MIL-HDBK-1038
provided from short circuits and ground faults and from motor overloads. Short
circuits are line-to-line faults; ground faults are line-to-ground faults.
Overcurrent protection from ground faults is not the same as ground-fault circuit-
interrupter protection for personnel. A motor overload is operation of a motor at
a current in excess of its nameplate rating which, if continued for a period of
time, would cause damage.
Fuses and circuit breakers can both be used for overcurrent protection.
Although faster acting, fuses must be replaced once they have performed their
function. They can be expensive and difficult to obtain, particularly those
intended to protect electronics. Recently, fuse manufacturers have reduced or
eliminated the DC rating of various types of fuses. The manufacturer should be
contacted before installing new fuses to protect a DC circuit; most manufacturers
have service representatives whose job it is to provide this information.
It
is important that selective coordination be provided between all protective
devices from the final overcurrent device protecting a branch circuit back to the
circuit breaker protecting the feeder. This ensures that the device closest to a
fault would interrupt the current first and that disruption to other branch
circuits is minimized. A study consisting of the time-current characteristics
curves for all of the devices involved, plotted on a sheet of full-log paper,
demonstrates that selective coordination is achieved for a branch circuit.
4.5.23.1 Feeder Protection. On bridge cranes, a circuit breaker is used as the
disconnecting means on the crane. It is selected based upon the load it is
supplying and is sized to protect the feeders. A fused disconnect switch may also
be used for this function. It is fed by the leads from the runway electrification
system; the runway electrification system is protected by equipment in the
building.
On portal, floating, and container cranes, the feeder supplied by the
generator is protected by a circuit breaker. It is selected based upon the load
it is supplying. This breaker also protects the generator itself.
4.5.23.2 Motor Branch Circuit Protection. It is required in NEC Section 610-42(a)
that motor branch circuits be protected in accordance with NEC Table 430-152.
There are exceptions to NEC Section 430-52 which permit ratings or settings
exceeding NEC Table 430-152's maximums so as to accommodate standard ratings of
protective devices and motors which have excessive starting currents. Standard
ratings for fuses and circuit breakers are listed in NEC Section 240-6. If a
circuit breaker is used for this protection, it is required to be an inverse time
type; an instantaneous trip breaker is not acceptable. If an inverse time circuit
breaker with adjustable setting is used to protect a motor branch circuit, the
breaker is required in NEC Section 430-110 to have a rating of at least 115
percent of the motor full load current.
The maximum rating or setting of protective devices is listed in NEC
Table 430-152 as a percentage of motor full load current. Full load current is
listed in NEC Tables 430-147 and 430-150 for DC and AC motors, respectively. Full
load current is required in NEC Section 430-6 to be used, rather than the
nameplate amperes of a specific motor, so that the motor can be replaced with
another motor, having the same horsepower rating, without necessitating the
replacement of overcurrent protective devices.
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