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Figure 1: Type MCTI 39 Relay withdrawn from case.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 |
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