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Topic Title: Bonding. Plastic
Topic Summary: Domestic
Created On: 29 September 2013 10:53 PM
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 29 September 2013 10:53 PM
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SKElectrical

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I've never understood this fully.

1. Is the purpose of Main Bonding to remove potential that is on other incoming services - ie create an equipotential scenario between all services?

2. Focusing on Water service:
2a) if a house is supplied in a lead pipe, and then is plumbed in copper throughout - it is safe to assume that all pipework will be at equipotential so long as the Main Bonding is in place... correct?

2b) If the house is supplied in plastic, Main Bonding isn't required because plastic don't conduct.

2c) If there is no Main Bonding on plastic water supply but the house is plumbed in copper, should there be any supplementary bonding? - if so, what to / between what?

2d) Scenario I'm faced with: Plastic supply pipe, feeds copper, then interchanges copper - plastic - copper - plastic etc. C/u is full RCD.

2e) If the c/u is full RCD, is any supplementary bonding required anyway?
 30 September 2013 01:07 AM
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DOUGIE1000

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A plastic incommer and copper house pipwork should be bonded at stopcock as per normal

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 30 September 2013 03:32 AM
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johnbyrne

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Not the most convincing argument Dougie, "as per normal". If not extranious or an exposed conductive part possibly a reason needed
 30 September 2013 10:28 AM
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AJJewsbury

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2b) If the house is supplied in plastic, Main Bonding isn't required because plastic don't conduct.

Unless the metallic "internal" plumbing can pick up an (earth) potential from somewhere other than the supply pipe - copper pipes buried under concrete floors, or even plastered into (slightly damp) outside walls can have a surprisingly low resistance to true earth. That's without the obvious things like a copper pipe going out and then underground e.g. to a garage or outside tap.

A plastic incommer and copper house pipwork should be bonded at stopcock as per normal

The guidance documents are I think a little in two minds about that still - one at least suggests an insulation test between the metallic pipework in question and true earth - over 22k Ohms and it's not extraneous.

- Andy.
 30 September 2013 02:59 PM
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SKElectrical

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OK but there is no requirement to earth metal pipework that isn't exposed, is there?
 30 September 2013 04:22 PM
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AJJewsbury

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OK but there is no requirement to earth metal pipework that isn't exposed, is there?

<picky> - no requirement to earth any pipework at all, it might need to be bonded though </picky>

If it doesn't introduce a potential (from outside the installation/location) then no.

If it does, that's a bit trickier to answer. Common sense suggests you're right, but there's nothing in the requirements of 411.3.1.2 or the definition of an extraneous-conductive-part that obviously agrees. Perhaps you could argue that pipework that isn't exposed doesn't really enter the space the electrical installation serves, so in that respect it doesn't introduce a potential into the installation.

Certainly if you can identify all points where an potential could be introduced and (if necessary) bond each of those, you don't need to worry about bits in between within the installation - if they happen to be in contact then they're bonded, if they're not they don't need bonding.

Same logic applies for supplementary bonding (where it's still required) - if all pipework is bonded as it enters the location (e.g. bathroom) then you don't need to worry about copper-plastic-copper-plastic etc. within the bathroom.

- Andy.
 30 September 2013 07:23 PM
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geoffsd

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Originally posted by: SKElectrical
OK but there is no requirement to earth metal pipework that isn't exposed, is there?

Depends what you mean by 'exposed'.


Originally posted by: AJJewsbury
<picky> - no requirement to earth any pipework at all, it might need to be bonded though </picky>

Not picky - fundamental.
 30 September 2013 07:31 PM
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UKPN

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plastic incomer? copper internal? exposed?
bonding reqd. but then we all know that dont we?

perhaps the xxxxamendment-amendment? will tell us.

who cares? bonding is still reqd.

Regards.

trust the DNO
 30 September 2013 07:53 PM
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slittle

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I've found a couple recently where it's plastic external, plastic internal but some bright person has put two inches of copper in after the plastic stop cock to allow them to put a BS951 clamp on the pipework.........

