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I have a backup power bank made of an inverter and car batteries. It charges when the grid power is on and turns on when the grid power turns off. However, my "electrician" repurposed the grounding wires from a switch to provide the power to select sockets and loads around the house, so that only they turn on — in an effort to converse energy.

I know how terrible an idea this is, but my "electrician" whose trade isn't regulated in my developing country does not. He doesn't understand the difference between AC and DC either, and believes that the grounding wire is redundant.

My question is: What is the right way to deliver backup power?

Should I install the output of the backup to where the mains power input is? If so, the backup is 2 car batteries, how do I protect appliances like my air-conditioner from being under-powered?

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  • It's important for us to know what country you're in. May 26, 2019 at 1:34
  • Where are you on this planet? May 26, 2019 at 2:29
  • Also, what make and model is your inverter? May 26, 2019 at 2:29
  • @ThreePhaseEel My starter for 10 is India...
    – Solar Mike
    May 26, 2019 at 3:37
  • 1
    This is not safe - I would supply particular circuits only at the fuse box or supply board and never interfere with the earth or ground.
    – Solar Mike
    May 26, 2019 at 3:40

1 Answer 1

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It's pretty easy with a simple backup power system like yours, which has a separate battery charger from the inverter.

You have two panels -- a main panel and a sub-panel.

The main panel is on utility power only. You put circuits in there which only power from the utility (don't ever need to be powered from battery). This includes the battery charger.

The sub-panel is on a transfer switch which can place it on utility power OR battery power, but never both at once. This is necessary for the safety of linemen, but also makes sure you aren't powering up the whole neighborhood, which would flatten your battery fast!

The utility side of the transfer switch is fed from a circuit breaker in the main panel. That protects the wiring to the transfer switch from overload.

The various circuits that you want to be able to power off battery, are installed in this sub-panel. *Note that you get to choose which circuits are powered by battery, simply by switching their circuit breaker off when you throw the transfer switch to battery mode.

The North American style transfer switch

Now in North America, our service panel designs allow you to have a subpanel with the transfer switch/interlock built right in; this is a Siemens/Murray design that straps 2 circuit breakers together, and then those breakers are back-fed. The interlock includes a bar that won't allow both breakers to be on at once. The kit costs only US$30 (plus two large breakers $10-40 each). Very economical. In your case, you could actually use exactly that same kit as a simple interlocked transfer switch, just using a very small 4-space subpanel that has just enough room for the interlocked breakers. Since North America uses 2-pole power, wire it Philippine style, using the 2 poles as hot and neutral. That way you are switching neutral for maximum safety.

The interlock can then feed onward to your usual DIN rail+RCD type subpanel. Remember you will need an RCD inside that subpanel, otherwise you will have no RCD protection when the device is thrown to "battery".

This only works for simple battery systems with a battery charger separate from the inverter. (the diodes in both of them prevent backfeeding onto the line). If you get a modern system like a Tesla PowerWall where the battery charger and inverter are integrated and use a single connection, this gets a lot more complicated. I would avoid that if I could.

The better way - straight battery

An inverter itself is a heavy load. It takes a lot of battery power simply to keep the inverter "spun up", even if it's not driving any loads at all.

Many loads can be run directly off battery voltage without the need for an inverter - lighting, modem, router, a few PC power supplies, some thermostat controls, etc. It's a good idea to do this as much as possible. If you can eliminate the inverter altogether, you'll get better use out of your batteries (need a smaller pack or get better runtime out of the pack you have).

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  • You can get a changeover/transfer switch in a standard DIN mount form factor. E.g. this if 32A is enough, or one of these May 27, 2019 at 3:06
  • There should be no need to switch neutral (and it is specifically forbidden to switch the mains neutral here in NZ, though you may switch inverter/gen neutral). Keeping it all in one board potentially reduces the multiple supplies hazards. May 27, 2019 at 3:07
  • It depends a awful lot on the neutral-ground bonding involved. You have to switch neutral if the alternate supply would create a second N-G bond. Often an issue with portable generators. May 27, 2019 at 4:48
  • At least here in NZ, that's forbidden and the rules require removal of any N-E link in the generator (and presumably inverter) if connected to a permanent installation. MEN link must be in main switchboard. May 27, 2019 at 5:29
  • Meanwhile, in Canada... That is a 2-breaker style interlock using 3-pole breakers (intended for 3-phase). The panel has an extra "breaker row" at the top; its stab goes to neutral. They swerve out of their way to switch neutral. The reason is probably that people are constantly using portable generators which have their own N-G bond. The same could apply to OP depending on his inverter. May 27, 2019 at 16:18

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