Argus 7 is a combined check and system synchronising relay. It can automatically select check or system synchronise from measurements of the relative phase angles between line and bus voltages. The relay will prevent closure of the circuit breaker if the phase angle, slip frequency or magnitude of the incoming and running voltages fall outside prescribed limits. If the parameters are within limits, the relay will issue an output which can be used to close the circuit breaker either directly or in conjunction with an autoreclose scheme.