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