Earth Bonding

[fidom]

Thanks for the replies (the on topic ones anyway ). Do you know if there is any right of appeal with these inspections or am I obliged to do whatever work they specify before they will issue a certificate? I get the impression that this firm are trying to generate work for themselves. For example they say the meter tails are only 10mm and should be changed. There are old tails from the meter to Henley blocks and newer ones from the blocks to the CU. I know for a fact that the newer tails are 25mm and I think the old ones are also 25mm but with thinner insulation.

 

[EFLImpudence]

An illustrative example has just occurred to me. A neighbour of mine has a metal oil tank sitting on a plastic/wood stand which is connected by a metal pipe (which does not go underground) into his house. I would personally say that should count as an e-c-p which needs main bonding, because it 'may' ('is liable to') introduce earth potential, even though it will not usually (if ever, certainly under dry conditions) introduce any potential. What would you say?

That is a decision the electrical installer has to make.

'Looking at it from here' I would think that a plastic and wood stand would be sufficient insulation to make the tank and the pipe not extraneous (given or inclined to introduce etc.)

However, I presume that the pipe connects to other pipes in the house which could mean it (the pipe) may require supplementary bonding in some locations (rooms) if simultaneously accessible to other metal parts (RCD considerations ignored) but not main bonding to the MET.

 

[JohnW2]

One dictionary defines liable as 'given or inclined to' which I would take as more than 'may' (which the definitions do not use - (may, that is)). I would say likely to. I suppose we could have a few pages on this.

I don't particularly want to get into a semantic discussion. However, more to the point, what do you have to say about the oil tank example. In that case, the oil pipe is NOT likely to to introduce a potential, but may possibly do so under some circumstances - so, however you care to intepret 'liable', I would want to bond it, wouldn't you? If so, does that affect your interpretation of 'liable to'?

... if a metal part is not already bonded or earthed then it does not need to be bonded.

Could you help me understand that?

If one earths two things by connecting them both to the same earth terminal (e.g. a MET) then one is 'bonding' them together,

This is where you're confused - If two exposed parts are earthed it would be at the earth bar in the CU (to operate a CPD). Extraneous parts connected to the MET would be bonded (to equalise potential in the event of a fault).

I'm not confused at all. I could just as easily have written 'CU earth bar' as 'MET' - and, indeed, in some installations the 'MET' is the earth bar in the CU. You appear to be trying to define the difference between earthing and bonding on the basis of whether the cable/CPC in question is connected to MET or CU earth bar (which, as I said, could be the same thing - and, even if they're not, will be joined with a short length of ≥10mm² G/Y), which seems all rather odd. As you know as well as I do, the difference relates to the reason/intent of the connection.

If you want to get philosophical, perhaps the oddest thing is use of the word 'earthing' to relate to connecting CPCs to a CU's earth bar (and hence the installation's MET). As we know, the whole reason for main bonding is that, under certain fault conditions, the 'earth' connection (whether DNO-supplied or local TT electrode) could rise to a poential well above true earth, taking the MET, CU earth bar and any bonded e-c-ps with it. What you/we are calling 'earthing' therefore would probably be more properly termed 'METing', or something like that!!

Kind Regards, John.

 

[Electrifying]

I really don't see why the confusion arises between earthing and bonding - it really is quite simple.

You 'earth' a part of an electrical installation that could become live under fault conditions - this includes casings, steel conduit, basket, metal accesories.......all part of an installation.

An incomming gas/water pipe is not part of an electrical installation - it is metalwork that could introduce a different potential to the 'earthed' metalwork of the installation.

This incomming metalwork is bonded to the MET in order to 'equalise' that difference in potential..............the gas/water pipe aren't, and never will be, 'earthed' - they don't need to be, they aren't part of the electrical installation.
The earthing of the installation metalwork gives a fault path back to earth, should that installation metalwork become live, in order to trip the protective device.
The gas pipe won't need earthing because it's not likely to become 'live'.

I think people tend to think of it the wrong way round - thinking that, it's connected to the MET, so it's earthed, but that's not what you are doing, you are connecting (bonding) the gas/water to your installation metalwork.......which just happens to be earthed.

In an 'earth free' location, you could still bond all metalwork within that location to maintain a potential - but none of the installation would be earthed.

As for the potential thats 'likely to be introduced'.
It is most likely to be earth potential but, if say, you lived in a first floor flat, and the gas supply entered the ground floor flat, and then continued on up to your flat - does the pipework need 'bonding' to your MET?

It's not going to introduce 'earth potential', as it's bonded to the MET in the ground floor flat, at the point that it enters the building.

But it could introduce a 'system' potential, differing from that of your installation and it could, of course, introduce a 'fault potential' if there was a fault in the ground floor flat..........so, yes, you would 'bond' the pipe to your MET.



PS As for your 'oil tank', John, I wouldn't bond the pipe!

