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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.

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    Is all the life support system 12 V DC? If so, why are you using an inverter? Why not backup with a 12 V battery? We'd still use your relay idea but at the 12 V level rather than mains. – Transistor Aug 03 '16 at 17:36
  • I thought this question sounded [familiar](http://electronics.stackexchange.com/questions/231383/two-identical-dc-power-supplies-in-parallel-for-redundancy). – Transistor Aug 03 '16 at 17:41
  • @Transistor - No, none of the equipment is 12v. All of it is 110VAC. And yes, that was my other question! I didn't want to switch the AC power at first, but there are too many variables in this setup for my liking. –  Aug 03 '16 at 17:43
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    There is absolutely no reason to switch ground. In fact, things will be safer overall to always keep the household ground connected. There's probably no reason to switch neutral, but it's ok if it makes you feel better. I'd use the extra pole to switch the battery feeding the inverter so it doesn't drain it when you don't need it. – DoxyLover Aug 03 '16 at 18:02
  • @DoxyLover - The only reason I was considering switching ground is because my inverter has fuses, and I assumed any fault would blow the fuses and stop power flow. If I grounded to my house ground and there was some kind of fault, wouldn't that ground my battery bank to earth on fault? –  Aug 03 '16 at 19:39
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    I would leave your inverter ground, load ground and house ground hard connected together. Unless you have a wiring fault, the only way your house ground could cause problems would be with an extreme event like a direct lightning strike and in that case, you're likely to have worse problems. – DoxyLover Aug 03 '16 at 23:30
  • @DoxyLover - Thanks for your input. I consulted my inverter's manual and discovered that there was a grounding terminal on the back side of the unit that is designed for connecting to house ground. I'm assuming now that the ground leg of the outlet would simply connect back to this ground. I believe the correct way to go is the way that you mentioned: connect the ground from all loads to house ground. Thanks again for this input. –  Aug 07 '16 at 14:30
  • @DoxyLover I have ended up implementing a system not unlike what KalleMP suggested. I have, however, took your advice and grounded everything to house ground. I am also using the third pole to switch the 12VDC to the inverter. I have to use another relay because the 3PDT inverter has a low current rating (in the context of 12VDC), but the system works very well. Thanks again for your input. –  Aug 27 '16 at 13:45

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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

KalleMP
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    Thanks for the input. The system I was envisioning is actually strikingly similar to [this](http://www.reuk.co.uk/OtherImages/auto-switch-from-inverter-to-mains-circuit.jpg), which came up as a result in the search you suggested. –  Aug 07 '16 at 14:32
  • That is about the simplest configuration. It is not ideal if you have large loads that are close to the maximum your inverter or DC supply can deliver as load induced volt drops may cause it to drop out the relay causing the voltage to rise and pull in the relay again, some deliberate hysterysis is always a good idea (a relay does have some, more when operated on DC). This will run through the inverter whenever it is on causing waste heat. If you drive the coil from the utility side instead the switching should be more positive as well. – KalleMP Aug 27 '16 at 19:17