Two CU's in 500 Year old house!

[FattyMatty]

Hello All, We are just about to start renovation work including an extension on a 500 Year old house, and I think the electrics must have been installed when it was first built they're that bad!

At the moment there is a new(ish) 100A Feed (Installed bout 6 years ago) marked "PME System". The earth comes from the Old wylex board, through the box marked "PME" and back out to an earth stake outside.

1) Is the earth stake necessary? I take it that the system is TN-CS but I didn't think you needed a stake?

2)
The old wylex board runs:
3x Ring main (32A)
2x Lighting main (6A)
1x Shower (32A)
1x Cooker (45A)

This board is going to be replaced with a proper CU with a 100A Breaker then daisy chain to a second CU in the extension.

Main CU MCB's
- 3x Ring Main (32A)
- 2x Lighting Main (6A)
- 2x Shower (32A)
- 2x Cooker (32A)
- Daisy Chain to 2nd Board (45A)

2nd CU MBC's:
- Outdoor Shed (10A)
- 2x Lighting Main (Outside) (6A)
- Outdoor Sockets (16A)
- Indoor Lights (6A)
- Ring Main (32A)
- burglar Alarm (6A)
- Door Bell (6A)

The two boards are 20m Apart, was going to use 10mm(sq) T&E to link the boards, + a 6mm(sq) Earth from CU to CU bonded mid way to new plumbing.

Does this sound Kosher? I understand it would be easier to use one big board but the first CU is at one end of a long thin house and the 2nd CU will be at the other end and don't fancy a rewire!

Cheers, Matt

 

[TopTrumper]

Matt, have you looked at the maximum load of this set up?
Also, your question about the earthing arrangements is not something a spark should ask?
have you got a test meter?

 

[Steve]

The "daisy chain" (AKA sub main) should be rated as high as you can manage in your CU. Use a Memera board, as these can take 63 amp accessories if i remember right! Then use 16mm to link the two boards protected by a C63 (normally ok on a PME earth - your electrician will verify).

A better arrangement would be to take both CU supplies to a henley block then to a main isolator.

The main switch in the main CU is just that - a switch, not a breaker of any sort.

It is normally better to have a seperate main earth terminal near your PME terminal, where all earths and bonding conductors go to.

Did you ask your electrician about RCD protection?

 

[ricicle]

Then use 16mm to link the two boards protected by a C63 (normally ok on a PME earth - your electrician will verify).

Why would you want to use a type C, Crafty?This would only be required for loads with inrush currents.

 

[FattyMatty]

Cheers for the responses.

I was thinking of doing as you said using a henley block, with 63A RCD's isolating each CU (Not using a DP isolator anywhere in the system). But I would have to use a 100A RCD between the henley block and the meter to stop the total max load !=> 100A (2x 63A).

Is this a valid way of doing it if I use 16mm Tails to the main CU and 16mm T&E to the 2nd CU (20m away)?

Cheers again

Matt

 

[Steve]

Then use 16mm to link the two boards protected by a C63 (normally ok on a PME earth - your electrician will verify).

Why would you want to use a type C, Crafty?This would only be required for loads with inrush currents.

Plenty of discrimination. And I dont do wishy washy submains Design it right to start with. Big and butch. Jus' 'cos I can

 

[Steve]

Cheers for the responses.

I was thinking of doing as you said using a henley block, with 63A RCD's isolating each CU (Not using a DP isolator anywhere in the system). But I would have to use a 100A RCD between the henley block and the meter to stop the total max load !=> 100A (2x 63A).

Is this a valid way of doing it if I use 16mm Tails to the main CU and 16mm T&E to the 2nd CU (20m away)?

Cheers again

Matt

How much electrical experience have you?

If you have equally rated RCDs protecting both CUs, you'll end up with two RCDs in series on the second CU. This is totally pointless.

You'll also have an RCD protecting the whole house. Which is also pointless.

And RCDs DONT react to overcurrent situations! This is the job of an MCB!

There is no need to put a 100 amp MCB device at the mainswitch, as the system is already protected by the suppliers fuse (why do you think its there?).

A MUCH better solution would be RCBOs on the circuits which will need them.

 

[FatGit]

The best solution for this guy would be get someone else in.

 

[FattyMatty]

Ok

Back to my original idea. The 2nd CU wont need to draw more than 45A, so put a 16mm T&E (to be on the safe side) from the 2nd CU to a 45A RCBO in the Main CU.

Bring all the earths to a central block then into the PME & Earth Stake.

Replace the main CU with a split load box protecting the rings on a 63A RCD with the rest behind the 100A isolator?

Sound Ok?

(As for the suppliers fuse, I wanted something we have have access to protecting the system before the suppliers fuse)

 

[Steve]

If you're going down the split load route, get 2 of them. And feed the submain from the NON-RCD side of the main CU.

Only sockets that could be reasonably expected to provide power outdoors need to be RCD protected. So ground floor sockets.

You dont need anything besides the supplier's main fuse to protect the system. An isolator is sufficient, so YOU have a single isolation point. The chances of a 100 amp device activating is VERY small in domestics anyway. Most people will never experience their main fuse blowing.

I ask again, do you have any experience of this sort of work? Do you know about part p?