Supply to OUTBUILDINGS

[acat]

Hi Folks

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


Thanks


Chris

 

[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

 

[WayneL]

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)

Sorry to butt in, but how would they become live?

That's a bit like saying - if the metal legs on my glass coffee table became live............ - should I bond them?

 

[pushrod]

Sorry to butt in, but how would they become live?

That's a bit like saying - if the metal legs on my glass coffee table became live............ - should I bond them?

Sorry but nothing like that at all! Surely electricians are always trying to pre-empt unlikely accidents, but as an example - we had a loft conversion done which required the mains fed water tank being moved and the plumber used speedfit? plastic pipe the feed from this rejoins copper pipes to the hot water tank. the cupboard containing
this has cabling for immersion heater , the pump and various other surface mounted cabling! I don't think it is much of an argument to stand up in court and say i never thought they would become live!

Can you answer my question or not, don't mind if i am wrong or not - would just like to know!

 

[martysparky]

We know if there was a fault current it would most likely disconnect.

How would a sink become live?

I cant see a sink being fed via plastic except push fit that most plumbers I know avoid due to future leak potential.

If you can get a back current from the neutral bar why could this not happen with the earth bar?

The hot water pipe comes from the boiler where it is bonded.

We know if we do an IR test and wrongly put the live tester probe on the earth bar it makes everything live so if the line conductor came into contact with a cpc or conductive part and lets just say it didn't disconnect why would it not be possible for the sink to become live. Can we assume that the current will take the path straight to the main earthing terminal without any flowing away through conductive pipe.


I know this is not very likely but just raising this thought.

Also maybe this should now be a different thread? its become outbuilding/sink

 

[geller]

Hello guys.

After reading your topic i have a query.

My garage (not attached to the house but only about 10 feet behind it) is wired from the consumer unit to a smaller C/u in the garage running 4 sockets and two strip lights.
Earth wise, it is connected with SWA from the house to the Smaller unit.

Q1 - Do i need to add an Earth electrode to this ?
Q2 - Is this set up regarded as a TT system ?

Thank you.

 

[WayneL]

Sorry but nothing like that at all! Surely electricians are always trying to pre-empt unlikely accidents, but as an example - we had a loft conversion done which required the mains fed water tank being moved and the plumber used speedfit? plastic pipe the feed from this rejoins copper pipes to the hot water tank. the cupboard containing
this has cabling for immersion heater , the pump and various other surface mounted cabling! I don't think it is much of an argument to stand up in court and say i never thought they would become live!

Can you answer my question or not, don't mind if i am wrong or not - would just like to know!

Hi Pushrod,

I'll try answer it the way I see it

Any metalwork entering your house (or outbuilding-to stay on topic), could introduce earth potential into the property, especially if originating underground.

In this case you would bond the metalwork as near to it's entry point as possible.

If this is done to all metalwork on entry (gas, water, oil, structural), then you create your equipotential zone.

If you touch two metal parts simultaneously, within this zone, you won't get a shock, as all metal will be at the same potential - whether that be 0 volts or 100 volts.

Supplementary bonding, which isn't always required now, is just additional links between metalwork to keep that equipotential zone going - e.g. between hot and cold taps

No other metalwork in the house will require bonding, as it can't possibly introduce a potential.

What will be required is earthing.

Exposed conductive parts need to be earthed - e.g. cooker casings, washing machine casings, boiler casing and pipework, metal wall lights - the list could go on and on - anything that could become live, under fault conditions, needs to be earthed - not, though, my coffee table

This ensures that the fault will be cleared by the protective device, rather than the exposed conductive part remaining live.

This is the best I can do - maybe someone can be clearer

 

[Deleted member 9648]

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)

The purpose of bonding as I and others have pointed out has nothing to do with 'earthing pipes so they cant become live'. This is a common misunderstanding which results in an awful lot of incorrect advise on bonding.

 

[acvc]

Hello guys.

After reading your topic i have a query.

My garage (not attached to the house but only about 10 feet behind it) is wired from the consumer unit to a smaller C/u in the garage running 4 sockets and two strip lights.
Earth wise, it is connected with SWA from the house to the Smaller unit.

Q1 - Do i need to add an Earth electrode to this ?
Q2 - Is this set up regarded as a TT system ?

Thank you.

Hi Geller

What is the earthing system in the house? And is there any connection between the main earthing conductor in your house and the one in your garage?

