In distribution systems where overhead networks with isolated neutral predominate, this type of transformers should be used as protection against ground faults. Basically they are three-phase Zig-Zag reactors, with accessible neutral for system grounding, of adequate power according to the expected ground fault current, as well as the time allotted for the respective relay to command the feeder breaker to trip.

The design current density in the winding is determined by the ability of the choke to withstand the mechanical stresses caused by the short-time fault current. The provision is offered in oil-bath machines.