All boils down to the accepted definition of extraneous conductive part
Seems to be a grey area
Seems to be a grey area
Explaining Main Bonding to customers
- Thread starter ceraunic_level
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BS 7671 defines:
An extraneous-conductive-part – “a conductive part
liable to introduce a potential, generally earth potential
and not forming part of the electrical installation”
I don't think the definition is a grey area, I just think that many people don't (make the effort to) understand it and just repeat someone else's incorrect interpretation they have heard or read somwhere.
An extraneous-conductive-part – “a conductive part
liable to introduce a potential, generally earth potential
and not forming part of the electrical installation”
I don't think the definition is a grey area, I just think that many people don't (make the effort to) understand it and just repeat someone else's incorrect interpretation they have heard or read somwhere.
Has no one noticed the error on the OP's picture,
sure he's an electrician,
sure he can make pictures,
sure he cant get work as he cant explain bonding to customers,
sure he doesn't even know which side of the gas meter and water stop tap to bond to !!!!!!!!!!!!!
sure he's an electrician,
sure he can make pictures,
sure he cant get work as he cant explain bonding to customers,
sure he doesn't even know which side of the gas meter and water stop tap to bond to !!!!!!!!!!!!!
The question I would be asking as a customer is "Why might the sink and cooker be at different voltages - the sink isn't even connected to the electrics"
I think the idea of a handout is a good idea- anything that sets you above the other tradesmen has got to work in your favour.
I think the idea of a handout is a good idea- anything that sets you above the other tradesmen has got to work in your favour.
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Has no one noticed the error on the OP's picture,
sure he's an electrician,
sure he can make pictures,
sure he cant get work as he cant explain bonding to customers,
sure he doesn't even know which side of the gas meter and water stop tap to bond to !!!!!!!!!!!!!
erm, to be fair, he did say
Am I right in saying that an extraneous conductive part is something metal that comes in to the premises from outside the equipotential zone (ie a water supply pipe) and thus may not be at the same potential as the MET. But an extraneous conductive part is NOT just something metal inside the premises that could become live due to a fault (such as central heating pipes), these are called exposed conductive parts?
I think the confusion arises because I have heard electricians refer to things entirely within the premises that could become live due to a fault as 'extraneous' conductive parts...
Many electricians struggle to understand earthing and bonding.
What about CH pipes buried in a concrete floor?
What about CH pipes which are connected via the boiler manifold to the water and gas supply pipes, which are, via the main bonding, at earth potential. Could they introduce an earth potential to the bathroom?
What about CH pipes buried in a concrete floor?
What about CH pipes which are connected via the boiler manifold to the water and gas supply pipes, which are, via the main bonding, at earth potential. Could they introduce an earth potential to the bathroom?
I want to be able to give the customer something to explain the need for such things as Main Bonding in a simple and clear way.
Of course there are different scenarios, but there must be a simple consensus on the basics that are required to make your standard domestic property safe and compliant.
People have difficulty understanding this, I want to produce a SIMPLE BASIC explanation that a granny would get. Stripped down to the very basics, but (obviously) factually correct.
What I have produced, is it incorrect or missing something
Of course there are different scenarios, but there must be a simple consensus on the basics that are required to make your standard domestic property safe and compliant.
People have difficulty understanding this, I want to produce a SIMPLE BASIC explanation that a granny would get. Stripped down to the very basics, but (obviously) factually correct.
What I have produced, is it incorrect or missing something
I am one of those who does not have a complete understanding of earthing, so I am ready to be corrected but the issues I can see with your diagram are:
The reason for MPB's is not so 'all pipework is at the same potential' (although this may be a consequence and desirable) it is so that these services do not introduce an earth potential different to that of the main earth.
Following on from that, there does not have to be a fault within the oven (or whatever) for you to get a shock between the earthed oven casing (at main earth potential) and the pipe (at introduced pipe potential), if MPB's are not present.
However that is not to say that the diagram is 'wrong' as if there was a fault as you have shown then you are right that the pipe being at the same potential as the oven casing would prevent you getting a shock between the pipes. (assuming a mains voltage fault you would probably still get a shock due to you providing another path to earth though, albeit at a higher resistance)
With RCD's and Automatic Disconnection of Supply, it seems to me that the main reason for MPB's is to stop the introduction of different potentials as described in the first 2 paragraph's above and also (maybe) as a back-up main earth (should the suppliers earth fail for some reason).
Unfortunately all the reasons and explanations for bonding are not simple so it's not going to be easy to produce a simple document for a 'granny' to understand, however a simple document which goes some way to explaining it, without being inaccurate or scaremongering would be great so I hope you can get something together (which you don't mind sharing with us!). Atm however I don't think you're quite there.
