Are we talking sprinklers to water the lawn, or sprinklers to put fires out?  I would have thought that the latter was a bit more safety critical!

Nicely put! :-)

I can see the point. If you have to take the UPS offline for maintenance, can you maintain the mains supply in the meantime?

I do not quite understand, "the feed goes straight through the UPS". Surely the mains and UPS should form the legs of a Y with the stem supplying the sprinkler system. Then if either is offline, a supply is maintained. Your 3-way switch needs to provide supply 1 on, supply 2 off; supply 2 on, supply 1 off; supply 1 and supply 2 on.

supply 1 and supply 2 on.

If that means effectively shorting the UPS output to the mains supply, I'd suggest avoiding that situation. Firstly unless the UPS is designed for parallel operation, there's no guarantee its a.c. output is synchronised to the mains (with a resulting flash, bang and magic smoke escape) when the switch is closed. Also you really don't want local generation potentially feeding back into the mains both for isolation and load reasons.

Another alternative to a manual change-over switch might be an automatic transfer switch.

   - Andy.

supply 1 and supply 2 on.

If that means effectively shorting the UPS output to the mains supply, I'd suggest avoiding that situation. Firstly unless the UPS is designed for parallel operation, there's no guarantee its a.c. output is synchronised to the mains (with a resulting flash, bang and magic smoke escape) when the switch is closed. Also you really don't want local generation potentially feeding back into the mains both for isolation and load reasons.

Another alternative to a manual change-over switch might be an automatic transfer switch.

   - Andy.

A good point. I was assuming that the UPS fires up only if mains power is lost, and closes down when mains power is restored.