 

[EFLImpudence]

Yes that is obviously so but only exposed (already earthed) and extraneous (already bonded because liable to be naturally earthed) parts need to be bonded to equalise potential in fault conditions.
Assuming a correctly installed installation, if a metal part is not already bonded or earthed then it does not need to be bonded.

Could you help me understand that?

I have inserted my whole quote.
All service pipes etc. are correctly bonded and all exposed parts are earthed.
So anything else will be isolated and does not need bonding.

Earthing operates the CPD to limit touch voltage duration.
Bonding equalises potential to limit touch voltage magnitude.

 

[JohnW2]

That is a decision the electrical installer has to make. 'Looking at it from here' I would think that a plastic and wood stand would be sufficient insulation to make the tank and the pipe not extraneous (given or inclined to introduce etc.)

As you say, it's a decision the installer would have to make. I personally would probably err on the side of caution, particularly since 'unnecessary bonding' doesn't really do any harm (particularly given, as you go on to say, that it is almost certainly going to be in electrical continuity with other pipework, which itself will almost certainly have a path to earth.

In the particular case, the pipe is almost certainly in contact with an (often wet) outside brick wall (metal saddle clips if I recall), which would probably strengthen my inclination to err in the bonding direction.

Kind Regards, John.

 

[JohnW2]

Yes that is obviously so but only exposed (already earthed) and extraneous (already bonded because liable to be naturally earthed) parts need to be bonded to equalise potential in fault conditions. Assuming a correctly installed installation, if a metal part is not already bonded or earthed then it does not need to be bonded.

Could you help me understand that?

I have inserted my whole quote. All service pipes etc. are correctly bonded and all exposed parts are earthed. So anything else will be isolated and does not need bonding.

That's what I thought you were saying, but it seemed to me so much of a 'statement of the obvious' that I thought I must be miunderstanding. So, to be clear, is it correct that what you are saying is:
[code:1]When everything which needs to be bonded is bonded, then nothing else needs to be bonded[/code:1]?
If so, I definitely do not disagree

Earthing operates the CPD to limit touch voltage duration. Bonding equalises potential to limit touch voltage magnitude.

That's quite a useful way to explain things, but it's obviously a simplification which obscures the inevitable overlap. In practice, given the relative CSAs concerned (but obviously depending upon wiring layout), a very substantial proportion of the fault current which results in the CPD limiting touch voltage duration may flow via pipework and main bonding conductors rather than CPCs. Conversely, given the multiplicity of exposed-conductive-parts, CPCs will inevitably form (parallel) part of many 'potential equalisation'(bonding) pathways (which limit touch voltage magnitude). ... as if often said, electrons cannot read the label of the conductor of which they are part

Kind Regards, John.

 

[JohnW2]

I really don't see why the confusion arises between earthing and bonding - it really is quite simple.

As I hope you realise, I understand that, but it doesn't alter the fact that people get confused by the 'overlaps'. As you go on to say yourself ...

I think people tend to think of it the wrong way round - thinking that, it's connected to the MET, so it's earthed, but that's not what you are doing, you are connecting (bonding) the gas/water to your installation metalwork.......which just happens to be earthed.

As you point out, even if it's not one's 'intention', there is no way of main bonding anything without also earthing it. I can see why some people get confused by the fact that, although one is inevitably earthing the bomded metalwork, "this is not earthing". It would probably be less confusing to them if we admitted that an MPB conductor does both things (even if we only put it there to do one of them).

In an 'earth free' location, you could still bond all metalwork within that location to maintain a potential - but none of the installation would be earthed.

One could, indeed, but in such a location there would obviously be no 'earthing' to get confused with!

As for the potential thats 'likely to be introduced'.... But it could introduce a 'system' potential, differing from that of your installation and it could, of course, introduce a 'fault potential' if there was a fault in the ground floor flat..........so, yes, you would 'bond' the pipe to your MET.

Exactly ... which is one of the reasons I challenged the suggestion that one only bonds together two things which are earthed.

PS As for your 'oil tank', John, I wouldn't bond the pipe!

You might possibly reconsider when you saw it clamped to a dripping wet brick wall. As I've just written to EFLI, I think I would err on the side of caution. Next time I'm in the neighbour's house, I'll have a peep to see what the installer decided!

Kind Regards, John.

 

[EFLImpudence]

[code:1]When everything which needs to be bonded is bonded, then nothing else needs to be bonded[/code:1]?
If so, I definitely do not disagree

When put like that it does make it look silly - but it was in reply to your statement of

but bonding something to something else which is earthed results in the first something being 'earthed', whilst earthing two things (to the same earth) results in them being 'bonded' - which is what results in a lot of the confusion, particularly given that when one bonds two things together, at least one of them will usually be 'earthed'!

My point being that the first something - either

already was or should not be.

