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Topic Title: Determining an extraneous conductive-part by measurement
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Created On: 12 April 2018 11:26 PM
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 13 April 2018 05:00 PM
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Zoomup

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Originally posted by: AJJewsbury

I think that I would prefer nice strong non-flammable metal gas pipes in my home as opposed to plastic ones.


Usual arrangement these days for gas is plastic out of the ground to an entry point above ground level then copper through the wall and internally (whether the meter is internal or external) - which would seem to satisfy both camps.

- Andy.


Copper through the wall eh? Nice and wet in the rain, is it then still not extraneous? I worked at an install where the copper gas pipe ran from the external ground level gas meter for about 25 metres held on plastic standoff pipe clips fixed onto an outside brick wall at low level. It then ran through a brick wall into the house. Extraneous when wet, not extraneous when dry?

Z.
 13 April 2018 05:34 PM
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geoffsd

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Originally posted by: Zoomup

Copper through the wall eh? Nice and wet in the rain, is it then still not extraneous? I worked at an install where the copper gas pipe ran from the external ground level gas meter for about 25 metres held on plastic standoff pipe clips fixed onto an outside brick wall at low level. It then ran through a brick wall into the house. Extraneous when wet, not extraneous when dry?

Was it in contact with the ground?

If water made that much difference we wouldn't have to bond water pipes would we?
 13 April 2018 10:30 PM
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yellowvanman

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If you are able to get access to NICEIC Connections Magazine, look at Autumn 2014. There they say its not extraneous if >6.67Kohm!

NICEIC should be 'singing from the same hymn sheet'!
 13 April 2018 11:19 PM
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geoffsd

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I would suggest that, as the OP was told, that is the calculation for applying supplementary bonding.

Edit - I was getting muddled. That's not correct because the Ia of devices is divided into 50V; not 230, so I'm not sure what it relates to.



After all, if it does apply to Main Bonding, what would have been the calculation before RCDs?
It would give a very low value, above which MPB is not required, for MCBs or Main Fuses.

Edited: 13 April 2018 at 11:39 PM by geoffsd
 14 April 2018 10:47 AM
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gkenyon

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Originally posted by: geoffsd

We were discussing Main Bonding, however:



Originally posted by: gkenyon



If we are talking about CH pipes, they may need protective equipotential bonding ("main bonding") for reasons other than they are extraneous-conductive-parts.


What other reasons would they be for Main Bonding?



For example, if you want to omit supplementary equipotential bonding in a bathroom in accordance with 701.415.2. CH pipes are extraneous-conductive-parts of the location (the bathroom)


Only IF they ARE extraneous-c-ps.



see specific wording of (vi) in 701.415.2. Hence if CH pipes are not connected to the protective equipotential bonding of the installation in accordance with 411.3.1.2, you would not be able to omit supplementary equipotential bonding in the bathroom - regardless of whether they are extraneous-conductive-parts of the electrical installation as a whole.


Yes, but effectively connected to the protective equipotential bonding means satisfying 415.2.2, namely R<=50/Ia which with a 30mA RCD means 7,666 Ohms (as in the OP) which is unlikely not to be so IF it IS an e-c-p - not an unnecessary 10sq.mm. cable to the MET.
Read carefully, 701.415.2.

CH pipes (metallic) that are extraneous-conductive-parts to the location (i.e. the bathroom), must be main-bonded as one of the conditions to omit bathroom bonding.

So, if the CH pipes are not extraneous-conductive-parts (they never come into contact with the ground outside, say, only connected to the boiler's cpc, and held on everywhere by plastic pipe clips for argument's sake), I could see there would be an argument they are not extraneous-conductive-parts for the electrical installation as a whole, and possibly an argument not to main bond them (but why not anyway).

However, once you get to the bathroom, they could introduce earth potential from outside the location (from the cpc of the boiler in this example), and therefore you would have to provide supplementary local equipotential bonding in the bathroom ... or the other option, if all of the other conditions are met for omission of bathroom bonding, would be to main-bond the CH pipes and omit bathroom bonding ...

