Protective Relay Circuits
Protective relays are special power-sensing devices whose job it is to automatically open or close circuit breakers in large electric power systems. Some protective relays are designed to be used directly with large electric motors to provide sophisticated monitoring, shut-down, and start-up control.
One of the features of these motor-oriented protective relays is start-up lockout. What this means is the relay will prevent someone from attempting too many successive re-starts of a large electric motor. If the motor is started and stopped several times over a short period of time, the relay will prevent the person from starting it again until a sufficient “rest” time has passed.
Explain why a large electric motor would need to “rest” after several successive start-up events. If electric motors are perfectly capable of running continuously at full load for years on end, why would a few start-ups be worthy of automatic lock-out?
A differential relay is a common type of protective relay used in power systems. One of the more common forms is the differential current relay. A very common example of a differential current relay - so common, in fact, that nearly every house is equipped with at least one - is the GFCI, or Ground Fault Current Interrupter. Explain what a GFCI is, and then in a larger context, explain what a differential relay protects against.
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