Testing Overcurrent Relay:

Test benches are used for proving the design characteristics of relays. These are provided with accurate timing apparatus and calibrated current and voltage supplies. The basic circuit for Testing Overcurrent Relay is shown in Fig. (8.1).

The harmonic content in the current wave form is kept to within 2% of the fundamental by adjusting the current through a choke which has an impedance at least six times that of the relays under test. Interchangeable matching transformers are used for differently rated relays, so that same burden is imposed on the control circuit for all current ratings.

The test current is adjusted to the desired level with the relay short circuited by a normally closed contact on the starting contactor. By energizing the starting contactor the short circuit across the relay is released and the timer starts. Finally the timer stops when the relay operates.

The method adopted for load adjustment allows sufficient time for the current to be accurately set without overloading the relay, and also gives a steady ammeter reading immediately on transference of the current to the relay since the pointer does not have to rise from zero.