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EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 14 April 2018 11:22 AM
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AJJewsbury

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Edit - I was getting muddled. That's not correct because the Ia of devices is divided into 50V; not 230, so I'm not sure what it relates to.

R <= 50V/Ia
relates the operation of a circuit protective device where only a limited fault current might be available to the resulting voltage difference a victim could be exposed to - i.e. if the fault current is too low to operate the protective device then the touch voltage won't exceed a (safe) 50V. If the fault current is higher then the touch voltage may also be higher than 50V, but disconnection is assured. Basically it's invoking the old something like the old 'alternative method' (which allowed extended disconnection times provided the resistance of the c.p.c. was low enough to ensure that the potential difference between the fault and MET remained below 50V). Remember that supplementary bonding isn't just for bathrooms, but also for situations where normal ADS may not work - e.g. equipment fed from inverters (e.g. IT equipment fed from small UPSs).

Basing Ia on 5s disconnection times might seem odd at first sight for places like bathrooms where much faster disconnection is typically required, but for most overcurrent protective devices there's a inverse relationship between fault current and disconnection times - so a setup that would result in 5s disconnection time for a touch voltage of 50V would probably manage less than 1s for 70V and less than 0.4s for 100V and so on - which still provides pretty good match for shock protection curves. So maybe think of 50V and 5s as being one point that identifies the required curve, that than the whole requirement as such.

Clearly where you have several circuits that might put a potential into the same group of supplementary bonded parts during a fault you need to consider the worst case from all such circuits - even if they supply equipment that's outside the location you're considering (the old example was an old immersion in a cupboard off the back bedroom, fed by an 15A BS 3036, tied to the bathroom pipework via the copper hot water system pipes, but isolated from main bonding by the plastic header cistern - ia would then be 43A rather than 30mA).

The calculation for main bonding isn't related to any protective device but simply checks how much current could flow through a victim in the worst possible case voltage difference between the candidate part (probably earthy) and the installation's earthing system (during a fault). On a TN system the MET typically rises to around half the line voltage during an earth fault (e.g. 115V), on a TT system it's anything up to close to the entire line voltage (230V). But with PME supplies, in the case of a broken supply CNE, it could again get to 230V. So in general we take the worst case voltage to be 230V. We then look for a resistance that is at least what would be needed to limit the current though the victim to an acceptable value (where the choice of between 10mA and 0.5mA etc comes in).

- Andy.
 14 April 2018 12:21 PM
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geoffsd

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Originally posted by: AJJewsbury

R <= 50V/Ia
relates the operation of a circuit protective device where only a limited fault current might be available to the resulting voltage difference a victim could be exposed to - i.e. if the fault current is too low to operate the protective device then the touch voltage won't exceed a (safe) 50V. ...

Yes, I realise that, but because of the advice the OP was given and the NICEIC article I was confusing the two.



The calculation for main bonding isn't related to any protective device

What calculation is there for MB?

but simply checks how much current could flow through a victim in the worst possible case voltage difference between the candidate part (probably earthy) and the installation's earthing system (during a fault). On a TN system the MET typically rises to around half the line voltage during an earth fault (e.g. 115V), on a TT system it's anything up to close to the entire line voltage (230V). But with PME supplies, in the case of a broken supply CNE, it could again get to 230V. So in general we take the worst case voltage to be 230V. We then look for a resistance that is at least what would be needed to limit the current though the victim to an acceptable value (where the choice of between 10mA and 0.5mA etc comes in).

Yes, that is what I said in my first reply.

So, what relevance has 230/0.03 (7666 Ohms)in relation to the need to Main Bond extraneous-c-ps?

If it were correct then with a 10mA RCD the limit would be 23,000 Ohms and for your quoted fuse (230/43) 5.4 Ohms.

This is clearly nonsense.
 14 April 2018 01:01 PM
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geoffsd

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Originally posted by: gkenyon

If we are talking about CH pipes, they may need protective equipotential bonding ("main bonding") for reasons other than they are extraneous-conductive-parts.

What other reasons would there be for Main Bonding?

Read carefully, 701.415.2.

I have. It states that
Local supplementary equipotential bonding according to Regulation 415.2 shall ...


