pplication Notes
P74x/EN AP/N
1, P742, P743 (AP) 6-
5. ISOLATION AND REDUCED FUNCTION MODE
The scheme permits maintenance on the busbar and, or busbar protection whilst maintaining
some form of protection if possible. Two maintenance mode levels in the Central Unit and
two maintenance mode levels in the Peripheral Units allow this to be possible. A command
to one or more of the affected units will then force the scheme to a selected (reduced)
operating mode. The levels are as follow.
5.1 Normal mode
5.2 Peripheral Units (P742 and P743)
Two levels of command can be applied selectively to each peripheral unit.
5.2.1 CB Failure Protection (50BF) disabled
In this mode, all tripping information from external relay are not taken into account. The
topology algorithm forwards the feeder positions to the central unit as normal. As the
peripheral unit continues to monitor the analogue values the central unit will maintain a
balanced condition with the remainder of the system still in normal operation. However, the
CB failure backtrip information will not be sent to the Central Unit. The local protections
(Dead Zone, Overcurrent, Earth Fault) are still enabled and the PU is able to retrip the
breaker. However, the Peripheral Unit is able to react to a fault condition by creating a CB
fail condition and back tripping the zone(s) if the CU sends a trip order (87BB or 50BF
backtrip), to clear a genuine fault).
Central Unit: Normal mode
Topology,
Service
Bay
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