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(c) Provide alternate circuits, automatic switching, and automatic reclosing devices,
where applicable, to minimize the duration and extent of outages.
4.1.2 Abnormalities.
4.1.2.1 Short Circuits. Short circuits may be phase-to-ground, phase-to-phase,
phase-to-phase-to-ground, three-phase, or three-phase-to-ground. Short circuits may range in
magnitude from extremely low current faults, having high impedance paths, to extremely high
current faults, having very low impedance paths. All short circuits produce abnormal current
flow in one or more phase conductors or in the neutral or grounding circuit. Such disturbances
can be detected and safely isolated.
4.1.2.2 Other Disturbances. Other sources of disturbances such as lightning, load surges,
and loss of synchronism, usually have little overall effect on system coordination and can be
handled on an individual basis for the specific equipment to be protected.
4.1.3 Protective Equipment. The isolation of short circuits requires the application of
protective equipment which senses abnormal current flow and removes the affected portion from
the system. The sensing device and interrupting device may be completely separate,
interconnected through external control wiring, mechanically coupled, or a single device.
4.1.3.1 Overcurrent Relays. Overcurrent relays are sensing devices only and must be used
in conjunction with an interrupting device to isolate the affected portion of the system. Action of
overcurrent relays may be either directional or nondirectional, while response may be
instantaneous or time delay.
4.1.3.2 Fuses. Fuses are the oldest and simplest of all protective devices. The fuse is both
the sensing and the interrupting device. Fuses are installed in series with the circuit and operate
when a fusible link melts in response to overcurrent. Fuses are one-shot devices, because their
fusible elements are destroyed in the protective process.
4.1.3.3 Circuit Breakers. Circuit breakers are interrupting devices only. They must be
used in conjunction with sensing devices to fulfill the detection function. The sensing devices
are usually separate protective relays or combinations of relays for circuit breakers rated to
operate at 1000 V or more. Low-voltage circuit breakers usually use direct-acting sensing and
tripping devices that sense the actual load current.
4.1.3.4 Fused Interrupter Switches. Fused interrupter switches are a hybrid form of circuit
protection which function exactly the same as fuses under short-circuit conditions. Under certain
circumstances, however, they may function as circuit interrupters. These switches will be treated
and coordinated as fuses and no attempt will be made to incorporate the interrupter in the
coordination scheme.
4-2





 


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