 

[pushrod]

Thanks for the detailed reply Wayne. I appreciate that bonding is to maintain an equipotential zone eg so that in the event of a fault, and before disconnection, if a person touches a live exposed conductor and at the same time an extraneous conductor eg cold watertap there will be no difference in potential ie they will have both risen to the same level and so the person will not get a shock. What i suppose i was getting my head around was that some of the pipe work that can possibly be both acting as an exposed conductor eg continuous with hot water pump body or an extraneous conductor capable of introducing an earth potential. My main concern is with large sections of pipework becoming isolated from there bonding conductor by the handywork of a plumber sticking in a bit of plastic pipe. Should the plumber then take responsibility and ensure that there is equipotential bonding if the plastic then goes back to copper? Or if the electrician comes across it should they do the same?

The purpose of bonding as I and others have pointed out has nothing to do with 'earthing pipes so they cant become live'. This is a common misunderstanding which results in an awful lot of incorrect advise on bonding.

This not what i was originally asking and the "quote" above is not mine and you are incorrect if you inferred it was meant.
Quote:
Originally Posted by wirepuller
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?"

The above is the main question that i am asking.

 

[Davey101]

Although I think Pushrod may misunderstand the definition of 'extraneuos' (I also misunderstood the exact definition before someone on this forum corrected me), I agree with him on his point about earthing pipework. If I saw a house with a copper heating system, which had been electrically dissconected from the boiler by a short section of plastic pipe, I would want the remaining copper pipework to be bonded.
There are far too many ways a pipe (and all the radiators and taps connected to it)could become live. I should think that there are enough sparks who have seen live unearthed metalwork to agree

It may not be covered by the regs, but it's good practice at the very least.

 

[martysparky]

I was told that the copper pipes should be tested for continuity.

 

[WayneL]

I was told that the copper pipes should be tested for continuity.

This is true, although it's irrelevant if all extraneous metalwork is bonded at its point of entry to the building - i.e. if it's continuous, it's already bonded - if its not continuous, it doesn't need bonding

If you have a section of pipe, and there's no continuity back to the MET (not bonded), you can always test it to see if it's extraneous - put your mind at rest

 

[Deleted member 9648]

I was told that the copper pipes should be tested for continuity.

I think you may have heard this when referring to supplementary bonding,it is permissible to use a copper pipe as a supplementary bonding conductor ,(NOT a main bonding conductor),in which case the electrical continuity of the pipe would have to be confirmed.

Pushrod......apologies if I misunderstood the posts.

 

[Deleted member 9648]

Although I think Pushrod may misunderstand the definition of 'extraneuos' (I also misunderstood the exact definition before someone on this forum corrected me), I agree with him on his point about earthing pipework. If I saw a house with a copper heating system, which had been electrically dissconected from the boiler by a short section of plastic pipe, I would want the remaining copper pipework to be bonded.
There are far too many ways a pipe (and all the radiators and taps connected to it)could become live. I should think that there are enough sparks who have seen live unearthed metalwork to agree

It may not be covered by the regs, but it's good practice at the very least.

I see what you are saying here....but you cant stop at just plastic sections,compression joints are electrically unreliable,are you going to do a continuity test on every single pipe in the building between each electrically suspect joint and perhaps link across compression joints?
A sledgehammer to crack a nut IMHO....just stick to the requirements of BS7671.

 

[pushrod]

Although I think Pushrod may misunderstand the definition of 'extraneuos' (I also misunderstood the exact definition before someone on this forum corrected me), I agree with him on his point about earthing pipework. If I saw a house with a copper heating system, which had been electrically dissconected from the boiler by a short section of plastic pipe, I would want the remaining copper pipework to be bonded.
.

I would be interested to know which of my posts make you think i don't understand the term "extraneous" conductive part, at least i can spell it

By the way in the OSG section 4.7 page 31 it says "supplementary bonding is not required to metallic pipes supplied by plastic pipes"

apologies as well if i have been a bit off with anyone!

 

[geller]

Hi Geller

What is the earthing system in the house? And is there any connection between the main earthing conductor in your house and the one in your garage?

I believe mine is a standard earth system (TNC-S !), and the SWA is connected to the garage c/u from my house c/u. Connected to a separate breaker on the rcd side of the unit and i presume the earth in the unit too.