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.