ABB Relion 650 Series REB650 Technical Manual page 162

Section 9
Secondary system supervision
156
The input signal DISCPOS is supposed to be connected via a terminal binary input to
the N.C. auxiliary contact of the line disconnector. The DISCPOS signal sets the
output signal BLKU in order to block the voltage related functions when the line
disconnector is open. The impedance protection function does not have to be affected
since there will be no line currents that can cause malfunction of the distance
protection.
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Technical manual