Differential current

Forward and return conductors in the summation current transformer

The differential current , measured at one point in an electrical system, is the vector sum of the currents of all active conductors at this point.

Active conductors are those conductors that carry current in regular operation, i.e. outer conductors ("phase") and neutral conductors ("zero"), but not protective conductors or equipotential bonding conductors . Ideally, the differential current would have to be zero, since the currents flowing into and out of the system should exactly balance each other out.

Fault currents (e.g. due to defective insulation of an extension cable that allows a current to flow into the ground) or leakage currents (e.g. due to line filters ) result in deviations from the ideal value.

This can be compared with leak monitoring of a pipeline , in which a flow meter at the beginning and end of the pipeline records the volume flow of the medium. If less comes out at the end than was pumped in at the beginning, there must be a leak. The easiest way to measure residual currents is to use summation current transformers , which are also used in residual current protective devices (RCDs) or in residual current monitoring devices (RCMs).