Protective relays are used in conjunction with medium voltage circuit breaker (above 600 volts) to sense an abnormality and cause the trouble to be isolated with minimum disturbance to the electrical system and with the least damage to the equipment at fault. They have the accuracy and sophistication demanded by the protective requirements of the primary feeder circuits and larger electrical equipment. Protective relays are designed to be responsive to an abnormal excursion in current or power flow, etc. Each relay application requires custom engineering to satisfy the parameters of its particular intended function on the system.
The more common protective relay is of the electromechanical type. That is some mechanical element such as an induction-disc and induction cylinder of magnetic plunger is caused to move in response to an abnormal change in a parameter of the electrical system. The movement can cause on contact in the control circuit to operate tripping the related circuit breaker. Protective relays should be acceptance tested prior to being placed in service and periodically there after to insure reliable performance. In a normal industrial application, periodic testing should be done at least once a year.
Relay Testing Procedure:
Visual check and inspection of relay
Electrical Testing
Primary Circuit Test -High Current / Low Current (Primary injection)