Also sink's are no longer bonded (for a long time) so using a sink in your diagram is confusing.
Would be good to get a collaborative effort to improve/redo the drawing because such a document would indeed be useful for explaining to customers. (I dont think ESC/NICEIC do one dedicated to bonding at the moment?)
The reason for MPB's is not so 'all pipework is at the same potential' (although this may be a consequence and desirable) it is so that these services do not introduce an earth potential different to that of the main earth.
Following on from that, there does not have to be a fault within the oven (or whatever) for you to get a shock between the earthed oven casing (at main earth potential) and the pipe (at introduced pipe potential), if MPB's are not present.
However that is not to say that the diagram is 'wrong' as if there was a fault as you have shown then you are right that the pipe being at the same potential as the oven casing would prevent you getting a shock between the pipes. (assuming a mains voltage fault you would probably still get a shock due to you providing another path to earth though, albeit at a higher resistance)
With RCD's and Automatic Disconnection of Supply, it seems to me that the main reason for MPB's is to stop the introduction of different potentials as described in the first 2 paragraph's above and also (maybe) as a back-up main earth (should the suppliers earth fail for some reason).
Unfortunately all the reasons and explanations for bonding are not simple so it's not going to be easy to produce a simple document for a 'granny' to understand, however a simple document which goes some way to explaining it, without being inaccurate or scaremongering would be great so I hope you can get something together (which you don't mind sharing with us!). Atm however I don't think you're quite there.
Also sink's are no longer bonded (for a long time) so using a sink in your diagram is confusing.
Would be good to get a collaborative effort to improve/redo the drawing because such a document would indeed be useful for explaining to customers. (I dont think ESC/NICEIC do one dedicated to bonding at the moment?)
Many thanks skenk
I see what you are saying about the sink, and people (electricians) getting hung up on the bonding issue. It could be any metal object, I just thought a sink would be a simple object to use.
Your comment
The reason for MPB's is not so 'all pipework is at the same potential' (although this may be a consequence and desirable) it is so that these services do not introduce an earth potential different to that of the main earth.
a subtle point but, yeah you could show the shock potential from the services at different voltages.
I guess the the key, stripped down point is that any metal coming into your house has the possibility of being at a different voltage to another piece of metal entering your house, by linking them together that possibility stops.??
I may be showing my ignorance here but..
If you touch a live object you will get a shock
If you touch a live object with one hand and some earthy metal with the other you may get a bigger shock as the fault path is better, and it will go across the chest.
The CPC is there to take a fault current to the protective device and clear the danger
The bonding is there to create an equipotential zone so metals do not have different voltages
But...
Say you have bonding but the CPC fails The metalwork throughout the house (That is connected) will rise to the same potential as the faulty item via the bonding. Thus no potential difference will exist between the metal work so you will not get a shock.
But because their is no fuse to clear, the fault it will remain.
If for some reason some earthy metal entered the house, when touch you would get a shock.
Oh and if we do come up with something useful, and people want to use it , great.
I see what you are saying about the sink, and people (electricians) getting hung up on the bonding issue. It could be any metal object, I just thought a sink would be a simple object to use.
Your comment
The reason for MPB's is not so 'all pipework is at the same potential' (although this may be a consequence and desirable) it is so that these services do not introduce an earth potential different to that of the main earth.
a subtle point but, yeah you could show the shock potential from the services at different voltages.
I guess the the key, stripped down point is that any metal coming into your house has the possibility of being at a different voltage to another piece of metal entering your house, by linking them together that possibility stops.??
I may be showing my ignorance here but..
If you touch a live object you will get a shock
If you touch a live object with one hand and some earthy metal with the other you may get a bigger shock as the fault path is better, and it will go across the chest.
The CPC is there to take a fault current to the protective device and clear the danger
The bonding is there to create an equipotential zone so metals do not have different voltages
But...
Say you have bonding but the CPC fails The metalwork throughout the house (That is connected) will rise to the same potential as the faulty item via the bonding. Thus no potential difference will exist between the metal work so you will not get a shock.
But because their is no fuse to clear, the fault it will remain.
If for some reason some earthy metal entered the house, when touch you would get a shock.
Oh and if we do come up with something useful, and people want to use it , great.
