Some designs have used (and may still use) two sets of current sensors,
or one set of sensors with dual secondary windings, in order to derive a
signal to monitor and another to act as the power supply. Additional sensors
may be required for a function that direct-acting trips could not offer—
ground fault. Both three-phase, three-wire; and three-phase, four-wire
ground fault detection systems are offered. The signals from either the three
or four sensors are processed to determine if all INST currents add up to
zero (Kirchoff’s current law applied to three-phase AC systems). Therefore,
it should be apparent that if grounding conductors are used, they should not
be included along with any neutral connections. When current returns to its
source via a ground conductor, the monitored currents no longer add up to
zero and the trip device activates. The connections and current sensors used
for a three-phase, four-wire, plus ground conductor on a feeder breaker are
shown in Figure 8.1.