FITTINGS
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
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
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
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.
5.6 DESCRIPTIONS, FEATURES, AND TEST CRITERIA OF
ENCLOSURES FOR HAZARDOUS (CLASSIFIED) LOCATIONS
(PER NEMA 250)
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
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
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.
5.7 TYPE 7 ENCLOSURES
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.
DESCRIPTION AND APPLICATION
Type 7 enclosures shall be capable of withstanding the pressures resulting from an internal
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
gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion,
hydrostatic, and temperature design tests.
5.7.2 FEATURES AND TEST CRITERIA
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
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
5.8 TYPE 8 ENCLOSURES
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.
5.8.1 DESCRIPTION AND APPLICATION
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
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).
5.8.2 FEATURES AND TEST CRITERIA
When completely and properly
installed, Type 8 enclosures:
1. Provide, by oil immersion, a degree of protection to a hazardous gas environment from
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
5. Are marked with the appropriate class and groups for which they have been qualified
5.9 TYPE 9 ENCLOSURES
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).
5.9.2 FEATURES AND TEST
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
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
5.10 UNDERGROUND FACILITIES
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
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
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
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
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
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,
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.
5.10.1 WORK ON ELECTRICAL EQUIPMENT
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
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
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
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
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.
5.10.3 POWER CABLES AND CONDUCTORS
Cables and insulated conductors shall be protected against physical damage, adverse
environmental conditions, and failure of adjacent
Cables and insulated conductors shall not be supported from or be in contact with pipelines.
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
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.
5.10.4 TRAILING CABLES
Trailing cables used in electrical systems of mines shall meet requirements expressed in 30
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
for LO/TO purposes per 30 CFR 57.12016, 57.12017, 75.511, and 77.501.
5.10.5 TROLLEY CIRCUITS FOR TRACK HAULAGE
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
of 30 CFR 57.12001 and 75.1001.
Track serving as the trolley circuit return shall be bonded or welded according to the
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.
Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
All trademarks, copyrights, patents, and other rights of ownership to images and text
used on the RF Cafe website are hereby acknowledged.
My Hobby Website: AirplanesAndRockets.com