Schematics Simple Single Phasing Preventer Circuit PCB | Electronic Circuits

Schematics Simple Single Phasing Preventer Circuit PCB

Many circuits of single phasing preventor (SPP) are available but the circuit suggested here is very simple and economical.

Three-phase motors and other appliances are widely used in all sectors of industry. These appliances are prone to damage due to single phasing. Apart from damage to the costly apparatus, it may also cause a production loss.

Working Principle
Circuit diagram of  Single Phasing Preventer : Click on image to enlarge

In this single phase preventer circuit diagram, Easily-available mains step-down transformers X1, X2, and X3 (230V AC primary to 0-12V, 500mAsecondary rating) are used with their primaries connected in star mode and secondaries in open delta mode. The characteristic of this type of connection is that when three-phase balanced input is applied to the primaries, no output across open delta secondaries will be available. But in case of major unbalance or single-phasing, some voltage, called residual voltage, is induced in the secondaries across points 1 and 6 shown in the circuit diagram. Three-phase supply is given to apparatus (load) through contactor C. While the primaries of transformers X1, X2, and X3 are connected ahead of the contactor. The contactor can be energised via N/C (normally closed) contacts of relay RL1 by pressing switch S1.

As soon as single-phasing or major unbalance occurs, 8 to 12 volts are induced across point P1-P2, which after rectification operates relay RL1. As a result, the supply to contactor coil is cut off and it de-energises, thereby protecting the apparatus. Lamp L(d) is also lit up (unless B phase has failed), indicating that SPP has operated. Lamps L(a), L(b), and L(c) indicate the healthiness of three phases R, Y, and B. After resumption of the balanced 3-phase supply, the contactor will automatically energise (with S1 closed) and supply to the appliance will be resumed.


1. In the actual circuit for warded by the author, the transformers X1, X2, and X3 primaries as well as switch S1 were connected after the contacts of contactor. As a result energisation of contactor was not feasible. Even when switch S1 was shifted to a ‘live’ phase, relay RL1 (as well as contactor C) was energising/deenergising in quick succession during single-phasing and causing sparking—for obvious reasons.

2. The relay was also changed from 12V to 6V rating, as 12V relay was not energising properly with single-phasing.

3. Proper polarity of the transformer connections has to be ensured in the above circuit. To determine proper polarity, connect the primary ends which are eventually to be connected to three phases, to any single phase (the other ends are connected to neutral). Now proceed to connect secondaries of two of the three transformers in series and measure the AC ouput across the unconnected ends. This should be double (24V AC) of the individual secondary output (12V AC). If it is not so, reverse one of the two secondary connections to get the required output. Similarly, connect the third transformer secondary in series with the other two secondaries. The output across the unconnected ends should now be treble (36V AC). If it is not so, reverse the connections of the third secondary. Now shift the primary ends (connected to single phase) to each of the three phases, as shown in the figure. The voltage across points P1-P2 will be nearly zero if all three phases are present.

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