The 7SR18 is a numeric three-phase differential protection used to detect in-zone phase and earth faults. The relay is applied to the overhead line and underground cable circuits as well as shorter circuits such as interconnections. The relays have integral backup overcurrent/earth fault protection and inter-tripping channels. Relays (and associated CTs) are installed at each end of the protected circuit. The relays are connected together with a fully supervised fiber optic protection communications link. Relays have four current inputs and are housed in E6 cases. Integrated logic is provided to allow the optimal configuration of relay functionality with the relays analog inputs and binary input/output circuits. Protection and instrumentation functionality is accessible via the data comms channel – a front USB, RS485, and IEC 61850 (optional) rear ports.