hello to everyone Distance relaying should be considered when overcurrent relaying is too slow or is notselective. Distance relays are generally used for phase-fault primary and back-up protectionon subtransmission lines, and on transmission lines where high-speed automatic reclosingis not necessary to maintain stability and where the short time delay for end-zone faults canbe tolerated. Overcurrent relays have been used generally for ground-fault primary andback-up protection, but there is a growing trend toward distance relays for ground faultsalso.Single-step distance relays are used for phase-fault back-up protection at the terminals ofgenerators, as described in Chapter 10. Also, single-step distance relays might be used withadvantage for back-up protection at power-transformer banks, but at the present suchprotection is generally provided by inverse-time overcurrent relays.Distance relays are preferred to overcurrent reIays because they are not nearly so muchaffected by changes in short-circuit-current magnitude as overcurrent relays are, and,hence, are much less affected by changes in generating capacity and in systemconfiguration. This is because, as described in Chapter 9, distance r