The most typical protective devices installed are overload, over current, reverse power, earth fault, differential proportion etc. Injection tests are carried out to check the operation of these protective relays and the breakers. There are two types of injection tests that are normally conducted – primary injection test and secondary injection test. Primary injection test is conducted by injecting the current that is required for the operation of the protective device power through a circuit breaker. The secondary injection test, on the other hand, is conducted when the circuit breaker, though closed is not carrying any current through the main poles.