Application Relay types MCTI 39 and MCTI 40 are three phase or two phase and earth fault instantaneous overcurrent relays and the MCTI 19 is a single phase relay. The relays have been designed to be applied as current detectors in breaker fail protection schemes. The MCTI 40 is particularly suitable for breaker fail applications where single/ three pole breaker tripping is possible, since control of the current detector is provided on a per phase basis. Simple application diagrams are shown in Figure 2. Figure 1: Type MCTI 39 Relay withdrawn from case. fault current under normal circumstances. A particular feature of the relay is a fast reset. This is important in a scheme such as that shown in Figure 2, Alternative ( b) since it will be possible to set the time delay closer to the breaker trip time. For details of a suitable time delay relay see Publication R6012, relay types MVTT 14 and MVTT 15. L B B F C A B D E In the substation single line diagram shown above: if breaker A fails to remove the fault at F, breakers B, C, D and E should be tripped simultaneously by breaker fail protection for breaker A, to isolate the busbar. The breaker fail timer relay should be set in excess of the time required for the breaker to clear the