The Reyrolle 7SR23 DAD is a high-performance numerical high impedance circulating current relay featuring four current inputs. Designed for unit protection applications, it delivers fast and dependable detection of phase and earth faults, making it ideal for safeguarding critical infrastructure such as busbars. 

A High impedance protection is the preferred solution for applications where faults must be cleared rapidly while maintaining secure discrimination. It offers highly sensitive fault detection, enabling lower operating settings and superior stability during through-fault conditions compared to many alternative protection schemes. 

The inherent stability of a high impedance scheme is achieved by ensuring the relay operating voltage exceeds the maximum voltage that can develop across the relay circuit during through-fault events. An external series stabilising resistor (RSTAB) is used to establish the required operating voltage and maintain dependable system stability. 

To further enhance protection system resilience, external non-linear resistors (Metrosils) are connected in parallel with the relay circuit, limiting transient overvoltages and helping to safeguard the relay and associated equipment. 

In built CT Supervision (CT50) continuously monitors CT secondary wiring and connections, helping to identify wiring faults and abnormalities before they impact protection performance. 

Typically used alongside three-phase differential protection schemes, CT supervision enhances system security by detecting faults in the CT secondary circuit that can create unwanted unbalance currents during normal operating conditions. By identifying these conditions early, it helps prevent spurious operation and unnecessary tripping of the protected zone, improving overall protection reliability and availability.