There's a word for it, but I'm struggling at the moment


Stu
 30 September 2013 08:11 PM
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cookers

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Stu, I bet its got a compliance certificate !
 30 September 2013 09:12 PM
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leckie

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Plastic incomer. Copper internal water pipes. Bond or not bond? I say bond. The reason being what Andy alluded to. Do an R2 test to water pipes that have a plastic incomer. Always low in my experience. Are they introducing an earth potential? Possibly not. Try proving that without dismantling cpc's to imm heater, boiler, etc, very difficult. As Andy said, pipes in walls, floors, etc., all likely to likely or at least possible to introduce an earth potential. So in my opinion bond. If you don't you will have the hassle of trying to convince NHBC, Buillding Control etc, why not. Not worth the hassle.
 01 October 2013 08:09 AM
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dickllewellyn

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If you're careful, you can sometimes use a self tapping screw in the side of the plastic pipe. You have to watch it doesn't split, but then with a bit of silicone to stop any leeks, and obviously a safety electrical connection tag, the water is bonded so it can flow through any pipe it likes in relative safety.

Now with my tongue no longer in my cheek....... Please don't actually do this!!!

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

"Insert words of wisdom and/or witty pun here"
 01 October 2013 10:04 AM
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davezawadi

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Right lets do this again...

Main bonding is to prevent an external potential from entering our installation. Nothing else.
Supplementary bonding is to ensure that various bits of an environment are at the same potential, nothing else.

The reason that main bonding conductors may need to be quite large is that currents from other installations may flow in them under external fault conditions, and these currents may not disconnect that fault, for example a lost neutral in the supply system with PME. Potentially all of the supply system neutral current could flow in the service pipes of all the connected installations through their main bonding conductors, and then back through the service neutral to the rest of the neutral isolated load installations beyond the fault.

So if we have a plastic service pipe no current can flow in it and we are safe from the situation described above.

Now the rest of the pipework, and whether it needs to be bonded. In a domestic installation it is likely that all the pieces which could gain a potential from a fault within the installation are bonded by the cpc of the connected electrical items like the boiler, shower and immersion heater. Other lengths of pipe which are not directly connected to mains appliances will either be connected by metal pipes or isolated by plastic ones, or plastic joints. As such these present no particular danger, and may be ignored. Lengths of pipe which are otherwise isolated but are close to exposed conductive parts may be bonded, but it is debatable whether this increases or decreases any risk. In the bathroom or other areas of enhanced risk because a person is likely to be standing in water, we ensure that we have a solid equipotential to every conductive object which could gain a potential with supplementary bonding, to all the local supplies and exposed metalwork within reach (Amdt 1: unless every supply is RCD protected at 30mA or less). Any services which travel outside the building and are metal (soil and drain pipes) should also be supplementary bonded to the bathroom system.

If both the gas and water service are plastic, then there is nothing to main bond, and you may consider supplementary bonding or RCD protection or both to your choice.

Adding short metal bits to plastic services to fit bonding is plain stupid, and any certs issued will undoubtedly be complete tosh, and such contractors should loose any registration they may have, particularly partP.

You will see that most installations will have multiple connections from pipework to earth, and that they are very safe. You may decide to add additional bonds between pipes (cross bonding to use the plumbers term) but adding such bonds in places where pipes are already solidly connected eg. by the boiler, is a waste of time and effort. I like to link the pipes at a sink, but not the sink itself, as an additional safety measure, but certainly bonding everything in a professional kitchen is foolish as it produces a huge conductive environment, besides making cleaning very difficult (which is a big risk).

-------------------------
David
CEng etc, don't ask, its a result not a question!
 01 October 2013 11:27 AM
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OMS

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Main bonding is to prevent an external potential from entering our installation. Nothing else.


I disagree totally David - at every level.

Main bonding won't prevent any external potential from "entering" - from the perspective of an external fault - it will provide a truly equipotential zone when referenced to the MET - ie it ensures that all extraneous metalwork rises to the same potential but it does not deny that voltage insult

Main bonding will also try to equalise any touch voltage arisng from an internal fault - it doesn't quite succeed in that as Ohms law gets in the way - but it can get pretty close to creating the so called equipotential zone (best to think of it as a zone of protection) - again, it does not deny the electrical insult, it simply serves to diminish (and in most cases, to significantly diminish) the magnitude of that voltage.

Bonding does nothing to prevent a voltage rise when refernced to the MET - it only controls the magnitude of the insult

Regards

OMS

-------------------------
Failure is always an option
 01 October 2013 09:43 PM
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SKElectrical

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Originally posted by: davezawadi
Main bonding is to prevent an external potential from entering our installation. Nothing else.

Supplementary bonding is to ensure that various bits of an environment are at the same potential, nothing else.



