Mains changeover switch to generator backup

[David]

Prompted by the power cut thread.

As far as I can tell if you want to be able to power your home from a generator if the mains power goes off then you need a changeover switch to isolate the mains before you fire up the generator.

A quick and nasty search of Amazon suggests that these may not be very expensive - perhaps around £100.

You would presumably then need an approved installation to provide a feed in from the generator and a guaranteed isolation from the mains. I assume a competent electrician could do this.

Any feeling for the cost of this installation?

Dodgy calculation suggests that a 60A mains feed would require roughly 13 kW of generator power to maintain supply, but I am assuming that this would not be the aim. Target to be able to run the fridge and freezer, lights, and central heating pump and controls.

Electric fires, electric cookers etc. not catered for.

If power is out then the internet is presumably out as well, and probably mobile phones, so basic amenities only required.

Cheers

Dave R

[Brian]

To run just ?essentials? ( keeping fridges / freezers going - although the latter will be ok for a reasonable time- lights, ch pump, perhaps a tv, computer etc, you don?t need a huge generator. Remember, not everything needs to be run at once. You can even have the generator converted to run on gas. I run mine on propane but I could run it on natural gas.

As for change over switches. They are easily available - manual and automatic. Some motorhome people fit them either to use with generators or inverters.

[Owain Lastname]

The difficulty is arranging load shedding so that unnecessary circuits are locked out when in generator mode.

And during a power cut you cannot rely on the supplier's earth so you may have to add a TT earth in conjuction with TN-S or TN-C-S (PME) from the supplier.

Owain

[ARW]

I used

outside on the last domestic genny supply I fitted and used 6mm T&E from it to the changeover switch.

I them made up an appropriate lead using arctic flex (with a NE link in it and fitted an earth rod.

Customer would then wheel the genny out the garage and plug it in when required. It could run their oven or a ring or two on the cooker. Electric fires not needed as the gas CH should still be on

[Jethro_uk]

Don't forget to check and test it ... monthly ? Weekly ?

Back in the 70s, my Dad rigged up a series of car batteries that drove the gas CH and some 12V workshop lights through the house. I remember him reminding my Mum to keep them charged when the power was on.

[Theo]

It would be possible to put the changeover downstream of the CU, so you're only feeding a specific circuit rather than all of them. But the problem is often that the circuits aren't laid out nicely, and there will be some loads on them that might cause issues - for example heaters or hot water on timers, as well as inrush current to SMPSUs. Those might trip a generator if they weren't locked out. Also there may be standby loads you don't want to power.

Another option would be to run a separate circuit for loads that want to be run in generator mode, but that would be disruptive. I wonder if you could do something instead with smart switches?

Theo

[Brian Gaff (Sofa]

I see no o reason not to keep the internet router and a low power computer of some kind. Hardly high wattage compared to heating and cooking. You might dedicate a separate ring for such devices and only have to switch that so there is no problem with needing some huge switch. I think I'd stick to a manual switch system, obviously break before make is needed. Brian

[Nick Odell]

I'm not your dad am I?

Back in the 70s when power cuts were almost daily occurences I had to rewire our house and so I took the opportunity to put in a parallel

12v system powered by car batteries stored under the floor in the cupboard-under-the-stairs.

I like to think that I single-handedly solved the UK energy problem because once the 12v system was up and running we didn't have another power cut for years.

Nick

[ARW]

How? I cannot visualise this at all. Enlighten me please.

[Mark Carver]

By killing all the breakers on the CU, and connecting the generator to a

13A socket (or whatever) on just a single circuit ?

The breakers would need to be double pole, or it could get 'tricky' ?

(None of that would be regs compliant of course)

[ARW]

That is sort of what I did at my parents for there pure sine wave inverter.

There is a 12V car battery that they keep charged in the garage powers the inverter. The then connect the extension lead I made and run it across the garden to an inlet as in my earlier post. On the inside of the house at the back of the inlet is a socket that only becomes live when this is done.

There is then another extension lead to run out from this socket with sockets on it at appropriate places such as a couple of table lamps, TV and finally the boiler that I swapped it's supply from a switched fused spur to a unswitched socket.

