Protective relays Inverse time

This is the simplest of all relay operating principles. Any current above, or voltage below, a set level may be taken to mean that a fault or some other abnormal condition exists inside the zone of protection. Figure 2 shows a definite time and an inverse time overcurrent relay. 

When high voltage motors are being considered, it is usually found that the minimum conductor size of the cable is determined by the let-through fault withstand capability rather than the full-load or starting current. Cable manufacturers provide graphical data for fault withstand capabilities of their cables, which are based on practical tests. These aspects are also associated with the protection system used for the motor, e.g. a contactor-fuse combination, a circuit breaker, the protective relay characteristics (thermal, inverse time with or without instantaneous or earth fault elements). 

The star-point connection to earth is provided with a current transformer and a sensitive relay. The relay (51 G) is of a definite time delay or inverse time delay type so that it can be graded as back-up protection to earth fault relays at downstream feeders and consumers. 

Inverse, in which the time of operation is dependent upon the magnitude of the current, and such relays can be arranged to protect equipment in one of the following ways ... 

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