Program Policy Manual - VOLUME V - Mine Safety and Health ...

MSHA PROGRAM POLICY MANUAL VOLUME V
mechanical rather than an electrical function. The function of a
proper electrical fitting for a cable entering a junction box,
electrical panel, termination box, or other electrical enclosure
is to prevent a strain on the electrical connections and to
prevent chafing or other movement of the cable that might allow an
energized electrical conductor to fault to the enclosure frame.
Proper fittings would permit box connectors, packing glands,
strain insulators, strain clamps, or metal or wood clamps, etc.
Electric circuits that are made up of individual insulated wires
that enter junction boxes, termination boxes or other electrical
enclosures need not have fittings, but must be provided with
insulated bushings.
77.506 Electric Equipment and Circuits; Overload and Short-
Circuit Protection
Section 77.506 requires "automatic circuit-breaking devices or
fuses of the correct type and capacity" on electric equipment and
circuits to protect against short circuits and overloads. Section
77.506-1 specifies what was considered to be the correct type and
capacity for circuit breakers and fuses at the time the standard
was enacted. It requires such devices and fuses to meet the
minimum requirements of the 1968 NEC. For certain excavation
equipment, requiring strict compliance with the terms of the 1968
NEC could prevent the use of circuit protective devices of
appropriate type and capacity. Therefore, Section 77.506-1 should
not be applied to some equipment which meets the requirements of
Section 77.506.
Alternating- and direct-current loop (feedback) systems and their
controls, which are used on large shovels, draglines and in-mine
hoisting installations are normally designed so that their
currents are limited to values below those which would cause a
harmful overload condition to circuits or motors. These circuits
on the equipment specified above comply with Section 77.506, and
will not be required to have short-circuit or overload protective
devices to comply with the terms of the NEC.
77.507 Electric Equipment; Switches
The intent of this Section is to require that all control devices
be fully enclosed to prevent exposure of bare wires and energized
parts. Improvised starting methods such as plug and receptable
devices, trolley taps and trolley wire "stingers" that are used to
start or stop electric motors are examples of noncompliance with
this provision.
77.508 Lightning Arresters, Ungrounded and Exposed Power
Conductors and Telephone Wires
Conductors that are provided with a metallic shield or are
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