Earthing arrangement when moving consumer unit far away from TN-S supply?

[ZEDEZ]

I've been doing a lot of research, and am hearing a lot of conflicting information on this subject. Currently working on an important job and want to get this perfect, and the time is coming soon to connect the new consumer unit up to the supply, so would be interested to get some more feedback here.

So to put it in hypothetical terms, lets say you had a 100 amp TN-S supply & meter located on the outside of the house, and the consumer unit were to be relocated in a room within in the house, 5 - 10 m away from the supply, with the cable disappearing underground. How would you do it in the most efficient and compliant way possible?

Here's what I'm thinking I'd do:

Install 100 amp metal clad switched fuse next to meter. Meter tails go into this. To get from here to the new CCU I'd use 2 core 25mm SWA, glanded and Earth nutted to switched fuse enclosure, with a 16mm copper flying lead connecting Earth nut to MET. The same same gland / Earth nut arrangement would apply at the CCU, connecting the Earthing bar to SWA armour. So no unsightly additional CPC tagging along with the SWA, and no unnecessary re purposing of a core of 3 core SWA.

Assuming disconnection times / adiabatic equations etc. all work out (which so far given optimal circumstances seem that they do), can anyone see any problems with this arrangement, or is there a better way? Also - on the schedule of results, if using this arrangement, what would the e̶a̶r̶t̶h̶i̶n̶g̶ ̶c̶o̶n̶d̶u̶c̶t̶o̶r̶ CPC of the distribution circuit (thanks for clarification of terms there davesparks) material be listed as, given that it's a combination of copper and steel?

Generally interested to hear how you'd do it / have done it, and reasons why. I am inexperienced but very keen to really get to grips with best practice & the theory behind it.

EZ

 

[ZEDEZ]

Bonding?

In this scenario, the SWA will be acting only as the CPC of a distribution circuit in effect, and will not also be providing bonding. Any bonding would go back separately to the MET.

 

[ZEDEZ]

I think what he wants to know is whether the armour will be required to perform the functions of both cpc and bonding conductor or just a cpc?
Where are you going to terminate the bonding conductors?

Ahh I see, (answered in reply to Simonslinline)

 

[ZEDEZ]

I'm struggling to see what this discussion is about and why it has even been brought up. Just do it in accordance with Bs7671,if you prefer a 3c with provision for a copper cpc do it that way,if you think it's unnecessary then use a 2c and SWA cpc. Whats to discuss?

What we're discussing is the most efficient way to correctly do something given a set of circumstances We've already got some conflicting answers, and the reason I'm here asking this question is because my experience level is far less than most on this forum, and I'm trying to really understand all the different approaches and view points so I can make up my mind on what I think. If it was as simple as "just do it in accordance with BS7671" I don't think any of us would be here! You know what I'm sayin' :wink_smile:

I'm not sure it's as simple as you say potato I say potaato, as it has been pointed out in some situations using steel armour as CPC is fine, given slight environmental changes it might be preferable to use an additional copper CPC. While this may be obvious to some, it's perhaps not so obvious to others (in this case me)

 

[Deleted member 26818]

I take it that the MET will be in the external meter cupboard?
Why are you intending to use SWA for the distribution circuit?

 

[amlu]

pull a 10mm single earth alongside the 2core armoured, just in case that armour is going to fail at some point....
Zs on the final circuits may be little lower too.

 

[davesparks]

pull a 10mm single earth alongside the 2core armoured, just in case that armour is going to fail at some point....
Zs on the final circuits may be little lower too.

Why waste cable like that? Under fault conditions the seperate earth will see next to none of the fault current

 

[davesparks]

I take it that the MET will be in the external meter cupboard?
Why are you intending to use SWA for the distribution circuit?

He mentions it going underground so SWA seems a logical choice

 

[Deleted member 26818]

Will the bonding also be going underground?

 

[ZEDEZ]

He mentions it going underground so SWA seems a logical choice

This is correct - it is going underground. If I weren't using SWA, I'd have to protect with an RCD.

 

[davesparks]

This is correct - it is going underground. If I weren't using SWA, I'd have to protect with an RCD.

Why would you need RCD?

 

[Deleted member 9648]

This is becoming farcical.

 

[ZEDEZ]

Why would you need RCD?

Yeah, shouldn't have said "have to", as I'm not entirely sure. Would've assumed it necessary if the cable had no mechanical protection, as the cable is underground at a shallow depth / runs up the outside of an internal wall.

Could well be wrong there though and have a feeling you're about to correct me davesparks!

 

[7029 dave]

pull a 10mm single earth alongside the 2core armoured, just in case that armour is going to fail at some point....
Zs on the final circuits may be little lower too.

Funny you say that, I wanted to, but don't want to get roped into it just what super has said.

 

[Deleted member 26818]

I'm a little concerned with the low Ze value.
It suggests that PME conditions apply.