pplication Notes
P74x/EN AP/N
1, P742, P743 (AP) 6-
3. ID>1 Alarm Timer to not operate for an external fault shall be greater than the longest
protection time (such as line, overcurrent, etc…)
4. Slope k2 (ID>2) recommendation is 65%
To be always stable in the worth CT ratio conditions (between the biggest CT and the
smallest CT). 60% is OK as long as the CT ratio is less than 5.
1. ID>2 shall be lower than 80% of the minimum phase to phase fault current to operate
sub-cycle for the minimum fault and higher than 120% Iload Max (120% to allow 20%
margin) and if possible equal to 6 x (ID>1) to be insensitive to the worth CT saturation.
2. IDCZ>2 same as ID>2
3. Slope kCZ (IDCZ>2) recommendation is 30%
The requirement is to be able to trip for a fault that is counted twice by the Check Zone (for
example one and half circuit breaker substation)
2.1.1.3 87BB SDEF CU Settings (High Impedance Earthed Schemes Only)
An Excel spreadsheet tool called “Idiff_Ibias_SDEF_HighImp “is available on request to
assure a reliable setting choice:
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