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Single Phase Preventor Circuit

We all know that for operating heavy electrical loads three phase power or AC is required in order to make the functioning efficient and viable. However this necessitates the presence of all the three phases at all circumstances. If any of the phases fail, can cause catastrophic consequences to the connected systems. The following artice offers a simple yet effective solution for tacking the above conditions.

As discussed above, a three phase load such as an industrial heavy motor will require the presence of all the three input AC mains phases for reliable and correct operations.

If there's any discrepancy with the presence of the input phases, the motor might start operating under heavily stressful and abnormal conditions.

This might cause huge current consumption, heating of the winding and ultimately burning of the motor parts.

The circuit of a single phasing preventor shown below can be effectively used for eliminating all kinds of undesirable consequences that might result from an abnormal three phase issues.

In the diagram we can see the use of three transformer/relay driver stages.

The transformers can be the normal step-down types, rated appropriately for switching the connected relays.

One of the input primary terminals of all the transformers are made common and connected with the neutral line.

While the other terminals of each transformer are joined to the respective first, second and the third phases of the input mains.

However the above connections are done cleverly via the relay N/O contacts of the subsequent relay assemblies for implementing the required single phasing prevention.

Initially when the set-up is integrated with the the three phases as per the given connections, the phases are remain cut off from the output load, because the relay contacts are all open.

On pressing the given push button, the particular phase in the line is allowed to reach the second or the middle transformer primary winding.

The middle transformer instantly operates its own relay, whose contacts just like the above relay connects the second respective phase with the primary of the bottom transformer, which finally operates its relay powering the top transformer.

Once this happens the entire system gets latched via the N/O contacts of the relays such that now even if the push button is released the system continues and sustains the voltages across the outputs and to the transformers.

Now suppose if any of the phases become low or fails, the particular transformer in line instantly deactivates its relay and the whole system of relays break down in sequence, immediately halting and disconnecting the output loads.

Thus the system effectively prevents the loads from operating under the absence of any of the phases making it sure nothing goes out in fumes.

The circuit was designed exclusively by me, I guess so, if it's already been discovered kindly provide me with the link:

single phase preventor circuit

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  1. Fine if the motor loses a phase winding. But that suggests the motor is stuffed anyway.

    If the grid loses a phase then the motor generates the missing phase. Which keeps the relays energised.

    So I don't think this works. An engineer might be able to confirm this for you.


    1. Thanks!

      I have seen in factories the use three different colored incandescent AC bulbs for indicating the different phases. When a phase goes, the particular bulb stops illuminating providing the necessary visual warning.

      So as per your suggestion does it mean if motors are involved the bulbs would remain ON even with missing phases??

  2. how can i power a small 10 - 6 watt a/c water pump with a 20 watt - 10 watt solar panel and 12v 7amp battery

    1. You can try this circuit....

  3. Can you please design me a pwm charge controller for solar panel that has lvd and over charge protection based on 555 ic should work on 12v 10 amp battery and above plz plz plz
    Thank you

    1. I'll try to design it, and inform you soon....

  4. Thank u so much by the way my name is deepak, will it still charge the battery if the voltage drops ?

    1. You are welcome Deepak!

      I cannot say right now, I'll have to assess the design thoroughly first.

  5. hi,thankyou for the ckt... can you please tell me the estimatted price for the transformers and the relays??


    1. Hi,

      The transformers might cost Rs.50 to 70/- each. Price of relays will depend on its contact current rating.....

    2. thankyou..

  6. dear Sir, i have know from someone, we can make communication using 230v power ac current line` can you give me an intercom circuit to talk with my friend using current line, there is about 5 km between our houses, plz

    1. Dear Thilana, 5km distance looks too large, I don't think that may easily to room communication may be possible though,

      I have posted one similar circuit here, which you can read for more info:

  7. Hi..... I have one doubt please tell answer. I have one single phase preventer, is it act as a over voltage protection...... If we give 485v to input, it will ON or not....please say answer for it anyone....

    1. Hi, no it will not protect from high will switch ON at 485v if all 3 phases are present


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