Overcurrent Protective Devices
The low-voltage overcurrent protective devices are direct-acting (electro- mechanical) trip (series trip) and static (electronic) trip. These overcurrent protective devices are used in the power circuit breakers as discussed above.
Direct-Acting Trip
The direct acting overcurrent trip device is also known as series trip, electro- mechanical and dashpot trip device This device utilizes the force created by the short-circuit current flowing through it to trip its circuit breaker by direct mechanical action. These devices are operated by (1) an electromag- netic force created by the short-circuit current flowing 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 field 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 nonadjust- able. 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 com- bination of the three types is available to provide protection for overcur- rents. 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.
The time delay bands are accomplished by the action of the solenoid’s pull against springs, pneumatic, or hydraulic devices. Since these elements are purely mechanical, different characteristics cannot be supplied in a single trip device. Although some calibration points and some effects on the time can be changed by adjustments, completely different delay bands can be selected only by physically changing out the trip devices with others of the desired type. Direct-acting trips are still used on some applications, and are still required on power circuit breakers used on DC unit substations.