ANSI C37.90 is the American National Standard covering relays and relay systems associated with electric power apparatus, and it is the document under which a protection relay intended for the North American market demonstrates its ability to survive the electrical environment of a substation. The family addresses the disturbances that reach a relay installed in switchgear or in a control house: the oscillatory transient produced when a circuit breaker or disconnector operates and the current in the station wiring is interrupted, the surge that arrives over the current transformer and potential transformer circuits from a lightning stroke on the line, and the radio frequency field present in the switchyard while transmitters are in use. The best known element of the family is the surge withstand capability test, in which a damped oscillatory wave is applied across the terminals and from terminals to earth and the relay must not change state or suffer damage. Companion documents add the radiated and electrostatic discharge requirements.