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.

If used for the protection of the supply side of a transformer, the risk of trip during energization must be considered.
For motor application, select according to motor starter coordination tables.