The 7SG18 is a numeric three phase differential protection used to detect in zone phase and earth faults. The relay is applied to overhead line and underground cable circuits as well as shorter circuits such as interconnectors.
The relays have integral back-up overcurrent/earth fault protection and intertripping channels.
Relays (and associated CTs) are installed at each end of the protected circuit. The relays are connected together with a fully supervised fibre optic or metallic pilot wire protection communications link.