• 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 9 1MRK 505 277-UUS C
Secondary system supervision
150
Technical Manual
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