CH pipes (metallic) that are extraneous-conductive-parts to the location (i.e. the bathroom), must be main-bonded as one of the conditions to omit bathroom bonding.

It does not say that. Why should parts which are not extaneous-c-ps to the building be main bonded?

So, if the CH pipes are not extraneous-conductive-parts (they never come into contact with the ground outside, say, only connected to the boiler's cpc, and held on everywhere by plastic pipe clips for argument's sake), I could see there would be an argument they are not extraneous-conductive-parts for the electrical installation as a whole, and possibly an argument not to main bond them (but why not anyway).

Or Why? but that's not what is being discussed.

However, once you get to the bathroom, they could introduce earth potential from outside the location (from the cpc of the boiler in this example),

They could - exactly the same as any other pipe or metal part.

and therefore you would have to provide supplementary local equipotential bonding in the bathroom ...

You would.

or the other option, if all of the other conditions are met for omission of bathroom bonding, would be to main-bond the CH pipes and omit bathroom bonding ...

I disagree. Would you Main Bond a metal light fitting?

"Effectively connected to the protective equipotential bonding of the system according to 411.3.1.2" does not mean fitting unrequired main bonding but complying with 415.2, in particular 415.2.2.
 14 April 2018 01:04 PM
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AJJewsbury

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What calculation is there for MB?

Applying Ohm's law to the above requirement - i.e. R = 230V / acceptable body current. (perhaps less body resistance, sometimes taken to be 1000 Ohms).

So, what relevance has 230/0.03 (7666 Ohms)in relation to the need to Main Bond extraneous-c-ps?

It seems to be the above, but on the basis that the maximum acceptable body current is 30mA (somewhat high for my liking, but I suppose it is in line with the general concept of specifying 30mA RCD protection, even though we're not presuming the existence of any RCDs when considering main bonding).

- Andy.
 14 April 2018 01:27 PM
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AJJewsbury

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Copper through the wall eh? Nice and wet in the rain, is it then still not extraneous?

Copper gas pipes though outside walls are usually run through an plastic sleeve (to prevent gas entering any wall cavities in the case of a leak) and often sheltered by the meter box or house entry tee. There may be some cases where the copper is in enough contact with something earthy to make it extraneous of course - and you'd have to take each of those cases on their merit - but I suspect that a significant proportion of modern installations would prove not to be extraneous if measured.

- Andy.
 14 April 2018 01:53 PM
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geoffsd

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Originally posted by: AJJewsbury

Applying Ohm's law to the above requirement - i.e. R = 230V / acceptable body current. (perhaps less body resistance, sometimes taken to be 1000 Ohms).

Exactly, that was in my first reply.


It seems to be the above, but on the basis that the maximum acceptable body current is 30mA (somewhat high for my liking, but I suppose it is in line with the general concept of specifying 30mA RCD protection, even though we're not presuming the existence of any RCDs when considering main bonding).

No.

It says in the article that it is based on the rating of the RCD - so clearly wrong.
 14 April 2018 02:44 PM
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geoffsd

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Having reread the NICEIC article, the text of it is, in fact, correct.

However, in its 'Example' it incorrectly substitutes the rating of the RCD for the value of current through the human body which should not be exceeded, what it calls IB.
This error is compounded by the apparent belief that a 30mA RCD limits any fault current to that 30mA.

If this is corrected and 10mA (or 5mA or whatever) is used then the correct value as I stated in my first reply is confirmed.
 14 April 2018 04:04 PM
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chrispearson

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Originally posted by: MichaelGranvilleLevens

I'm looking to confirm best practice. I have a situation in a house where the lower ground floor kitchen has a screed floor and a small plant room containing the main water inlet (metal) two hot water tanks and various metal pipe work going into the screed floor.

An NICEIC assessor advised me not only the main water inlet which was already main bonded with 10mm but also the central heating pipes all needed to be main bonded too as they go into the ground, which I have now done.


Originally posted by: geoffsd

... you are not going to have to make a choice between 22,999 and 46,001. It will either be quite a low value or very high.


