DIY auto transfer switch?

[leadbelly]

Any experience with making one's own transfer switch between 2 say 120VAC 15A lines automatically on fault? I've tried to google this but haven't found anything other than examples for gensets, and I would think it would be much simpler and less expensive when scaled down.

 

[ticknpop]

You can use two oversized relays with an extra pole on one of them.

The "preferred" AC source powers the coil of Relay A. The closed Contact of 1 pole of relay A powers the load.
The extra contact of relay A is in series with the coil of relay B and is wired to be open when the relay A AC source is present. The open contact of Relay A prevents the coil of relay B from being powered by the B power source.

When "A" source fails the auxiliary contact on relay A closes allowing the B source to power the B relay and power the load from the B source.

Whenever the "A" source returns the load is transferred back to the " preferred" AC source. All transfers are automatic and the break time between sources depends on how fast the relays open and close - about 50 to 100ms is common