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

 

[davesparks]

For the sake of say £20 on 5-10M run.. It really is a no brainer lads.! Are u seriously pricing jobs that close.. I pride myself on being a local spark and here for the long run on the local work i do! not it complies now and sod the future degrading and then its some1 else's problem! Didnt mean to offend you davesparks non ment, but reading back i see my bluntness!

If it is installed properly then there will be no issue with degrading. And if the armour does become corroded then the cable will no longer be fit for purpose regardless of how many unnecessary conductors are installed.

I look after installations with 2 and 4 core SWA submains dating back to the 1970s, where the armour is undamaged and fully working.

 

[davesparks]

In the original scenario? As said in reply to Andy78:

MET at the supply, located on outside of the house, same place the SWA eventually terminates (via Earth nut & copper fly lead of course).

So the earthing route from CU to Earth is, CU Earth Bar -> 16mm copper fly lead -> Earth nut -> steel armour -> Earth nut -> 16mm copper fly lead -> MET -> DNO Earth

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?

 

[7029 dave]

When outside SWA glands take a right hammering over time, they do degrade and the resistance of the steel wire Armouring goes sky high because of this. 3 core for me too.

 

[Deleted member 9648]

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?

 

[sam400]

Unsure ... In this scenario what are the differences between the two?

The xlpe is smaller OD and won't comply if your sizing cpc using table 54G on a 25mm 2c, could still be compliant by calculation (adiabatic)

sam

 

[davesparks]

When outside SWA glands take a right hammering over time, they do degrade and the resistance of the steel wire Armouring goes sky high because of this. 3 core for me too.

Then the cable is no longer fit for service as the armour is compromised.

 

[bigspark17]

Davesparks im not questioning your reasoning or ability, im adking in this scenario over a 5-10M run of 25mm ur talking minimal cost.. Shud it be a 95mm over 1/2 mile! Seems a no brainer!

 

[7029 dave]

Then the cable is no longer fit for service as the armour is compromised.

Yes correct if the armour is being used as an cpc, if the 3rd core is used then why?, the armouring will still be earthed, the cpc is not compromised.

 

[davesparks]

Yes correct if the armour is being used as an cpc, if the 3rd core is used then why?, the armouring will still be earthed, the cpc is not compromised.

The main protective function of the armour is to ensure that any metal object penetrating the cable becomes reliably connected to earth before it makes contact with a live conductor.
If water has come in to contact with the armour then it can/will spread along the length of the cable and rot more than just the bit of armour at one end. This the cable is compromised and no longer fit for service.

I can't be the only person who has cut in to an old SWA and had water drip out of the cut ends and found the armour rusted?

 

[ipf]

3core swa everytime for me.. Also get ur zdb reading as low as possible! Seems lazy doing 2c and unprofesional.. What part of country are you, i live by coastline and seasalt in the air can do some serious damage to swa glands overtime especially as ur terminating in outside box!

What if it's a 3 phase and n supply? Is installing a 4 core lazy?

 

[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?