 

[JohnW2]

When put like that it does make it look silly - but it was in reply to your statement of

but bonding something to something else which is earthed results in the first something being 'earthed', whilst earthing two things (to the same earth) results in them being 'bonded' - which is what results in a lot of the confusion, particularly given that when one bonds two things together, at least one of them will usually be 'earthed'!

My point being that the first something - either already was or should not be.

Yes, but we've been over that before, and it's down to the semantic question about interpretation of 'liable to introduce'. If, for example, I decided to bond that oil pipe on the grounds that it may at some point in time introduce an earth potential (but currently isn't earthed) that would be an example of a situation in which your assertion doesn't quite work.

Kind Regards, John.

 

[EFLImpudence]

Sorry about the diversions - we're like that.

One of the items was "No main earth to gas or water". Most of the water pipes are plastic

There should be a 'Main Bonding Conductor' from the metal incoming gas and water pipes to the 'Main Earthing Terminal' in the Consumer Unit.
They should be on 'your side' of the stop-cocks.
If the water pipes are plastic when they enter your property then, obviously, plastic does not have to be bonded.

but I now realise there were odd short lengths of copper like the washing machine pipes which were not earthed. I can rectify this

These do not have to be bonded.

The only thing I'm wondering is if these earth cables should go to an earth block outside the CU? They currently terminate at the earth block inside the CU.

That's OK.

The earthing was also checked for the gas safety certificate and was passed.

I suspect that refers to the 'Supplementary Bonding' on the pipes entering the boiler.

Do you know if there is any right of appeal with these inspections or am I obliged to do whatever work they specify before they will issue a certificate?

I would think the only way out would be a second (or third) opinion but obviously expensive.

The Gas Safety Check is mandatory but you do not have to legally have the electrical installation checked to let your house (unless it is to be a house of multiple occupancy - several independent lets, like students)

You do have a duty to ensure the electrical installation is safe, though.

Have a good look at where the pipes run and see if you can see any green/yellow cable.
Why not take some pictures of the gas meter and water entry and post them on here

 

[EFLImpudence]

Yes, but we've been over that before, and it's down to the semantic question about interpretation of 'liable to introduce'. If, for example, I decided to bond that oil pipe on the grounds that it may at some point in time introduce an earth potential

That, as I said is up to the installer to decide.

(but currently isn't earthed)

It doesn't have to be earthed

that would be an example of a situation in which your assertion doesn't quite work.

Not sure what you mean.

 

[Electrifying]

.

PS As for your 'oil tank', John, I wouldn't bond the pipe!

You might possibly reconsider when you saw it clamped to a dripping wet brick wall. As I've just written to EFLI, I think I would err on the side of caution. Next time I'm in the neighbour's house, I'll have a peep to see what the installer decided!

Kind Regards, John.

And you might reconsider when, under a fault condition that, for whatever reason, doesn't clear, the installation metalwork and any metalwork bonded to it, rises to 230V.

This would take the oil-tank, which is outside, to 230V.
The man coming to fill said oil-tank, standing on true earth, would not be impressed......especially if he's dead.
You see, 'unneccesary bonding' can do harm.

I work to the Regs - and they are written by some very intelligent people...........and the guidance in them is there for a reason.

The pipe coming into the house from the oil tank is not part of the electrical installation - so it's not an Exposed Conductive Part and doesn't require earthing.

The pipe is also not an Extraneous Conductive Part - this could be tested and proven - so it doesn't require bonding.

If you really wanted to 'bond' the pipe to the MET, then you should first sink an earth rod and earth the tank - that way, if the tank were to go 'live' due to your 'bonding', there would be minimal risk to the operative filling the tank.

Thoughts???

 

[JohnW2]

..and my apologies to the OP for the diversions, too - as ELFI said, "we're like that!

The earthing was also checked for the gas safety certificate and was passed.

I suspect that refers to the 'Supplementary Bonding' on the pipes entering the boiler.

This is an interesting point. When I had some gas work done here a few months ago, checking the Main Bonding of the gas supply was certainly something that was done as part of the paperwork/certification of the gas work. Admittedly this was LPG, but I doubt it's any different for natural gas - so I would have rather expected that Main Bonding would also be checked as part of the gas inspection the OP had done. If he has paperwork from that inspection which indicates that the Main Bonding was satisfactory, this would obviously be something he could take up with those who performed the electrical inspection.

[my recollection of the Main Bonding here being checked is vivid, since I was dealing with a serious Jobsworth of a gas fitter. The MPB conductor to the gas entry pipe is long and inaccessible for a small part of its run. He would not believe me that the G/Y connected to the MET was the same one connected at the gas pipe near to entry into the property, and was not satisfied until he had disconnected both ends and undertaken a continuity test on the cable!]

Kind Regards, John.

 

[ban-all-sheds]

Jest ye not - it's a big problem in my part of the sticks.

I know.

He shouldn't really liven up the tank though.

At least they haven't found a way of helping themselves to my LPG yet (although it might be fun to watch them trying!)!