Automatic generator changeover switch (with schematic)
This is a classic bigclivedotcom teardown of an automatic generator changeover switch – which is used, as the name implies, to automatically switch from a failed mains supply – to a generator, once the generator has fired up. The (main) clever detail of the switch is that it isolates the two supplies from each other – so that the generator supply can never pass back onto the mains circuit – which could have life-threatening consequences for any utility workers attempting to restore mains electricity to a customer.
Overall, I think Clive was impressed by the simplicity/efficiency of the device’s design and construction. As always, he advises against using a device bought from a potentially untrustworthy supplier/manufacturer – in a commercial environment as liability, in the event of a failure, could be impossible to pass back to the supplier/manufacturer. For a garden shed installation, by someone that accepts those limitations, this device may provide a cost effective solution.
This was a lot of fun to reverse engineer. Mainly because it’s very old-school inside. It’s an automatic changeover switch for a backup generator supply. When power fails and the generator starts, the switch will automatically transfer over to the generator. When the power returns it will switch back over to the main supply.
The issue with the relay being energised all the time and to make it generator-priority could be addressed by simply swapping the sides the main and generator power are attached to. That way the generator would take priority in the event of a power failure and not switch back until it was turned off, and the relay would only be energised when the generator was running.
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