•
The magnitude of the phase-ground voltage has been above
VPPU
for more than 1.5
cycle
•
The magnitude of DV is higher than the setting
DVPU
•
The magnitude of DI is below the setting
DIPU
and at least one of the following conditions are fulfilled:
•
The magnitude of the phase current in the same phase is higher than the setting
50P
•
The circuit breaker is closed (52a = True)
The first criterion means that detection of failure in one phase together with a current in
the same phase greater than
50P
will set the output. The measured phase current is used
to reduce the risk of false fuse failure detection. If the current on the protected line is low,
a voltage drop in the system (not caused by fuse failure) is not necessarily followed by
current change and a false fuse failure might occur.
The second criterion requires that the delta condition shall be fulfilled in any phase while
the circuit breaker is closed. A fault occurs with an open circuit breaker at one end and
closed at the other end, could lead to wrong start of the fuse failure function at the end with
the open breaker. If this is considered to be a disadvantage, connect the 52a input to
FALSE. In this way only the first criterion can activate the delta function.
Section 10
1MRK 506 335-UUS A
Secondary system supervision
338
Technical manual
Содержание REL650 series
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