Supplying outbuilding, bonding and earthing

[kropaske]

Hi Guys,

Just join this great forum so lets get straight to the point...

Soon we will be working on property (TNS) with 1 outbuilding on the back of the garden which have kitchen, bathroom and 1 extra room. Also there is gas central heating with combi boiler. At the moment they feed this outbuilding from kitchen ring on a 30amp fuse. They spur off to the outside socket with 2.5mm then 6mm swa from that socket to the CU in the outbuilding(cable not even burried). CU have 2x20amp and 1x6amp circuits as per image (bit of a mess). House CU is approx 20mtrs away from outbuilding CU.
We going to rewire kitchen and outside bit and also have to reconnect outbuilding back all this will go into 3 or 4 way CU for now. So the questions are:

1. 6mm swa gland at house CU from 32 amp mcb without rcd to the outside plastic waterproof junction box to reconnect to existing 6mm swa. Will that be ok?
2. Would you gland both swa cables at the outside junction box and reconnect armour with banjo or is there better solution?
3. At the moment armour is used as the earth, should i connect 3rd wire back to earth as well?
4. There is no bonding provided in the outbuilding, mains water is in plastic externally but all inside is copper. Wouldit be acceptable to bond gas and water back to outbuilding CU? I read it somewhere that it may be different scenario with PME supply, but TNS should be fine.
5. And finally do i need to retest whole outbuilding or just the sub main cable?

Lucas

 

[davesparks]

No need to up the size of main bonding just because you happen to use a size of earthing conductor above the minimum required. 544.1.1 refers.

I'll look in the book tomorrow, but I thought it had to be half the size of the earthing conductor as opposed to half the calculated value.
So for example if the calculated value is 20mm the earthing conductor would actually be 25mm (nearest standard size), so the main bond would be 16mm not 10mm.
The OP was suggesting that if it calculates at 12mm then a 6mm bond would be ok, yet I make that to be a 16mm earthing conductor and therefore a 10mm bond.

 

[westward10]

544.1.1 states bonds should not be less than half the size of the required earthing conductor and not less than 6.0 for a TN-S system.
To assess whether the armour of the cable can support bonding refer to GN8, Section 5.2.1 where a ratio of 8 is to be applied where the material is steel. So if your main bond is 10.0 apply the ratio which gives you a size of 80.0 for the armour. Refer to Appendix B of GN8 which gives you the actual csa of the armour for SWA cables which for the cable in question is 23.0, way below the required 80.0. To calculate how the armour in question actually relates to copper for bonding divide 23.0 by the factor 8 which gives you approximately 2.88.
The adiabatic equation is used to calculate cpc size and not whether a conductor can support main protective bonding, so I believe anyway.

 

[Midwest]

.
4. Check resistance between met and pipework in outbuilding and find out if the pipe are extraneous-conductive-part. If not no bonding required if yes, bond it.

But also, dependant on the installation of the internal non electrical metal services and electrical services, reg 528.3.4 needs consideration.

 

[Wilko]

I agree Westy, after your help recently I revisited the good book and there are several regs that set limits - bonding - buried - pme etc. The simplest way to ensure compliance for a small service once bonding is involved appears to be 16mm if buried (sleeved but not mechanically protected from Table 54.1) or 10mm if Cu conductor within SWA say (from Table 54.8). If the 16mm is used here it will future proof the installation against bonding requirements and if DNO converts to PME, I think

 

[Barry White]

OMG look at the main earthing conductor wrapped in the green n yellow tape looks like its actually some of the steel wire armorings no gland there then!

 

[westward10]

OMG look at the main earthing conductor wrapped in the green n yellow tape looks like its actually some of the steel wire armorings no gland there then!

It is indeed.

 

[Octopus]

I think I'm losing the plot on this thread.... is the OP planning to run a cable back to the CU shown in the first post?

 

[westward10]

I think I'm losing the plot on this thread.... is the OP planning to run a cable back to the CU shown in the first post?

No the cable is running to the CU in the post from the CU at the origin.

 

[Octopus]

No the cable is running to the CU in the post from the CU at the origin.

But didn't he say that the outbuilding was spurred off the kitchen ring - or is this CU the one in the outbuilding?

 

[westward10]

But didn't he say that the outbuilding was spurred off the kitchen ring - or is this CU the one in the outbuilding?

Yeah sorry the CU in the post is coming from a socket via the SWA in question.

 

[Octopus]

So if is was me..... I would quote to rewire the kitchen, quote to inspect the outbuilding, quote to run a new submain to from the main CU to the outbuilding and if the EICR on the outbuilding shows up anything, that would be included too.
I'm guessing that this "outbuilding" will be let to some unsuspecting tenant so I'd be VERY careful about what signed off.

and if the client didn't like my quote, then I wouldn't be shedding any tears.

If they can afford a new kitchen, they can afford to sort out the fiasco of a submain and the outbuilding too.

Anyone who contemplates much less shouldn't be selling their services as a competent spark IMHO.

 

[westward10]

If the standard of work is like the CU it is likely to be a mess.

 

[kropaske]

Ok Guys,
Went there today to talk about options and they want to redo this outbuilding so it's only for storage. They going to remove boiler and water and gas services will be terminated. So now it's only the case of new small consumer unit for lighting and few sockets.
Testing will confirm whether to reuse some of existing wiring or rewire all.

I did some resistance test between met and metal services in outbuilding and the resistance was below 2000 ohms so definitely needed bonding.
Adiabatics also gave me csa of earth 5.31mm. With that in mind would I still have to run another 10mm bonding or 3rd core would work fine parallel with armour as combine earth/bonding?

 

[westward10]

Ok Guys,
Went there today to talk about options and they want to redo this outbuilding so it's only for storage. They going to remove boiler and water and gas services will be terminated. So now it's only the case of new small consumer unit for lighting and few sockets.
Testing will confirm whether to reuse some of existing wiring or rewire all.

I did some resistance test between met and metal services in outbuilding and the resistance was below 2000 ohms so definitely needed bonding.
Adiabatics also gave me csa of earth 5.31mm. With that in mind would I still have to run another 10mm bonding or 3rd core would work fine parallel with armour as combine earth/bonding?

See Post 17.

 

[Midwest]

I did some resistance test between met and metal services in outbuilding and the resistance was below 2000 ohms so definitely needed bonding.
Adiabatics also gave me csa of earth 5.31mm. With that in mind would I still have to run another 10mm bonding or 3rd core would work fine parallel with armour as combine earth/bonding?

I seem to remember that you cannot combine swa & core, as form of earthing or bonding, but I might be wrong. What was the exact reading you obtained for suspect extraneous conductive parts?