Mains power restored alarm, non contact

If you have installed a transfer switch and have access to a backup power source such as a generator or a battery bank and mains inverter, then you can safely switch power for the whole property between a grid supply, and your backup source. The transfer switch does this safely by ensuring the power transfer is "break before make" - forcing the disconnection of one source before the other can connect. This ensures there is no danger of your local power source attempting to back feed the grid supply - something that at best would overload your local generation capability, and at worst could kill or injure someone working on the grid supply.

One shortcoming of using a transfer switch is that once the grid connection is disconnected, how can you tell when the power cut is over? This article explores a small project to make identifying when the grid mains power is restored easy.