It gives them light, TV, heat (and as it's a combi HW) and just enough juice to power a travel kettle or the microwave on a 700W setting for long enough to make a brew or hot meal. The battery will of course not last long - however they can recharge it from their cars in the garage.

The idea of this one was not for long term power cuts with 24/7 power but to heat the house have a cup of tea and something warm to eat and if Emley Moor is still broadcasting watch shit on the TV.

The main thing in their case was to keep the house warm and keep the lights on. It could also probably give the freezers a boost if needed.

[ARW]

I was thinking of regs compliant!

[Mike Humphrey]

That's quite straightforward. Disconnect the circuit from its circuit breaker, and wire it instead to the output of the changeover switch. Wire one input of the changeover switch back to the breaker in the CU. Wire the other input to the generator.

When the power goes off, move the switch over. That circuit is now connected to the generator (and not the mains). Everything else is still connected to the mains (only) and so is dead until power is restored. There's no cross-connection as the mains input of the switch is isolated from the generator input.

Perfectly regs compliant as long as you remember:

- provide a local earth rod

- RCD on the generator circuit (you're now running TT earthing)

- MCB suitable for the circuit on the generator feed (or an RBCO to also address the point above)

- the required warning label about multiple supplies.

You could also go for an in-between option - split the supply to two CUs. One is fed direct from the mains, the other via the changeover switch. This allows you to have lights on a separate circuit to the boiler etc while still having both fed from the generator when needed.

Mike

[Fredxx]

Is this still necessary where the incomer is TN* variety? As long as the earth is experted to the generator.[1]

Most Consumer units will be wholly RCD protected, or even RCBO. Does the generator need another?

I might at least suggest a visual means of knowing power had been restored.

[1] In the case of a known neutral fault then obviously this won't work. and the earth will need to be isolated from the network.

[Dave Plowman (News]

It's what I've done with the inverter which only feeds the boiler radial.

Used a changeover relay which disconnects it from the CU, and connects to the inverter output.

[Andrew]

Just use these :-)

[John J]

Maybe it would be an option (granted more work involved) to have a second consumer unit serving only critical circuits such as boiler, lights and a suitably placed socket(s) for kettle, and IT equipment? A changeover switch of sufficient rating between this and the primary consumer unit would enable easy connection of an alternative supply in the event of a power cut. I used to have an old 30 Amp 3 pole MEM ironclad changeover switch lying about in the shed but it went missing years ago. Our oil boiler and controls are connected by a 13A plug so I have the option of dragging out the generator to outside the window and feeding the oil heating via an extension lead if a power cut looks like dragging on for an extended period. Being sans mains gas we've currently got a propane gas fire and gas hob for cooking if required as my geny is only 1 kW so an electric kettle wouldn't be practical unless I bought a bigger geny. I've not looked into the inverter options but oil boilers do take a brief higher current demand during the ignition phase which might be more than cheap inverters can handle?

[tony sayer]

or

JFK Electrical have them on Amazon cant do a link;(..

07962109794 137 Bush Road coalisland Dungannon Tyrone BT71 6QQ GB

[John Rumm]

With TN-C-S and a loss of supply, it is possible to lose the combined Neutral and Earth as well (imagine a tree bringing down a LV overhead wire). That could leave your main earth terminal floating.

(typically they use PME with multiple earth connections along the route to try an ensure this never happens)

Likewise a TN-S setup could get cut and you lose the separate earth connection to the substation. (although depending on where the cut is, the cable sheath alone might make a reasonable TT earth if it's an old metal clad cable)

Not necessarily... unless you need to protect the main feed from the generator against earth faults.

Earth connections should not generally be "switchable". If going the separate CU route then making that whole CU TT might be an option (if possible - it can be tricky when you introduce main bonds to water supplies etc if they are in metal pipes - you might just find you have got a defacto TN-C-S earth again via someone else's TN-C-S neutral earth bond).

[ARW]

Yes that is one option

And that has

Short circuit, overload and overheat protection

and

Please note: Ensure the device is well within the inverter output power rating. We recommend using an inverter capable of supplying a minimum of

3 times the device's rated power to avoid damage to the inverter by any current surge (especially for fridges and freezers which can surge up to 7 times the rated power)