Connecting consumer units

[epager]

Hi All
I need to locate a new consumer unit approx 10 to 12 metres away from the mains supply. The existing old unit is less than 50cm from the supply.
My initial thought is to replace the existing old unit with a small unit housing just a 100A double pole isolator switch and run tails from this unit to the satalite unit, which will then act as the main distribution point.

However, everything I've ever known about tails is that they do not [usually] exceed 3 metres.

What is the generally accepted method of achieving my objective on this project (within the confines of current rules)?

Any suggestions gratefully received.

Thanks
Mike

 

[Jon Bridges]

The supply needs to be able to be isolated within 3 meters. No limit to length of tails after that. The only thing to look out for is that if they are to be burried in the wall they need to be protected by a 30ma RCD or mechanically protected. Obviously if you put a 30mA RCD on the CCU as well you get problems with discrimination.

 

[nickblake]

Put a small CDU by your incommig main ,you could protect it with an RCD but it would have to be time delayed to avoide neusance tripping

 

[Sav]

Hi,
I would put an isolator by incoming mains supply, then swa all the way to new ccu !!

Hope this helps,
Best regards,
Sav

 

[Guest123]

An isolator would not suffice as it's additional overcurrent protection via a switchfuse, thats needed as the DNO will not guarantee theirs over 3M.

 

[JUD]

An isolator would not suffice as it's additional overcurrent protection via a switchfuse, thats needed as the DNO will not guarantee theirs over 3M.

Thought that myself Lenny. Was just looking for some old threads to back it up.

 

[Sav]

Hi again,
As I understand it, when the new c/u is more than 3 meters away, then its upto the electrical installation designer to design a suitable distribution circuit (sub main) to connect to the c/u to the origin of the installation. This will normally be accomplished by providing, close to the origin,a device such as as a switch-fuse to permit the requirements of BS7671 for isolation, overcurrent protection and discrimination to be met for the distribution circuit.

Maybe I was not very clear by saying isolator ??
Thanks and regards,
Sav

 

[Guest123]

Terminology is key.

 

[epager]

Hi Guys, thanks for your replies.

I have asked this same question elsewhere and there appears to be favour for the use of an isolator switch with time delayed RCD. But here the suggestion is that this is not sufficient and should instead require a switchfuse. With such a device, isn't that just doubling up on the existing mains fuse and therefore technically unnecessary?

 

[Guest123]

As I said the electricity supplier will not guarantee the effectiveness of the main supply fuse over a generally accepeted distance of 3M, hence they insist that we install our own overcurrent protection.

 

[JUD]

Hi Guys, thanks for your replies.

I have asked this same question elsewhere and there appears to be favour for the use of an isolator switch with time delayed RCD. But here the suggestion is that this is not sufficient and should instead require a switchfuse. With such a device, isn't that just doubling up on the existing mains fuse and therefore technically unnecessary?

As Lenny said, DNO will only guarantee protection upto 3 meters.

An isolator and RCD will not provide protection against overcurrent and L-N short circuits.

A switch fuse rated lower than the main cut-out should be used to provide discrimination.

 

[epager]

Switchfuse it is then.
Thanks All

 

[epager]

Just when I thought I'd got it sussed...
There now appears to be healthy discussion on the cable type to connect from the switchfuse to the CDU.
So, taking a poll here, is swa necessary or would you consider standard tails cable acceptable?

 

[JUD]

Would depend on routes. If they're going to be buried in walls <50mm etc. I'd go with SWA otherwise you would need RCD/mechanical protection.

 

[Sintra]

IMO go for SWA to eliminate the need for additional protection for the sub main unless your tails would be run surface and do not require additional protection. Another option if additional protection is not required on the sub main is to use a split concentric cable.

 

[epager]

Picking up on a comment in a previous thread on this forum, is there any point running 25mm to the new CU if the incoming mains if fused at 80A ?
16mm would seem perfectly adequate.

(cable will be surface and under-floor)

 

[robsparx]

SWA every time. Really can't see any advantage in using tails

 

[SirKit Breaker]

Hi Guys, thanks for your replies.

I have asked this same question elsewhere and there appears to be favour for the use of an isolator switch with time delayed RCD. But here the suggestion is that this is not sufficient and should instead require a switchfuse. With such a device, isn't that just doubling up on the existing mains fuse and therefore technically unnecessary?

The idea of a switch fuse is that it will be fused at a lower rating than the supply fuse, therefore this will blow first and so protect the suppliers equipment.

Cheers.......Howard

 

[epager]

SWA every time. Really can't see any advantage in using tails

Cost would seem one advantage.