ABB Relion Technical Manual page 342

Section 11
Secondary system supervision
336
• 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.
1MRK 502 048-UUS A
Technical manual