Going back to the head of the thread, the answer seems to be that if a potentially extraneous conductive part has an impedance of around 50kohms or more, it isn't extraneous and, therefore need not be bonded. As ever, there is a range of opinion, and some would not bond at lower impedances - so be it.

I would be very cautious in concluding that a metal pipe which emerges from below a solid floor is never extraneous unless the value is an order of magnitude about the 50kohms. Vapour barriers allow the layer above to dry out, but in particularly wet situations, or if the ventilation is impaired, the floor may become damp. At one time, my cellar floor was dry most of the time, but water would appear after sustained rain. Unblocking the ventilation bricks to the exterior solved the problem, so it is a matter of equilibrium.

If you are in doubt about this, leave some polythene sheeting on your garage/cellar/basement floor overnight and see whether any condensation appears.
 14 April 2018 09:36 PM
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gkenyon

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Originally posted by: geoffsd

Originally posted by: gkenyon



If we are talking about CH pipes, they may need protective equipotential bonding ("main bonding") for reasons other than they are extraneous-conductive-parts.


What other reasons would there be for Main Bonding?



Read carefully, 701.415.2.


I have. It states that

Local supplementary equipotential bonding according to Regulation 415.2 shall ...





CH pipes (metallic) that are extraneous-conductive-parts to the location (i.e. the bathroom), must be main-bonded as one of the conditions to omit bathroom bonding.


It does not say that. Why should parts which are not extaneous-c-ps to the building be main bonded?



So, if the CH pipes are not extraneous-conductive-parts (they never come into contact with the ground outside, say, only connected to the boiler's cpc, and held on everywhere by plastic pipe clips for argument's sake), I could see there would be an argument they are not extraneous-conductive-parts for the electrical installation as a whole, and possibly an argument not to main bond them (but why not anyway).


Or Why? but that's not what is being discussed.



However, once you get to the bathroom, they could introduce earth potential from outside the location (from the cpc of the boiler in this example),


They could - exactly the same as any other pipe or metal part.



and therefore you would have to provide supplementary local equipotential bonding in the bathroom ...


You would.



or the other option, if all of the other conditions are met for omission of bathroom bonding, would be to main-bond the CH pipes and omit bathroom bonding ...


I disagree. Would you Main Bond a metal light fitting?



"Effectively connected to the protective equipotential bonding of the system according to 411.3.1.2" does not mean fitting unrequired main bonding but complying with 415.2, in particular 415.2.2.
The answer to all these questions is very simple.

BS 7671 permits the 0.4 s disconnection time to be exceeded under certain conditions.

If the CH pipes are (fortuitously or otherwise) connected to an extraneous-conductive-part for which disconnection times are exceeded, and say supplementary local equipotential bonding is provided to sort that out, then the potential could be transferred to the bathroom.

The wording in 701 is very specific for a reason.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 15 April 2018 09:53 AM
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Zoomup

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Originally posted by: geoffsd

Originally posted by: Zoomup



Copper through the wall eh? Nice and wet in the rain, is it then still not extraneous? I worked at an install where the copper gas pipe ran from the external ground level gas meter for about 25 metres held on plastic standoff pipe clips fixed onto an outside brick wall at low level. It then ran through a brick wall into the house. Extraneous when wet, not extraneous when dry?


Was it in contact with the ground?



If water made that much difference we wouldn't have to bond water pipes would we?


Hello Geoff,
the gas meter sat outside in a box and sat on the ground. The copper pipe ran horizontally about 300mm above ground level on an outside wall. It was clipped and held by plastic stand-off pipe clips. It then entered the building through a brick wall at low level.

I don't understand your statement about bonding water pipes and the difference water makes. Please clarify.

Bye,

Z.
 15 April 2018 10:43 AM
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Zoomup

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Ref. 701.415.2. (Bath and shower rooms) The regulation does say that supplementary equipotential bonding may be omitted where ALL of the following conditions are met:

(iv) All final circuits of the location comply with the requirements for automatic disconnection according to Reg. 411.3.2.

(v) All final circuits of the location have additional protection by means of an RCD in accordance with Reg. 701.441.3.3.

