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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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 12 April 2018 11:26 PM
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MichaelGranvilleLevens

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Joined: 03 January 2016

Hello Hello,

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.

I did advise that the lower ground floor had been tanked then insulating foam layered before screed was poured. He advised that I would need a test reading of 50,000ohms ( basically the non conducting location regs) in order for it not to be required.

However I found a link on a the Voltium website stating to prove wether earth potential is liable to be introduced by a conductive part that is connected to the general mass of earth, a measurement should be made between conductive part and the main earth terminal of the intallation. (which makes sense to me)

It then gives a calculation example: 230V (50Hz) supply, an item having a resistance Rcp would not be considered an extraneous conductive-part where Rcp exceeded 7.67kohms as shown:

230/0.030= 7.67kohms

This value is clearly a lot lower...

Any brains out there that can provide me with some critical thinking it would be much appreciated.

Mainly because another lecturer I had backed me up on my understanding that steel conduit is best for running in concrete due to its closer contraction and expansion rates to concrete as apposed to plastic which could cause cracks in screed etc. However If I have to run 10mm bonding to it every time I'm less inclined to use it. Others use high gauge plastic to no detriment...
 12 April 2018 11:54 PM
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geoffsd

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It's not the 30mA of the RCD which is the determining value.
It is the touch voltage and the current which the human body can withstand and still be able to let go, that is the muscles do not contract.

This is generally taken as 10mA leading to 230/0.01 = 23kOhms (although, of course, the actual voltage may be higher).
Some believe it should be limited to 5mA - 46kOhms. Not far from the assessor's 50kOhms.
This is a nominal value, anyway, as 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.

The Voltimum calculation value is appropriate for Supplementary Bonding in bathrooms etc.

You would, of course, have to disconnect the pipes from any electrical device to test decisively as they will likely be connected to a CPC so you may as well Main Bond them.

Also, while CH pipes emerging from the ground must be bonded, they might be effectively connected to the incoming pipe or each other anyway.
The Main Bond on the water inlet can be linked to the CH pipes.
 13 April 2018 07:39 AM
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dickllewellyn

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This has confused me a little.

I'm quite familiar with the 23kOhm Reading, but I'd question the need for a main bond to the central heating pipes. My assumption is that they run inside the house, and don't enter from outside. Would this not mean a supplementary bond will suffice? We use the measurement for bathroom pipe work, but we don't then take a main bond to the bathroom do we? We use supplementary bonding.

Maybe I've misunderstood, but I always thought we use main bonding for metallic services entering from outside the equipotential zone likely to introduce a potential. Are we now saying the equipotential zone only stretches as far as finished floor level? I guess it's not really black and white. An old building with clay tiles sat straight on dirt will be different to a new concrete slab on a layer of insulation sat inside a vapour barrier.

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 13 April 2018 10:15 AM
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MichaelGranvilleLevens

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Yes I have witnessed the installation of Pipeworks and know they stay inside the building. My view is yes the main water bond provides the continuity protection of the extraneous CH pipe work, even supplementry bonding is not required due to Zs values being adequate for ADS and RCBO protection.
 13 April 2018 10:19 AM
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MichaelGranvilleLevens

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Back to my original question which measurement and value can I use to categorically prove that an extraneous conductive part isn't bring in a different potential from earth?
 13 April 2018 11:25 AM
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Zoomup

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Guidance note 8 used to say that a conductive part may not be an extraneous conductive part if its resistance to the main earthing terminal exceeds 459k Ohm or 22k Ohm ( depending upon the 0.5 mA threshold of perception or 10.0 mA let go threshold current shock values). The guide also says that the designer's decision about a conductive part not being an extraneous conductive part should also consider the likely stability of the resistance of the conductive part of the installation. Could it become more "earthy" over time due to say dampness increasing in the area?

