In residual current monitoring or RCM, all three phases and the neutral conductor are passed through a summation current transformer. The summation current is ideally zero or close to zero.
If a residual current flows to ground, a residual current monitoring device detects this and can trigger an alarm or report it accordingly. Even slowly rising residual currents that are still too small to trigger alarms or fuses can be detected. This allows even insidious faults to be detected.