Again, my understanding and terminology are not 100% by any means so someone please correct me if i'm off/way off track here but:
Not necessarily subtle, we're talking about 2 different things here: a) getting a shock via fault or b) getting a shock from an introduced earth potential
Yes and the regs define where you stop earthing bits of metal which might provide a better return path than you (Main bonding, supp bonding with conditions etc)
Sounds fine to me
The route is via main earthing conductor/rod and earth at the transformer
If the cpc fails, yes. But if the MAIN (SUPPLIERS) EARTH fails, often a gas or water pipe will have a low enough ELI to 'clear a fault' assuming it is bonded and therefore connected to the fault via cpc/MET.
When explaining about bonding I usually tell people something along the lines of -
"You're not earthing the pipe (it's already earthed-probably), you're making it the same earth as the main earth."*
"It can provide an alternative earth if the main earth were to fail."*
Right enough, most people meet this with a bemused/don't know what-your-on-about look but further explanation can be given if necessary although I don't think any of them actually understand it.
This approach however dodges the issue of equipotential bonding so that the 'fault on the oven casing' doesn't give you a worse shock because you are 'holding the sink' *
Remember though, this only actually happens for a very short time because if the casing and sink are bonded together then (99.9 times out of 100) there will be an effective path to earth which will trip the breaker very quickly (in domestic situations to 17th). You actually have to be holding the radiator and touching the casing while the fault develops.
*these are the 3 concepts which, in my opinion, need to be conveyed on the labelled drawing/drawings and explained alongside in better language than I can manage.
I'd be willing to bet there are electrical contractors/engineers out there who have the kind of document we're looking for but I doubt if many (any) of them would be willing to share it.
Not necessarily subtle, we're talking about 2 different things here: a) getting a shock via fault or b) getting a shock from an introduced earth potential
Yes and the regs define where you stop earthing bits of metal which might provide a better return path than you (Main bonding, supp bonding with conditions etc)
Sounds fine to me
The route is via main earthing conductor/rod and earth at the transformer
fine
If the cpc fails, yes. But if the MAIN (SUPPLIERS) EARTH fails, often a gas or water pipe will have a low enough ELI to 'clear a fault' assuming it is bonded and therefore connected to the fault via cpc/MET.
When explaining about bonding I usually tell people something along the lines of -
"You're not earthing the pipe (it's already earthed-probably), you're making it the same earth as the main earth."*
"It can provide an alternative earth if the main earth were to fail."*
Right enough, most people meet this with a bemused/don't know what-your-on-about look but further explanation can be given if necessary although I don't think any of them actually understand it.
This approach however dodges the issue of equipotential bonding so that the 'fault on the oven casing' doesn't give you a worse shock because you are 'holding the sink' *
Remember though, this only actually happens for a very short time because if the casing and sink are bonded together then (99.9 times out of 100) there will be an effective path to earth which will trip the breaker very quickly (in domestic situations to 17th). You actually have to be holding the radiator and touching the casing while the fault develops.
*these are the 3 concepts which, in my opinion, need to be conveyed on the labelled drawing/drawings and explained alongside in better language than I can manage.
I'd be willing to bet there are electrical contractors/engineers out there who have the kind of document we're looking for but I doubt if many (any) of them would be willing to share it.
Bit late on this one, but if you go to the ESC website there are two PDFs that explain Earthing and Bonding and RCDs. You can phone the ESC and they will post you these leaflets, you can ask for 50 at a time, even if you are not registered with NICEIC.
http://www.esc.org.uk/safety-in-the-home/safety-leaflets.html
The two to look out for are...
Why do earthing and bonding need to be checked?
What is a Residual Current Device?
Whatever your views of the NICEIC or ESC these are helpful as they provide some independant advice to the consumer.
http://www.esc.org.uk/safety-in-the-home/safety-leaflets.html
The two to look out for are...
Why do earthing and bonding need to be checked?
What is a Residual Current Device?
Whatever your views of the NICEIC or ESC these are helpful as they provide some independant advice to the consumer.
Exposed conductive parts are "conductive parts of equipment which can be touched and which is not normally live but may become live in the event of a failure of basic insulation". Equipment in the regs is an abbreviation of "electrical equipment" hence for it to be an exposed conductive part the item needs to be part of an electrical system.
Items such as metal conduits/trunkings, electric cooker metal casings, metal socket fronts, metal luminaires, metal light switches etc are exposed conductive parts.
I wouldn't class central heating pipes as an exposed conductive parts.
They may be extraneous i.e. can introduce a potential via the water or gas supply, however as the route making the central heating pipework extraneous should already be bonded (PEBs to Gas + Water) this isn't a problem. The boiler will have electricity feeding the control side (the boiler is therefore an exposed conductive part). This should be connected to the MET via circuit protective conductors so isn't a problem either.
If the central heating pipework is buried in the ground then it may be extraneous in its own right and require bonding.
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