by the short-circuit current fl owing through it to trip its circuit breaker by
direct mechanical action. These devices are operated by (1) an electromagnetic
force created by the short-circuit current fl owing through the trip device
coil (the trip coil is usually connected in series with the electrical circuit or in
some instances to the secondary of current transformers) or (2) a bimetallic
strip actuated by the head generated by the fault current. The bimetallic strip
is usually connected in series with the circuit.
A combination of thermal (bimetallic strip or equivalent) and INST magnetic
trip is commonly used on molded-case breakers to provide time delay operation
for moderate overcurrents (overloads) and INST operation for high- magnitude
of short-circuit current. The thermal trip is usually nonadjustable in the fi eld or
there are some devices that have limited range of adjustment, such as 0.8 to 1.25,
whereas the instantaneous (INST) trip is available as adjustable or nonadjustable.
The adjustable-trip range varies from low to high with several intermediate
steps. The number of steps available may vary for different designs and sizes.
Direct-acting trips on insulated-case and heavy-duty power circuit
breakers are of the electromagnetic type. Three trip devices are available:
(1) long time delay (LTD), (2) short time delay (STD), and (3) INST. Any combination
of the three types is available to provide protection for overcurrents.
A trip device is installed in each phase of the electrical circuit. The
LTD, STD, and INST trip devices are available in minimum, intermediate,
and maximum time bands to facilitate the coordination of various trip
devices in series. All these units have adjustable settings.