Primary and secondary current injection tests are normally conducted to check the operation of circuit breaker and their protective relays/devices.

The protective devices installed vary from circuit to circuit depending on the protection needs and philosphy but typical relays/devices include overload, over current, reverse power, earth fault, differential protection, etc., etc.

Secondary injection testing normally involves disconnection of the protective device from it’s normal VT/CT and connection to a specialist test set that can inject and measure/record the required operating signal directly into the protective device relay to cause it to operate the circuit breaker.