Co-ordination principles and methods using fuse and relays

Protection objectives are, first to detect abnormal conditions, second to initiate action to isolate the faulty part of the system, and third to provide system information. The key factors are speed, discrimination, dependability, security, cost and post-fault switching, this paper being concerned with the first four of these. There are three types of overcurrent relay: (a) instantaneous; (b) dependent time delayed; and (c) independent time delayed. The advantages and disadvantages of each are outlined in relation to discrimination and co-ordination