2.3 Phase-Loss or Single-Phasing Detection

Single phasing, i.e. phase loss, is a very common electrical fault experienced by three-phase systems and it occurs when any phase of the three-phase power supply is lost. The phase-loss fault occurs due to blown fuse, thermal overload, broken wire, wire contact or mechanical failure. In case of failure to detect phase loss in the three-phase systems in time, the connected loads and infrastructure can be seriously damaged.

The phase loss of the three-phase supply can be detected either by measuring the Root Mean Square (RMS) voltage of each phase or by monitoring the zero-crossings of the phases using the ZCD peripheral.

If the RMS voltage of any phase of the three-phase supply is 0, it is deemed as phase loss. In case of ZCD-based implementation, if the zero-crossing is not detected for any of the phases in a specific time duration, it means loss of the phase.

Figure 2-2 below shows the signal phasing on three-phase supply. L2 phase is lost, L3 phase lags L1 by 240° and the three-phase signal is 120° out of phase.

Figure 2-2. Three-Phase Signal with L2 Phase Loss