ABB RET650 Technical Manual page 254

Section 11
Secondary system supervision
248
• The magnitude of the phase current in the same phase is higher than the setting
IPh>
• The circuit breaker is closed (CBCLOSED = 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
CBCLOSED input to FALSE. In this way only the first criterion can activate the delta
function.
1MRK 504 135-UEN A
Technical manual