IEC 60255-22-7 specifies the test method used to verify the immunity of measuring relays and protection equipment to power frequency disturbances. Such disturbances arise in service when a relay is installed close to a busbar carrying heavy current, or when the auxiliary circuit runs parallel to a power conductor over a long distance; capacitive and inductive coupling then imparts a 50 Hz or 60 Hz voltage onto the signal and auxiliary wiring. The standard defines the test source, the series injection arrangement and the values of induced voltage that constitute the severity levels, distinguishing between common-mode and differential-mode application. It describes the test layout, the terminal connections for each protected port and the duration of each applied level, and it requires that the relay function be monitored continuously so that any spurious operation is detected. Acceptance is expressed through the performance criteria declared by the manufacturer. The test is normally performed together with the 1 MHz burst, fast transient, surge and high-frequency conducted tests that make up the complete disturbance programme for protection equipment.