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kropaske

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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

Supplying outbuilding, bonding and earthing 20161116_134655 - EletriciansForums.net

Supplying outbuilding, bonding and earthing 20161116_134655 - EletriciansForums.net
 
Each SWA should be glanded at each end of the cable, not bridged the way it is at the moment in the CU you have shown.
The CU you have shown should be put back to being in accordance with the manufacturers spec instead of the current bodge up.

1- personally I don't like SWA terminated into a plastic box, I would prefer to fit a proper SWA joint.
2-earthing nuts are a better solution than banjos these days, or even just use a decent metal enclosure.
3-Yes any spare core of cable which contains live conductors should be shorted to earth at one end to prevent ghost voltages appearing.
4- is it extraneous? Test it rather than guessing. Whatever the earthing system a main bond must be at least equal to the minimum size required from the MET through to its final point of connection to the extraneous part. So no you cannot just assume that due to it being TNS you don't need a separate bonding conductor, establish the minimum size of main bond and then calculate whether the armour has a suitable copper equivalent CSA.
5- looking at the state of that board I'd want to do a proper inspection of the installation before starting,
 

Depends whats in the kitchen, bathroom and extra room. Wall heaters, electric shower, oven?


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? See 2

2. Would you gland both swa cables at the outside junction box and reconnect armour with banjo or is there better solution?
Personally Id rather have one cable without joints but if needs must and it needs jointing then the armour must be earthed on both SWA's. If outside i would definitely use piranha earth nuts and a suitable fly lead.

3. At the moment armour is used as the earth, should i connect 3rd wire back to earth as well?
Is the existing cable 3-core? if not then there's not much point installing part of the run in 3-core apart from an extra parallel path.

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. If mains water and gas is plastic and there are no extraneous conductive parts in outbuilding then nothing needs bonding.

5. And finally do i need to retest whole outbuilding or just the sub main cable?
I'd retest the lot
 
As Dave states that CU has been lashed up, incorrect protective devices which are not linked by way of a suitable busbar and the rcd shouldn't be there. Replace it after giving it a proper test. The armour of the cable is definitely inadequate for bonding purposes and so is the separate core.
 
Hi - reading this, just on the bonding - it may be an outbuilding, but it sounds like a house, so I'd like to run a properly sized conductor all the way back. Tin hat on I just don't like a building with water, gas, boiler, CH, kitchen and bathroom and no possibility of bonding. Or perhaps convert outbuilding to TT?
 
Thanks for all the input Guys, really appreciate.
I know this installation is far from perfect, and most of it should be redone. But you know what the clients are like.
I want to make sure is safe so plan of action will be:

1. Recommend to replace whole swa first. If not then swa to external plastic properly terminated - not perfect but safe and acceptable. New and existig swa would be 3 core.
2. Feed swa from 32 non rcd.
3. Test existing wiring in the outbuilding replace CU with metal RCD protected.
There is hardly any load on there anyway - only few sockets, 3 lights and boiler. No heaters, oven or shower.
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.
Option for bonding - if adiabatics confirm that 6mm conductor is enough, am i allow use 3rd core of swa, or this would not be acceptable?
Alernatively i can run separate conductor (externally) from the met from the house to the outbuilding.
5. Test all.

Is that sounds good?
 
It wants testing before hand to determine if there are any extraneous conductive parts. As for the board that's already there, that existing swa feeding it wants terminating properly. You might have already covered this so apologies if so.
 
Thanks for all the input Guys, really appreciate.
I know this installation is far from perfect, and most of it should be redone. But you know what the clients are like.
I want to make sure is safe so plan of action will be:

1. Recommend to replace whole swa first. If not then swa to external plastic properly terminated - not perfect but safe and acceptable. New and existig swa would be 3 core.
2. Feed swa from 32 non rcd.
3. Test existing wiring in the outbuilding replace CU with metal RCD protected.
There is hardly any load on there anyway - only few sockets, 3 lights and boiler. No heaters, oven or shower.
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.
Option for bonding - if adiabatics confirm that 6mm conductor is enough, am i allow use 3rd core of swa, or this would not be acceptable?
Alernatively i can run separate conductor (externally) from the met from the house to the outbuilding.
5. Test all.

Is that sounds good?

1 - Is there something wrong with the existing cable? Why are you giving them only the options of the apparently over the top total replacement or the utter Bodgeit plastic box? Why not joint the SWA with a proper SWA joint?

4 - this is a seriously scary question to be coming from someone who claims to be competent and niceic registered, I think it just goes to prove how far the niceic standards have dropped these days. The adiabatic equation is not used to calculate the size of main bonding and as to whether or not you can use a copper conductor as a main bond, well I'm not going to answer that if you can't work it out!
 
1 - Is there something wrong with the existing cable? Why are you giving them only the options of the apparently over the top total replacement or the utter Bodgeit plastic box? Why not joint the SWA with a proper SWA joint?

4 - this is a seriously scary question to be coming from someone who claims to be competent and niceic registered, I think it just goes to prove how far the niceic standards have dropped these days. The adiabatic equation is not used to calculate the size of main bonding and as to whether or not you can use a copper conductor as a main bond, well I'm not going to answer that if you can't work it out!

I may just word this slightly incorrect. What i meant is use adiabatics to determin csa of earth then bonding needs to be at leaset half of required earth conductor but not less than 6mm for tn system. So assuming required earth is less than 12mm then 6mm bonding is fine.And of course you can use copper conductor as bonding but i am just not sure if is acceptable to use one core from swa.
 
I may just word this slightly incorrect. What i meant is use adiabatics to determin csa of earth then bonding needs to be at leaset half of required earth conductor but not less than 6mm for tn system. So assuming required earth is less than 12mm then 6mm bonding is fine.And of course you can use copper conductor as bonding but i am just not sure if is acceptable to use one core from swa.

Main bond needs to be not less than half the size of the earthing conductor if I remember correctly, not half the size of the minimum permissible earthing conductor by calculation.
 
Main bond needs to be not less than half the size of the earthing conductor if I remember correctly, not half the size of the minimum permissible earthing conductor by calculation.
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.
 
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.
 
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.
 
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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.
 
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 :)
 
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!
 

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