generator change over switch

[Stephen]

Hello,

Does anyone know of a change over switch to switch between grid electricity and a local generator? TLC sell two, one is:

at 32A; the other is rated at 63A.

The plan is to have two consumer units. The main one would be grid connected only and supply most circuits. The second, smaller, CU would be switched between grid and locally generated power. This second CU would have three 6A MCBs: one for lights upstairs, one for lights downstairs, and one for the CH boiler.

Now set-up like this the maximum load is 3*6=18A. Since this is well below the 32A rating of the switch, can I use this switch? My concern is that the CU RCD is rated at 80A as are the tails to the CU. If someone added more MCBs in the future, for the freezer for example, this could take the load above 32A and the switch would go bang before everything else.

Should I protect the switch with a 30A fuse to prevent this? Or would it be the responsibility of whomever added to the system in the future to make the necessary changes and either add a fuse or upgrade the switch?

I feel a 80A switch would be the best solution, if you know of one?

I appreciate this kind of work needs a qualified electrician and I will do so but it's nice to understand what is involved so that I can understand when I get quotes and different people tell me what they will do and for how much. It also means I will know what they are on about and whether what they say needs doing or whether I am being talked into extra unnecessary work!

Thanks.

[Miichael Allen]

Stephen

Take a look at this switch:

more expensive option, but designed just for the job by those who know about generators.

I use this switch (with a 6.5kVA generator) wired into my main Consumer Unit. When the power fails, I switch all circuits (via their MCBs) off, power up the genny, switch over, and then select which circuits to turn on.

Obviously lights up and down, plus the ground floor ring main (serves freezer and boiler ).

url:

[Dave Osborne]

The 32A switch is fine as long as: -

1. The mcb in the grid cu is no larger than 32A.
2. The mcb in the generator is no larger than 32A.

This means that a maximum of 32A can be drawn through the switch irrespective of the current carrying capacity of the cu or its tails.

However, since you would power the generator CU from a 32A outgoing way on the grid CU (via the changeover switch, natch), you wouldn't need

16mm2 or 25mm2 tails on the generator cu; you could just use 4mm2 T&E.

HTH Rumble

[Stephen]

Thank you. I wasn't thinking of doing it like that: I was going to take the tails into a Henley block and then run separate feeds to the grid CU and small CU. The problem then was that the feed from the Henley block would be fused at 80A, much higher than the rating of the switch.

Your solution is much tidier: I will not need the Henley block, nor a fuse before the switch, so less boxes to put on the wall. And like you say, I can use, smaller, more manageable cable and run from a spare

32A MCB in the grid CU.

Thanks very much,

[Stephen]

Sorry to add to my own reply but I have just thought: isn't 4mm^2 T&E frowned on in this group because it has a small CPC? Would 6mm^2 T&E be better? I know CPC size is important; I remember reading the "taking electric outside" in the FAQ but OTOH we're probably talking a couple of metres at most running from one CU to the other via the switch.

One disadvantage of running the second CU from the first is that both have RCDs. Wouldn't this mean that if something caused the small CU to trip it would also trip the first CU? I suppose I could specify the second CU to have just a switch rather than an RCD but that would still result in the main CU RCD tripping on faults in the second CU, and by not having an RCD in the second box it means there's no RCD protection when fed from off-grid electricity. I suppose the ideal solution would be to upgrade my main CU to a split load one and feed the second CU from the un-RCDed part.

At the moment it is one RCD for everything, which I know this group doesn't like. How do split load CUs fit in with the 17th edition? Isn't everything supposed to have an RCD now?

Thanks, Stephen.

[Dave Osborne]

Over a short distance it makes no odds. Use 6mm2 if you prefer.

I know CPC size is important; I remember reading the

No. Under some circumstances, they would both trip; under others, only one would trip (and it could be either!).

I suppose I could specify the

True, but that's the way it works now, so you would be no worse off.

True, but since you're probably only going to do lighting, the boiler and the freezer from your genny, you may consider that having the genny RCD protected is not a high priority. Also, you could protect the genny by an RCD before the changeover switch if you wanted.

I suppose the ideal

Absolutely, if you chose not to bring your CU installation up to 17th standards.

Only because of its potential nuisance value.

How do split load CUs fit in with the 17th edition?

Well, notionally, yes, all final circuits do under the 17th. However, the feed to the second CU is a sub-main, so no RCD protection is required (unless you have a TT supply of course).

John Rumm or Owain will hopefully be along in a bit to comment further(I'm not really up to speed on the 17th).

[Stephen]

I was going to look up the equation and double check, but that's what I thought because it's such a small length.

Which could be a nuisance, though hopefully RCD trips should be very rare.

But perhaps now is the time to fix that since I'll be having the other work done at the same time. OTOH how much of a problem is it having everything on the one RCD because RCDs shouldn't trip that often.

That's true but two boxes (CU and RCD) probably will cost twice having one RCD'ed CU so it might be simpler to do it all in one box.

Can you get such an upgrade now thought? Surely everything has to be done to 17th ed. now?

I've had a quick look through Screwfix and it looks as though some

17ed boxes have two non-protected circuits before the RCD. I think you are supposed to use RCBOs in those though.

I notice Screwfix sell Crabtree type-B MCBs but their RCBOs are type C. Why is that? Why aren't they both the same type?

I am also puzzled because there seem to be split load boxes with two RCDs. How does one trip without taking out the other?

Are they electricians? I never knew that. I'll pay special attention to their posts.

Thanks, Stephen.

[Dave Liquorice]

I was going to mention screwfix as a flyer came through the door showing= a Wylex split load CU complete with 2 RCDs, Main switch, and MCBs plus som= e white electrical bits single pole double sockets and pendant fitteings f= or =A3100...

'cause they aren't wired in series? I would assume that the incoming mai= ns goes through the main switch then splits to each RCD. This gives you the= required 30mA shock protection on all circits whilst maintaining some circuits if one RCD trips.

[Dave Osborne]

Have a look at this document:

is no general requirement that you have to bring an electrical installation up to the current regulations, however, there are circumstances in which you have some obligation to improve your installation.

In particular, if you renew your consumer unit, you have to do so in compliance with the latest version of the regualtions and at the same time ensure that your main equipotential bonding is up to scratch.

They're pretty much de-rigueur.

Well, yes, but only because of a new requirement for circuits to be buried 50mm deep or have mechanical protection or be RCD protected means that for replacing CU's, RCD's/RCBO's galore is the way forward.

Maybe. As I said before, only final circuits need to be protected. Sub-mains don't.

Up until the 17th edition, RCBO's were almost exclusively used in industrial environments, where Type C MCB's are very common.

I suggest you consider

for your electrical supplies.

The RCD's are not daisy-chained. There is effectively a Henley block inside the consumer unit.

John is a clever bugger who is up to speed on a number of subjects.

Owain is (afaik) an electrician who I for one never seem to disagree with.

If Christian McArdle were still around, I would recommend him as well, but he dropped out some years ago. Anyone know what happened to him?

[Stephen]

Thanks. I haven't got a split load box so hadn't realised the busbars were in parallel; that explains it.

[Stephen]

Thanks for the informative post. So it would appear that though other aspects of the installation need not be upgraded, you must always upgrade CUs to the latest regs?

Can only the electricity company upgrade the earth connection from the CU to the meter?

I was thinking of the old split-load with one half without a RCD but if I have to go 17th ed, these are now out of the question (so why are they still being sold?)

I am already a happy TLC customer ;)

Thanks, Stephen.