Supply to OUTBUILDINGS

[acat]

Hi Folks

Please post all your ideas and suggestions here so we only have one place to look


Thanks


Chris

 

[WayneL]

- we are constantly being told different things- one lecturer says one thing and another says something different with regard to all sort of things .
cheers.

Exactly the same at my college, mate, when doing the 2330 - you could literally go from one lesson to the next and be told two totally different things regarding the same topic.

I think it's a combination of some misinterpretation of the 'regs' and some passing on of wrong information.

That's why I think it's important to give sources of information (e.g. Reg numbers) when making statements on topics like this - it's all to easy to repeat totally wrong information

 

[Deleted member 9648]

That's why I think it's important to give sources of information (e.g. Reg numbers) when making statements on topics like this - it's all to easy to repeat totally wrong information

But bear in mind the fact that there isnt a reg stating that outbuildings need to be on a TT.What the regs dont say is just as relevant as what they do

 

[WayneL]

But bear in mind the fact that there isnt a reg stating that outbuildings need to be on a TT.What the regs dont say is just as relevant as what they do

That's exactly what I mean, mate

It's no good coming on here stating that an outbuildings has to be TT'd, without giving a reference to back a statement like that up.

The statements that I have made, I have backed up with reference to GN8.

 

[dixon9]

I think i'ts worth pointing out here just what an extraneous part is as an awful lot of people are confused on this.
In bs 7671 an extraneous part is defined as-
"A conductive part,liable to introduce a potential,generally earth potential,and not forming part of the electrical installation"

The key part of this is underlined. Therefore a metal sink wholly within the building is not generally an extraneous conductive part. Pipes coming into the building from outside and metallic structure are likely to be extraneous conductive parts. The only time a metal sink,radiator etc is likely to be an extraneous conductive part is if it is fixed to the metallic structure of the building.

Err... So are you saying the metal sink is unlikely to be piped in from the outside?

Edit: If the pipes are plastic then fair enough - if not then the sink is extraneous....

 

[dixon9]

As an aside, this gives a pretty good explanation of the situation and also provides references to regs:

wiring matters supply to outbuilding - Google Search

 

[Deleted member 9648]

Err... So are you saying the metal sink is unlikely to be piped in from the outside?

Edit: If the pipes are plastic then fair enough - if not then the sink is extraneous....

No,it is the pipes coming into the building which introduce an earth potential from outside,not the sink.
The pipes should be bonded at the point of entry,if they are plastic then no bonding is required.
The sink itself cannot introduce an earth potential therefore it is not an extraneous conductive part.

 

[dixon9]

Fine ....but if the metal sink is joined to the metal pipes then the sink IS extraneous!

 

[Sintra]

An Extraneous Conductive Part is a metal part that is not part of, but in proximity to the electrical installation and is liable to introduce earth potential. p24
All extraneous conductive parts in an installation must be connected to the main earthing terminal by main protective bonding conductors. This applies to the metallic sheath of a telecommunications cable where permission from the owner of the cable must be obtained. 411.3.1.2


It is not generally required to supplementary bond the following :
kitchen pipes, sinks, draining boards, metallic kitchen furniture, boiler pipes, metallic parts supplied by plastic pipes or metal pipes to hand basins or wc's ( excluding metal waste pipes in contact with earth ). OSG p31

 

[Deleted member 9648]

Fine ....but if the metal sink is joined to the metal pipes then the sink IS extraneous!

Er....um....only if the incoming pipes (the real extraneous conductive parts) were not bonded at the point of entry would the sink be an extraneous conductive part by virtue of being connected to the pipes. Once the pipes are bonded ,everything on the building side of the bond cannot introduce an earth so is not an extraneous conductive part

 

[martysparky]

All the pipes are bonded to earth if there is continuity???

If an earth fault was to arise when you had your hands in the water in a sink, wouldn't that continuity give rise to danger?

Didn't Part P come about because a politicians daughter got electrocuted to death in her kitchen via an extraneous conductive part?