The reason that main bonding conductors may need to be quite large is that currents from other installations may flow in them under external fault conditions, and these currents may not disconnect that fault, for example a lost neutral in the supply system with PME. Potentially all of the supply system neutral current could flow in the service pipes of all the connected installations through their main bonding conductors, and then back through the service neutral to the rest of the neutral isolated load installations beyond the fault.



So if we have a plastic service pipe no current can flow in it and we are safe from the situation described above.



Now the rest of the pipework, and whether it needs to be bonded. In a domestic installation it is likely that all the pieces which could gain a potential from a fault within the installation are bonded by the cpc of the connected electrical items like the boiler, shower and immersion heater. Other lengths of pipe which are not directly connected to mains appliances will either be connected by metal pipes or isolated by plastic ones, or plastic joints. As such these present no particular danger, and may be ignored. Lengths of pipe which are otherwise isolated but are close to exposed conductive parts may be bonded, but it is debatable whether this increases or decreases any risk. In the bathroom or other areas of enhanced risk because a person is likely to be standing in water, we ensure that we have a solid equipotential to every conductive object which could gain a potential with supplementary bonding, to all the local supplies and exposed metalwork within reach (Amdt 1: unless every supply is RCD protected at 30mA or less). Any services which travel outside the building and are metal (soil and drain pipes) should also be supplementary bonded to the bathroom system.



If both the gas and water service are plastic, then there is nothing to main bond, and you may consider supplementary bonding or RCD protection or both to your choice.

You will see that most installations will have multiple connections from pipework to earth, and that they are very safe. You may decide to add additional bonds between pipes (cross bonding to use the plumbers term) but adding such bonds in places where pipes are already solidly connected eg. by the boiler, is a waste of time and effort. I like to link the pipes at a sink, but not the sink itself, as an additional safety measure, but certainly bonding everything in a professional kitchen is foolish as it produces a huge conductive environment, besides making cleaning very difficult (which is a big risk).



Great summary. Thanks.



Originally posted by: OMS

Main bonding is to prevent an external potential from entering our installation. Nothing else.



Main bonding won't prevent any external potential from "entering" - from the perspective of an external fault - it will provide a truly equipotential zone when referenced to the MET - ie it ensures that all extraneous metalwork rises to the same potential but it does not deny that voltage insult
OMS


Good point!
 01 October 2013 10:31 PM
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dg66

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I wouldnt rely too much on immersion,boiler or connected items CPCs as a means of trying to equalise any potential, as these are fortutitous contacts that are installed to protect against earth faults, nothing to do with bonding, which as OMS points out is to limit the magnitude of a fault, ie keeping touch voltages to a minimum.Regardless of plastic or metal service pipes, bond it, unless the internal pipework is plastic.

-------------------------
Regards

Dave(not Cockburn)
 02 October 2013 07:48 AM
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dickllewellyn

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An appliance earth or a cpc is perfectly acceptable for supplementary bonding though Dave.

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Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 02 October 2013 12:17 PM
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davezawadi

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Ha OMS, I was expecting you to say that!

I was attempting to make the reasoning as simple as possible, and using my arguably somewhat inaccurate terminology was to provide a degree of seperation between the internal and external fault scenarios.

I concur with some of the points about wheteher the pipes can be considered extraneous, but as we already have proper earthing (via CPCs) of any connected electrical items, it is only isolated, and insulated bits which really need consideration and their ability to import an external potential is pretty limited compared to incoming metallic services. Probably the greatest safety feature would be an earth electrode for every installation, even if TNS or TNCS, which would help to equalise the MET potential to the local structures and surroundings.

There is also the point about touch voltages inside the equipotential zone which OMS makes, and whilst we may be able to reduce this a bit with a substantial main bond from any exposed pipework to the MET, it is not particularly important in a domestic as we have a 0.2 seconds max disconnection time and it will usually be substantially faster given the common use of MCBs, and also the very low probability of short to earth faults in the connected water appliances such as the immersion or boiler. Use of RCD protection on all circuit will reduce the risk somewhat further.

I came accross an interesting case of "cross bonding" yesterday in a kitchen with an undersink water heater. The heater has solid metal between the input and output compression connectors, and the tap connections were via flexible hoses. The "electrician" (not actually that because there was no EIC for the new circuit) had bonded across the heater and to the sink, but not the tap, and had not connected the bonds to the incoming water (metal) which was insulated by a push fit coupler, or the local CPC. I wonder what manual he was following?

