10
SEL Application Guide 2020-04
Date Code 20200326
DCB O
VERVIEW
A DCB scheme is a communications scheme in which an asserted overreaching element at the
local terminal must receive a blocking signal from the remote terminal to block a trip. When a
forward-overreaching zone (Zone 2) at the local terminal asserts, it trips at a high speed unless it
receives a block signal from the remote end. A reverse-looking zone (Zone 3) at the remote
terminal sends the blocking signal.
As
shows, if a fault occurs at Point F1, the overreaching Zone 2 element at Relay 1
asserts, and Zone 3 deasserts. Because Zone 3 deasserts, Relay 1 does not send a block signal to
Relay 2. The assertion of Zone 2 starts TIMER 1 (see
). If Zone 2 asserts for longer than
the programmed timer delay (coordination time delay [CTD]) in TIMER 1, the top input of AND 1
becomes true. If the top input of AND 1 is true and Relay 1 has not received a block from the
remote end, Relay 1 issues a trip. At Relay 2, Zone 2 asserts and Zone 3 remains deasserted.
Figure 9
Simplified Wiring Between the SEL-421-5 and SEL-311C-1 Relays
Figure 10
Simplified Wiring Between the SEL-421-5 and SEL-321-1 Relays
52A
SEL-311C-1
(Partial)
A01
A17
OUT101
IN101
A02
A18
52A
52TC
Circuit
Breaker
(Partial)
(+)
(+)
(–)
52A
SEL-421-5
(Partial)
B01
B33
OUT201
IN201
B02
B34
52A
52TC
Circuit
Breaker
(Partial)
(+)
(+)
(–)
52A
SEL-321-1
(Partial)
218
201
OUT1
IN1
217
202
52A
52TC
Circuit
Breaker
(Partial)
(+)
(+)
(–)
52A
SEL-421-5
(Partial)
B01
B33
OUT201
IN201
B02
B34
52A
52TC
Circuit
Breaker
(Partial)
(+)
(+)
(–)