(vi) All extraneous-conductive parts of the location are effectively connected to the protective equipotential bonding according to Reg. 411.3.1.2. (Main bonding).

So, if the extraneous metal bits and pieces in the bathroom are connected to the MAIN bonding system then no supplementary bonding is required. That makes sense to me. (iv) and (v) above being complied with normally in modern installations.


Z.

Edited: 15 April 2018 at 11:04 AM by Zoomup
 15 April 2018 12:43 PM
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geoffsd

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Hello Z

Originally posted by: Zoomup

I don't understand your statement about bonding water pipes and the difference water makes. Please clarify.


I should have said that we would not have to bond water pipes at point of entry. merely at the nearest point to the MET.

You were saying that a pipe which ran outside which was not an extraneous-c-p became so when it rained. If it were a water pipe and rain made that much difference then the water inside the pipe would be a large conductor and we wouldn't need to use a copper wire to connect the MET to the point of entry.

Also, if true then the water inside would make all copper water pipes inside the building extraneous-c-ps.

Edited: 15 April 2018 at 12:55 PM by geoffsd
 15 April 2018 12:53 PM
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geoffsd

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Originally posted by: Zoomup

(vi) All extraneous-conductive parts of the location are effectively connected to the protective equipotential bonding according to Reg. 411.3.1.2. (Main bonding).

A problem is that a lot of people read 411.3.1.2 as it stands - NOT realising it only applies to the listed parts WHEN they are extraneous-c-ps.

Also, they mistake the meaning of "effectively connected to the protective equipotential bonding".
 15 April 2018 06:21 PM
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gkenyon

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Originally posted by: geoffsd

Originally posted by: Zoomup



(vi) All extraneous-conductive parts of the location are effectively connected to the protective equipotential bonding according to Reg. 411.3.1.2. (Main bonding).


A problem is that a lot of people read 411.3.1.2 as it stands - NOT realising it only applies to the listed parts WHEN they are extraneous-c-ps.



Also, they mistake the meaning of "effectively connected to the protective equipotential bonding".
No-one is making you main bond CH pipes.

However, the Section 701 consideration is very important. Whilst they may not be extraneous-conductive-parts of the Installation, they may well be extraneous-conductive-parts of the location [containing the bath or shower]. Hence the use of the wording "of the location" ... and they can be extraneous-conductive-parts of the location by means of earthing / bonding outside the location (say at a boiler, or by main bonding).

This is important for the reasons I stated, transfer potential into the bathroom, in cases where disconnection times can't be met.

So, if they are extraneous-conductive-parts to the bathroom (as per my previous example), but not main bonded (because someone decided there was no need ... which as you say, may be because they are not extraneous-conductive-parts of the installation), then supplementary local protective bonding cannot be omitted.

Simples.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 15 April 2018 06:57 PM
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geoffsd

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Originally posted by: gkenyon

No-one is making you main bond CH pipes.

You are.

However, the Section 701 consideration is very important. Whilst they may not be extraneous-conductive-parts of the Installation, they may well be extraneous-conductive-parts of the location [containing the bath or shower]. Hence the use of the wording "of the location" ... and they can be extraneous-conductive-parts of the location by means of earthing / bonding outside the location (say at a boiler, or by main bonding).

Yes, we know that.

This is important for the reasons I stated, transfer potential into the bathroom, in cases where disconnection times can't be met.

Why have you brought up where disconnection times cannot be met? and
Why does the fact that it enters a special location affect the disconnection times?

If disconnection times cannot be met then supplementary bonding may not be omitted.

However, we were discussing a situation where ALL the conditions for omission of supplementary bonding are met and you said the part would have to be main bonded even though it is not an extraneous-c-p to the building.

So, if they are extraneous-conductive-parts to the bathroom (as per my previous example), but not main bonded (because someone decided there was no need ... which as you say, may be because they are not extraneous-conductive-parts of the installation), then supplementary local protective bonding cannot be omitted.

Please show where the regulations state this and why it should be the case.
IET » Wiring and the regulations » Determining an extraneous conductive-part by measurement

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