Garage electrical supply: advice welcomed

[Bert Coules]

Well, I suppose if you're going to be an MCB with a flaw, you might as well make it a whopper.

Bert

[Andy Burns]

Counterfeits are scarier ...

[ARWadsworth]

I would have been a lot easier if you had asked yeasterday.

I had a second hand SAIP box that I fitted this morning:-(

[Tim Lamb]

Yes. Ta.

regards

[Bert Coules]

Ah, timing is everything... Thanks for the info about the SAIP unit using a standard DIN rail fitting. I checked out the MCBs today's: they're 80mm high x 18mm wide x 67mm deep. The central raised portion on the face (the bit that sticks through the aperture in the faceplate) is 45mm high. So all I have to do is look at the various makes of MCB to find one with the same dimensions.

There is one thing about the wiring of the garage CU which I'd like to check: at the bottom of the plastic casing is a nut and bolt and a securing plate. The earth wires for the garage lights and sockets are both bolted to this connector, and so is a substantial individual earth cable which runs outside to a ground pole. But the incoming earth - the one on the cable from the house CU - isn't connected to the same terminal. Instead, it's looped around one of the screws which hold down the DIN rail.

I assume this is so that the metal parts of the garage CU are earthed via the house, since they have no connection at all to the earth terminal in the garage CU. Is this correct?

If my description's not clear, please do say and I'll post a sketch.

Many thanks,

Bert

[ARWadsworth]

Yep, and I'll measure a few to get some recommendations.

Have a look at

seems that your workshop CU has been installed as a TT supply.

Is the workshop CU a plastic one? How far is the workshop from the house CU and are there any buried metal parts in your workshop (eg waterpipes or structual steel)?

[Bert Coules]

Thanks for the link: I'll go through the article in detail (though even on a cursory glance, I see that my earth rod should be protected by "a suitable enclosure and a warning notice", neither of which it has.

Yes, it's all plastic. The only metal components (apart from those in the RCD and MCBs and the various securing screws) appear to be the DIN rail and the nut/bolt/plate earth fixing.

At its nearest point, 13 feet. At its farthest, 29 feet.

No, nothing like that at all.

Bert

[ARWadsworth]

Well that bit is easy. I have just gone to my stores and sat there was a SAIP 2 way CU.

Now the 32A MCB in it is made by BG, it fits perfectly and it's all yours if you want it.

[Bert Coules]

Adam, that's enormously kind of you: I've sent you a private email.

Bert

[ARWadsworth]

Very few are:-)

The DIN rail would not normally be earthed.

So the workshop would not need to be TT due to equipotential bonding.

The workshop could have been made TT to provide discrimination between the workshop and house RCDs.

Hopefully someone else can offer other reasons why you have such a set up. There does not seem to be anything wrong with the setup (indeed it seems better than most apart from the 16A MCB and only one socket).

Finally, can you comfirm that the 4mm T&E runs all the way from the house CU to the workshop?

[<me9]

A dimension you might come unstuck on is busbar height, manufacturers vary.

[Bert Coules]

Well, the MCB will be replaced with one of a higher rating (thanks again) and I can upgrade the radial circuit to a ring, as you suggest. So as long as the present earthing setup will still be satisfactory, I shall be happy to keep it.

Yes, it definitely does, in one unbroken cable.

Bert

[Bert Coules]

Ah, right; thanks.

Bert

[ARWadsworth]

It should be fine. You can also use a 4mm 32A radial if you wish.

So that means there is no problems regarding the house MCB overloading.

[Bert Coules]

I think I'll go with the 2.5mm ring: the layout of the garage is suitable and the smaller cable is slightly easier to work with.

Excellent. Many thanks.

Bert

[Bert Coules]

Thanks to the generosity of Adam Wadsworth, my garage/workshop electrics are now up and running: with the old 16A radial circuit upgraded to 32A, my chop saw no longer trips the MCB on startup.

Now I'm all set to plan out the new socket placements and new lights and install the extended wiring for the ring circuit.

What a splendid resource this group is.

Bert

[Bert Coules]

Perhaps I could ask one more slightly naive electrical question:

I like the answer of installing a master-switch to kill the power to all the sockets on my planned 32A workshop ring circuit. Is there any reason why I shouldn't put a 45A cooker switch with a neon indicator into the ring? Would that work?

Many thanks,

Bert

[John Rumm]

"In" a ring it would only cut power to one of the legs. Although slightly unconventional, you would need to wire a radial to the switch and both legs of the ring from its output.

A more conventional approach for this sort of setup is to use a contactor in the CU with the output from it being latched via one or more emergency stop buttons. That way you can hit any button to kill power to the sockets.

(I thought about doing this in my workshop, but concluded in the end that it was probably not worth it, since I am usually working alone and each machine has its own NVR switch where required).

[Bert Coules]

Naive question again, but wouldn't that be enough to prevent any plugged-in machine from being accidentally activated? That was my main concern.

I'll be in much the same position, so it's possibly not worth doing for me, too.

Thanks for the speedy reply.

Bert

[ARWadsworth]

Would it not turn it into a radial and not cut any power?

You could also take the ring from the CU and put both "legs" into the supply side of the switch with the workshop ring supplied from the load side.

I wondered if a switch would be a little OTT.