Setting the Instantaneous Overcurrent Protection (I or ANSI 50)
Rules for Isd also apply to the Ii threshold.
|
Application |
Principle |
Usual value |
|---|---|---|
|
Secondary side of MV/LV transformer (switchboard main incomer) |
Selectivity with downstream circuit breakers |
Ii mode: OFF if other MasterPact circuit breaker downstream Ii = 15 x In if Compact NSXm, Compact NSX, or PowerPact B-, H-, J-, L-frame circuit breaker only downstream |
|
Feeder with selectivity with other MasterPact, Compact NS or PowerPact P- or R-frame circuit breaker downstream |
Same rule as for Isd |
Ii mode: OFF |
|
Feeder without selectivity with other MasterPact, Compact NS or PowerPact P- or R-frame circuit breaker downstream |
– |
Ii mode: ON Ii tripping mode: Standard Ii = 10-15 x In |
|
Primary side of LV/LV transformer |
– |
Ii mode: OFF |
|
Generators |
– |
Ii mode: OFF |
|
Power electronic (for example, uninterruptible power supplies, variable speed drives, photovoltaic inverters) |
Lower than minimum short-circuit or ground-fault at the end of the protected circuit. Lower setting possible as no selectivity or transient current is expected. |
Ii mode: ON Ii tripping mode: Fast Ii = 2 x In |
|
Motor |
Lower than minimum short-circuit or ground-fault at the end of the cable. Lower setting possible above starting current. |
Ii mode: ON Ii tripping mode: Fast Ii ≥ 13 x Full load current of motor |
Ii setting allows normal transient overcurrent inrush current for transformers:
A 1st peak 10 to 25 x In
Motor direct on line starting current:
NOTE: MasterPact MTZ1 L1 type circuit breakers are equipped with an additional fast instantaneous trip set at 10 x In.
oIf used for the protection of the supply side of a transformer, the risk of trip during energization must be considered.
oFor motor application, select according to motor starter coordination tables.