 

[Des 56]

I agree, mate.

The funny thing is, by rodding the outbuilding, you are probably giving it a worse earthing system (i.e. Ze/Ra) than you would be by using the supply earth

I think comparing Tnc-s and TT and saying better and worse earth is meandering away from the safety reasons
A Tnc-s system although having lower Ze than TT is a potentially poorer earth system than the higher TT

The outbuilding earthing choice,ought to be about whether danger exists by using the potentially dangerous Tnc-s or not, rather than" its better earthing than a rod"

Assessing the particular outbuilding and taking any bonding factors into account and deciding from there whether to use TT or Tnc-s

 

[WayneL]

The funny thing is, by rodding the outbuilding, you are probably giving it a worse earthing system (i.e. Ze/Ra) than you would be by using the supply earth

I wasn't comparing them directly, Des, which is why I said 'i.e. Ze/Ra'.

If your outbuilding was 2 meters from your house, and you could get a Zs at the outbuilding DB of, say, 0.5 ohms using the supply earth (TN-C-S), why on earth would you want to put a rod in and get an 'unpredictable' 50 ohms.

The outbuilding earthing choice,ought to be about whether danger exists by using the potentially dangerous Tnc-s or not, rather than" its better earthing than a rod"

Assessing the particular outbuilding and taking any bonding factors into account and deciding from there whether to use TT or Tnc-s

Which is probably why I said this, earlier:

The one time I would seriously consider installing an earth rod, and seperating the outbuilding earthing system, is if the outbuilding was steel framed and the main earthing system was TN-C-S.

The requirement to bond to the frame of the outbuilding would mean that if there did happen to be a loss of the supply neutral, the whole of the frame of the outbuilding could rise to 230v.

This would obviously cause a shock hazard to anyone standing outside the outbuilding and touching the frame. TT'ing the outbuilding, in this instance, would be safer.

 

[Deleted member 9648]

All the pipes are bonded to earth if there is continuity???

If an earth fault was to arise when you had your hands in the water in a sink, wouldn't that continuity give rise to danger? It shouldnt because within the equipotential zone you should not be in contact with any other earth potentials and so not subject to a potential difference

Didn't Part P come about because a politicians daughter got electrocuted to death in her kitchen via an extraneous conductive part?I thought it was a cooker cable drlled while fixing something on the wall

It is the introduced earth which gives rise to a danger,as when an earth fault is in progress there will be a rise in voltage on conductive parts (earthed parts of the electrical installation) relative to the general mass of earth untill the protective device clears the fault.
A person touching an earthed appliance and an unbonded extraneous part may recieve a shock while the fault is in progress, as they are effectively touching a live part while being in contact with the ground. As soon as that extraneous part is bonded there can be no potential difference between conductive and extraneous parts and the shock risk is reduced.
It doesnt matter about ensuring continuity of pipes to sink etc beyond th position of the bond as once inside the building they can no longer introduce an earth potential.

 

[pushrod]

It doesnt matter about ensuring continuity of pipes to sink etc beyond th position of the bond as once inside the building they can no longer introduce an earth potential.

So if copper water pipe comes into the house and then there is a section of plastic pipe which is effectively separating other sections of copper pipe you are saying that they do not need to be bonded? If those unearthed pipes became live, wouldn't they just stay live and not trip an mcb because the fault current would have no where to go? (assuming no rcd)

 

[Des 56]

Which is probably why I said this, earlier:[/QUOTE]


Originally Posted by WayneL


The one time I would seriously consider installing an earth rod, and seperating the outbuilding earthing system, is if the outbuilding was steel framed and the main earthing system was TN-C-S.

The requirement to bond to the frame of the outbuilding would mean that if there did happen to be a loss of the supply neutral, the whole of the frame of the outbuilding could rise to 230v.

This would obviously cause a shock hazard to anyone standing outside the outbuilding and touching the frame. TT'ing the outbuilding, in this instance, would be safer.



Lowering my headwith a sorry Waynel