I have a battery backup setup for my aquarium that I'm trying to make more robust. I'll go ahead and ask my question first, then you can decide if you want to read all the background info.
Is it safe to use a 3PDT relay to switch between grid power and the output of an inverter? My plan would be to attach the load to COM, the inverter output to NC and the grid to NO. Under normal operation the coil would be powered and the relay would be open, supplying grid power to COM. On grid failure, the relay would close, supplying inverter power from NC to COM. I would plan on using a 3PDT relay so I could switch ground, hot and neutral to minimize the chance of grid power backfeeding into the inverter. If all three are switched at once, my thoughts are that it's either all inverter or all grid going to COM, never both.
Also, sort of an aside, are mechanical relays designed to be used in this manner? The load on COM with different inputs feeding NC and NO? I don't see why it wouldn't work, but I'm not an electrician and I've never done this before, so I don't know!
Now for the background. The basic idea is that an inverter powers the critical life support equipment on my aquarium. The inverter is powered by a 12V power supply when the grid is on. On power failure, a relay switches the inverter's DC source to a battery. When the power comes back on, the relay switches back to the 12V supply, the battery is recharged by a battery maintainer, and all is well.
My problem with this design is there are several weak points. First, the inverter which powers the critical life support equipment could fail. This renders the entire system inoperable. Whether the grid is on or the batteries are charged, it doesn't matter, because the inverter failed. The second problem is the switching relay. If the relay fails on NC, then the system runs the batteries flat and fails. If the relay fails on NO and the grid goes out, the system fails. The third source of failure is the 12V power supply that powers the system when the grid is up. If this fails, the system will switch to battery (detecting this as an outage) and drain the batteries, at which point the system fails.
Switching the AC outputs of the inverter and the AC from the grid directly would solve a lot of the headaches mentioned above. The only question would be if it's safe to do so.
Best Answer
If you are considering connecting anything that might send power even accidentally into the mains network you want to be on your best electrical behaviour.
Generally what are used is two switches or contractors that are mechanically (and or electrically) interlocked so that your and the utility supply cannot ever connect to each other simultaneously. It prevents many dangers.
Check out the following picture search and see if something looks interesting. There will be DIY, off the shelf (OTS) and custom (PLC if you like) systems to do what you want.
https://www.google.com/search?q=automatic+mains+backup+relay&tbm=isch