Z.
 13 April 2018 11:41 AM
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Zoomup

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Reg. 411.3.1.2 says that in each installation MAIN protective bonding conductors complying with Chap. 54 shall connect to the main earthing terminal extraneous conductive parts including the following;

i. Water installation pipes.

ii. Gas installation pipes.

iii. Other installation pipework and ducting.

iv. Central heating and air conditioning systems.

v. Exposed metal structural parts of the building.

Now, what exceptions are mentioned in the Regs?

Z.
 13 April 2018 11:44 AM
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AJJewsbury

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but I'd question the need for a main bond to the central heating pipes. My assumption is that they run inside the house, and don't enter from outside.

It's not really a matter of whether the pipes physically run outside the building, but whether they can be held at a voltage that might differ from the installation's protective conductors (which of course may be significantly different from the potential of true earth at times).

In an ideal situation with completely insulating building construction (including floors) and the only conductive paths to anything else being metallic pipework (or the DNO's earth facility) then the problem does equate to looking for pipework entering (or exiting) the building (at least where the extent of the electrical installation is the same as the extent of the building).

However in a less than ideal situation, with say pipework embedded into damp walls or floors, such assumptions aren't necessarily safe (I've had significantly low readings from lengths of lead pipe plastered into the inside face of outside walls in old buildings and very low readings from a length of abandoned lead pipe buried under the floor of a cellar but not actually running beyond the perimeter of the building).

Back to my original question which measurement and value can I use to categorically prove that an extraneous conductive part isn't bring in a different potential from earth?

The regulations don't give a simple single answer to that. Basically it's down to the designer to decide on a maximum shock current they're happy with in the particular circumstances. GN 8 I think suggests 10mA as being reasonable maximum for many circumstances (for most people that's perceptible but shouldn't cause any long term harm directly) - others prefer 0.5mA (below the level of perception). If you're in a situation where a non-lethal shock might never the less cause significant harm - e.g. by causing someone to instinctively pull back and so perhaps cause a fall from a ladder or raised platform then you might well be happier with a 0.5mA limit; likewise if there was something like a swimming pool involved you might feel more comfortable with a lower current. So anything between 22k Ohms (10mA - 23k Ohms, less 1000 Ohms to account for body resistance) and 460kOhms (0.5mA) could be acceptable - you decide.

- Andy.
 13 April 2018 11:49 AM
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AJJewsbury

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Now, what exceptions are mentioned in the Regs?

Only where the part isn't an extraneous-conductive-part - hence the OP's question.
- Andy.
 13 April 2018 12:06 PM
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Zoomup

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"I'm quite familiar with the 23kOhm Reading, but I'd question the need for a main bond to the central heating pipes. My assumption is that they run inside the house, and don't enter from outside. Would this not mean a supplementary bond will suffice? We use the measurement for bathroom pipe work, but we don't then take a main bond to the bathroom do we? We use supplementary bonding."

Section 415 concerns Additional Protection. "Additional protection in accordance with Section 45 may be specified with the protective measure. In particular additional protection may be required with the protective measure under certain conditions of external influence and in certain special locations (see the corresponding section of Part 7)."

Bath and shower rooms are special locations. 415.2 covers "Additional protection:Supplementary equipotential bonding". This is where supplementary bonding is considered an addition to fault protection.

Z.
 13 April 2018 12:09 PM
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mapj1

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And note that in the draft 18th the rules looked like they might be changing again, so services don't need a main bond if they are metal with insulating breaks where indoors meets outdoors, so if like like most gas mains, if it is plastic until it gets indoors, no bond will be needed in future - suggesting it is the terra-firma voltage coming in that is the concern.
You are not alone in wondering about this, and opinion on earthing and bonding, either main or supplementary has rocked back and forth in various editions of the regs pretty much since the first edition . The answer is that is not clear cut - there are situations where leaving it alone is best, and other where it is not, and plenty where you may as well toss a coin.
Not that any edition of the regs has suggested that approach, yet anyway.

