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I have two UPS 10kva 220 volt used for controlling system. I need one ups online and the other one as backup. If the Grid supply out the main ups pick up the load and if the main ups fail to do that the backup ups will. So how do I setup this configuration electronically instead of electro-mechanically? Note: for now I am using two electromagnetic relays connected as interlock.

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  • Welcome to EE.SE. Most electrical codes make it mandatory to use mechanical relays to prevent backfeed into the power grid. – Sparky256 Sep 28 '18 at 23:10
  • Yes. An interlocking relay will also prevent both systems from shorting outputs to eachother. – K H Sep 28 '18 at 23:33

You want an automatic transfer switch

The device you want is called an automatic transfer switch (ATS for short) and should be readily available through an electrical supply house (or equivalent) in your area. Note that this device will need to be approved by your local electrical authorities.

A proper ATS not only has electrical protection, but mechanical interlocking of the contacts, making it impossible for the two sources to be inadvertently coupled together. It also has logic to avoid transferring between sources when they are not phase-synchronized with each other, which can cause serious problems to loads that are expecting a smooth sine wave instead of a sharp "jump", such as AC motors.

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