Differential feeder protection requires a comparison of the currents entering and leaving the protected zone.
Two MBCI relays are therefore required, one for each end of the feeder. A pair of pilot wires is used to transmit information between the two relays so that each may be able to compare the current flowing at its respective end with the current the other.
The pilot wire carries analog circulating current - allowing correct operation even for low specification or ageing pilots.
When applying this protection to overhead lines, the limiting factor is generally the length of the pilot circuits: for cable feeders the limiting factors are more likely to be the level of line charging current and the method of system earthing.