-------------------------
regards Mike
 13 April 2018 12:20 PM
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geoffsd

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If CH pipes re-enter/emerge from the ground they may be extraneous-c-ps and might require (main) bonding, if they don't they probably aren't.
You cannot question it apart from asking is it an extraneous-c-p ? The original post's question.


Supplementary bonding is not required except in special locations.
 13 April 2018 12:41 PM
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gkenyon

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Originally posted by: geoffsd

If CH pipes re-enter/emerge from the ground they may be extraneous-c-ps and might require (main) bonding, if they don't they probably aren't.

You cannot question it apart from asking is it an extraneous-c-p ? The original post's question.


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

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) ... 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.

Supplementary bonding is not required except in special locations.
Or also where required to comply with Regulation 411.3.2.6

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

Web-Site: www.gkenyontech.com

Edited: 13 April 2018 at 12:47 PM by gkenyon
 13 April 2018 01:14 PM
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geoffsd

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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.
 13 April 2018 02:16 PM
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chrispearson

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Is it better (i.e. less risky) to over-bond or under-bond?

In a new build with services entering in plastic pipes, and nowadays probably plastic CH pipes, no problem. But what of the rewire with supplementary bonding everywhere (including the kitchen sink ;-) ) and let's say a plastic water pipe to the stop-cock, but a metal gas pipe, and copper pipes throughout? Naturally, the DNO has PME'd the supply at some stage.

Do you replace the (undersized) MPBC to the gas and water pipes? Do you bond the CH?

On the one hand, it seems to me that all the supplementary bonding plus the joints at the back of the boiler, etc. mean that the network of metal throughout the property amounts to one large extraneous mass and there can be little harm (and only modest expense) in a belt and braces approach.

On the other hand, you might want to minimize the quantity of touchable metal which would see a voltage rise in the event of a broken PEN conductor. It's that outside tap that bothers me.
 13 April 2018 02:36 PM
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geoffsd

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Originally posted by: chrispearson

Is it better (i.e. less risky) to over-bond or under-bond?

Both wrong.

In a new build with services entering in plastic pipes, and nowadays probably plastic CH pipes, no problem. But what of the rewire with supplementary bonding everywhere (including the kitchen sink ;-) ) and let's say a plastic water pipe to the stop-cock, but a metal gas pipe, and copper pipes throughout? Naturally, the DNO has PME'd the supply at some stage.
Do you replace the (undersized) MPBC to the gas and water pipes? Do you bond the CH?
On the one hand, it seems to me that all the supplementary bonding plus the joints at the back of the boiler, etc. mean that the network of metal throughout the property amounts to one large extraneous mass and there can be little harm (and only modest expense) in a belt and braces approach.

You seem to be answering all your own questions. It's all a compromise as to which you think the lesser of two evils.

There is no stock answer for "Does this [whatever] require bonding?" apart from "IF it is an extraneous-conductive-part it must be Main Bonded".

On the other hand, you might want to minimize the quantity of touchable metal which would see a voltage rise in the event of a broken PEN conductor. It's that outside tap that bothers me.

You definitely do.
A better approach would be the insertion of lengths of plastic pipe so that all the metal pipes were isolated from any earth contact.
 13 April 2018 03:23 PM
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Zoomup

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"A better approach would be the insertion of lengths of plastic pipe so that all the metal pipes were isolated from any earth contact."

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

Historical use of internal plastic gas pipes: http://www.hvnplus.co.uk/gas-s...t-off/3100812.article

Z.
 13 April 2018 04:21 PM
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geoffsd

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I was, of course, referring to the electrical considerations.

Although just having one earthed metal pipe in the premises from entry point to boiler would be a huge improvement.
 13 April 2018 04:24 PM
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AJJewsbury

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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.
 13 April 2018 04:31 PM
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geoffsd

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Just thought -

As there might be an insulating section at the point of entry, there could be another in the boiler.
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