Protection for two consumer units

[jim_in_sussex]

In the last stages of upgrading the supply to the garage/workshop.

The elec board tails (25mmsq) come into an existing 100A fuse & isolator (mine not the board's) which then go to the house CU (about

3m away). I put in a Henley block after the isolator to split off the garage supply. From the Henley block I put in a new 100A isolator & 63A MCB (type C) then ran 16mmsq SWA cable (3 core including earth) to the garage. At the garage end this will go into a new 100A isolator & then into a new split load CU.

Result is 5 queries bothering me:

A type C 63A mcb was used on the assumption that it would discriminate against the type B MCBs in the CU (& also because TLC didn't have a or couldn't supply a 100A MCB). Is type C correct here?

Should the old 25mmsq cable to the house CU be protected at the Henley block end? To do that would need a 100A MCB: What's best here? [there's no room for a bulky switch & fuse).

Is the garage end (new isolator & then CU) OK? or is further protection needed to protect the new garage supply at the incoming point?

The old 100A fuse & isolator at the incoming end is large. Could this be replaced by a 100A isolator & MCB? Or some other alternative?

Can any (split load) CU be used for the garage end or does it have to be special - weather proof etc? [BTW the garage is dry, but what do the regs require?].

TIA for all opinions

[BTW nothing is connected yet beyond the new 100A isolator after the Henley block]

[Lurch]

On 30 Dec 2004 15:16:00 -0800, jim_in [e-mail] (jim_in_sussex) strung together this:

You don't need any discrimination between parallel MCB's. I shouldn't need to explain the fundementals of current flow here, should I?

No. It's protected by the company fuse, as it was before the Henley block went in.

Yes, that's fine.

You could use a 100A DP switch and 2 module DIN enclosure. do them.

Anything will be fine.

[Christian McArdle]

As I understand it, you have:

1 2 3 4 5 +---+ +---+ +---+ +---+ +--------+ | |--| |---| |----| |----| | +---+ +---+ +---+ +---+ +--------+ | +---+ +---+ | |=========| | +---+ +---+ 6 7 1 = supplier's fuse 2 = supplier's meter 3 = your 100A switchfuse 4 = henley block 5 = house consumer unit 6 = C63A MCB + enclosure + 100A isolator 7 = garage CU

In which case, I would say that the C63A MCB is not required and should be removed, due to the risk of lack of discrimination. Provided the cable marked '====' is good enough to be short circuit protected by the 100A switchfuse, you'll be fine. Should this not be the case, then I would calculate the protection required for short circuit protection and use an HRC fuse carrier to provide it, as it is more likely to discriminate.

Another alternative, which reduces the chances of the garage taking out the switchfuse and bringing the house into darkness is to put the Henley block before the switchfuse and have another switchfuse to support the garage supply. This could use your existing C63A MCB, but I'd be tempted to use a

63A HRC fuse unit.

I would use an RCD incomer, or alternatively RCBOs for every circuit, but see below.

You could find a smaller switchfuse.

If you're going for split load, then replace the isolator incomer with a

100mA time delay RCD and use TT earthing. If the garage is physically attached to the house, then this is not necessary. If the garage is dry, then any consumer unit will do. If TT earthing is used, then it must be an insulated plastic type, not metalclad.

Christian.

[John Rumm]

The way I read this, the garrage CU will be downstream of the 63A MCB. If that is the case then there will be a need to discriminate.

I have just been playing with these numbers for my own setup. Have a look at the thread from a counple of days back called "CU Wiring scheme suggestions"