Regards
David

-------------------------
David
CEng etc, don't ask, its a result not a question!
 02 October 2013 01:54 PM
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OMS

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Originally posted by: davezawadi

Ha OMS, I was expecting you to say that!

Am I that predicatable, David

I was attempting to make the reasoning as simple as possible, and using my arguably somewhat inaccurate terminology was to provide a degree of seperation between the internal and external fault scenarios.

OK - for simplicity we have two situations - faults outhwith the installation boundary - ie outside the equipotential zone - and those faults being generated inside the equipotential zone.

In both cases, without bonding, you can get a hell of a zap from earthed metalwork and unbonded extraneous parts - with bonding, almost without exception you will diminish that zap to often a quite small value.

In the former case - lets take something simple like a faulty install up the road from you and no or faulty RCD protection - your MET is going to rise to some voltage - an unbonded metallic water pipe at true earth is going to sit some way below that. Your MET is connected via CPC's to a class 1 appliance in your kitchen which you can touch as well as the tap - you are going to get a hell of a belt from the apploiance case at say 100+ volts to the tap at zero volts. That bonding also puts the tap at 100V and you efectively have equal potential at both hands and thus no shock.

In the latter case, assume it's your class 1 appliance that's faulty - everything else being equal, you will be exposed to a touch voltage from that appliance case to your water pipe - it will be in the order of magnitude of perhaps 180V and it will be effectively derived from:

Vtouch = Ifault x R2.

If you don't bond that pipe, R2 is your circuit CPC and the earth path right back to the transformer star point.

If you do bond, R2 is now just your circuit CPC.

As Ifault isn't going to change much with or without bonding, then when R2 is "small" so is the touch voltage - if it's big (er) then the touch voltage increases - simples

We can't make it much simpler than that



I concur with some of the points about wheteher the pipes can be considered extraneous, but as we already have proper earthing (via CPCs) of any connected electrical items, it is only isolated, and insulated bits which really need consideration and their ability to import an external potential is pretty limited compared to incoming metallic services.

Fridge and gas lines from an external heat pump, earth tube cooling duct, any variety of other services - metallic wastes etc - stuff diving into and out of the floor slabs, stuff diverting outside and back in again ?



Probably the greatest safety feature would be an earth electrode for every installation, even if TNS or TNCS, which would help to equalise the MET potential to the local structures and surroundings.

I disagree - do the sums on say a 40A load in the house - you'd need an electrode less than 2.5 Ohms resistance to get under 70V let alone 50V


There is also the point about touch voltages inside the equipotential zone which OMS makes, and whilst we may be able to reduce this a bit with a substantial main bond from any exposed pipework to the MET, it is not particularly important in a domestic as we have a 0.2 seconds max disconnection time

Actually we have 0.4 second disconnection time (Column 4 of table 41.1)

and it will usually be substantially faster given the common use of MCBs, and also the very low probability of short to earth faults in the connected water appliances such as the immersion or boiler.

I've seen plent of L-E faults in immersion heaters when they've got a bit hot - and we haven't yet got rid of fuses in a lot of installations - but we do now have a lot of installations where indicriminate swapping of MCB's to deal with other issues has served to ensure that they won't trip any faster than fuses anyway - limiting Zs is too high

Use of RCD protection on all circuit will reduce the risk somewhat further.

For sure, but it's not a mandatory requirement - there are plenty of circumstances even in a domestic setting where an RCD isn't required - there are many more outside of a domestic arena, but in household or similar locations.

I came accross an interesting case of "cross bonding" yesterday in a kitchen with an undersink water heater. The heater has solid metal between the input and output compression connectors, and the tap connections were via flexible hoses. The "electrician" (not actually that because there was no EIC for the new circuit) had bonded across the heater and to the sink, but not the tap, and had not connected the bonds to the incoming water (metal) which was insulated by a push fit coupler, or the local CPC. I wonder what manual he was following?


Regards

David


Regards

OMS

-------------------------
Failure is always an option
 02 October 2013 06:00 PM
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dg66

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Originally posted by: dickllewellyn

An appliance earth or a cpc is perfectly acceptable for supplementary bonding though Dave.


Agree, but i was more along the lines of equipotential bonding, and trying to create an equipotential zone.

-------------------------
Regards

Dave(not Cockburn)
IET » Wiring and the regulations » Bonding. Plastic

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