I installed a relay for switching purpose With main supply and Generator. When there is no electricity from main supply relay change its coil to the generator. I want to know which will be the source of electricity when both the generator and main supply is providing electrical energy to relay.

The person installed a relay is electrician. I was preferring the Changover Switch (manual) for generator, main supply and cut out power (OFF). But he said after starting the generator you have to change the changover switch manually. By relay you only have to ON or OFF the generator and Relay will automatically trip the coil when there is main line power and if there is no main line power by starting generator it trip the coil and power will be of generator. I am confused that when both main line and generator are giving power to relay what will relay decide, which power it will transfer to home appliances.

I am confused what should I do now, Am I at risk what are the cons or what he did is right? enter image description here


  • What does the datasheet say?
    – PlasmaHH
    Apr 21, 2017 at 10:12
  • 1
    It looks to me like main power will activate the relay to feed main power to the house, and a lack of main power will mean the generator is connected to the house instead. So if both are available you'll get main power to the house and the generator will be wasting fuel. But without the relay data sheet that's just an educated guess.
    – Finbarr
    Apr 21, 2017 at 10:15
  • 2
    Your electrician is incompetent. An automatic transfer switch is much more than just a DPDT relay. (Besides which, that particular relay doesn't look anywhere big enough to handle an entire house --100 to 200 amps.) In order to protect the loads in your house, a transfer switch requires logic to evaluate the quality and stability of both power sources, and implement suitable time delays in order to prevent excessively frequent switching. Of course, if you install a manual switch, that "logic" is you!
    – Dave Tweed
    Apr 21, 2017 at 11:53
  • Since we do not know how the system is wired there is no way to say someone was incompetent. I have seen reversing motor contactors used in a manual system that prevent both from being turned on at the same time using a relay for control power to the contactors. In both cases they were approved by the AHJ.
    – Ed Beal
    Apr 21, 2017 at 13:18

2 Answers 2


This is not a listed transfer switch and shouldn't be used for automatic transfer of power.

The "electrician" that sold you this should know better.

Manual transfer can be accomplished without this relay but if you want to do automatic transfer you need a listed transfer switch.

Good luck!


If the electrician sold this system to you, you should have made sure that you understood what you were buying before buying it. The electrician should have supplied some instructions in writing. However, the description you gave is probably complete.

The relay is simply energized by main power. That connects the house load to main line power if main power is available. If there is no main line power, the relay is not energized, causing the house load to be disconnected from main power and connected to the generator.

The only thing that is unclear it whether or not the generator has its own relay to prevent generator power from being connected to the transfer relay until the generator output is fully established. That may or may not be necessary. The generator manual should say something about that. If not, ask the electrician or the generator supplier.

Which current will pass through the relay when the generator and main supply are on?

If the main supply is is on, it will always be connected to the house load. It does not matter if the generator is on or off.

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