UNIT PROTECTION SYSTEMS

Unit protection systems are of special significance for arc flash reduction. Differential relaying is one form of unit protection system and is discussed first in this chapter. It is noteworthy that the overlapping of the differential zones of protection is achieved by proper location of the current transformers, and constructing a zone of protection so that one zone overlaps the other and no area is left unprotected. The importance of arc flash reduction with differential protections is evident. Electromechanical single‐phase high impedance relays have been popular in the industry for the last 50 years. Electormechanical low‐impedance relays have been in used in the past for protection of generators. The chapter shows that a bus differential zone is created by a current transformer (CT) on the main incoming circuit breaker source side and the CTs located on the load side of the feeder circuit breakers.