cross bonding

[JohnW2]

In this thread it has been a lot of tongue in cheek. But it does seem strange that cable installers cannot see that if two wires are connected to the same length of pipe they are both connected to the same galvanic (metal) node and a node can only have one designation.

As you will realise, that has been my point.

A cable installer with a book of regs says a water pipe to a boiler cannot not be earthed but must be bonded to the MET.....

I'm not aware of anything in the regs that says that such a pipe 'cannot be earthed'. Indeed, as you say, the boiler itself, which is in continuity with attached metal pipework, would (assuming it had an electrical supply) normally have to be 'earthed' (I've never heard of a Class II boiler). If you're looking for an anomaly (in engineering terms), the requirement to 'earth the boiler' (connect it to the CPC of its supply) exists even when the attached pipework is connected to the MET (via a 'bonding' cable). However, I guess that this is just 'belt and braces', and also to ensure that the boiler remains 'earthed' even if a plumber disconnects all pipes from it whilst it is still connected to an electricity supply!

Kind Regards, John.

 

[JohnW2]

Yes they are, but they are probably still more clued up than many practicing electricians on earthing and bonding.

It occurs to me that, as with trying to work out which of the people in a psychiatric hospital are staff, and which are patients, or even a Turing Test, it would be interesting to know what conclusions an 'outsider' would draw as to which of the posters in this forum were electricians, which were DIYers and which were 'special cases' .... I wonder how many they would get right!

Kind Rgards, John.

 

[bernardgreen]

~~ which of the posters in this forum were electricians, which were DIYers and which were 'special cases' .... I wonder how many they would get right!

You forgot three types, cable installers, engineers ( electrical ) and people with common sense. Though some individuals would fit into two or more types thus complicating the regulations as to which ward or staff room the individual must be sent to.

 

[dingbat]

Why bother to try and rewrite the rules when all you need to know is in this book:

http://www.amazon.co.uk/Guidance-No...6160/ref=sr_1_5?ie=UTF8&qid=1306861132&sr=8-5

You imply you are somehow trying to make it easier for the layman to get to grips with it all - why not do as the IET do and separate the principles of 'earthing' and 'bonding', as per diagram 2.1 of BS 7671 and forget all of this diversionary drivel?

I'm sure you would all have a lovely time discussing your theories with one Mr David-911-Cockburn, who is, after all, the expert:

http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=40836&enterthread=y

 

[JohnW2]

You imply you are somehow trying to make it easier for the layman to get to grips with it all ...

If I appear to have implied that, then I must have chosen my words poorly.

It's not actually the laymen who stimulate my comments. I just find it a bit tiresome to see non-laymen so often seemingly trying to score intellectual points by wheeling out a 'you don't understand the difference between earthing and bonding' lecture, often simply because a layman has written 'earth' when (s)he should have written 'bond', given the inevitable overlap which exists between the concepts in terms of actual wiring - and particularly given that some 'non-laymen' appear not to have a very good understanding of the underlying concepts.

Kind Regards, John.

 

[Electrifying]

There is a difference and it's really quite simple -

EARTHING limits the duration (time) that a voltage will exist on exposed metalwork during an earth fault (by Automatic Disconnection of Supply)

This is why 'Boiler Casings' etc are 'earthed' - if the line conductor touches the casing causing it to go 'live', then the CPC (earth) connection will carry the fault back to source (the Earth Fault Loop Path), activating the MCB/RCD.

BONDING limits the level of the Potential Difference (voltage) between two simultaneously-touchable conductive parts (pieces of metal) during the 'fault', thereby reducing the risk to a person of serious shock before the protective device trips.

Now there may be overlaps in the real world - i.e. combined bonding/earthing conductors etc etc.
BUT, I think it's important to understand the difference between the two - even if it's just to know what the hell it is you are actually doing!!!

 

[JohnW2]

There is a difference and it's really quite simple - ... etc. etc.

As I wrote yesterday, everyone (other than the op) involved in this thread fully understands all that.

As for how simple it necessarily is, consider the (admittedly rather unusual) situation I have in my house, which I described again yesterday ....

...I have a cable which, under L-E fault conditions within my installation would carry more than 99% of the fault current to earth, and hence more than 99% of the current that would result in the operation of a protective device. If you didn't know (or guess!) the background to the question, would you describe that cable as providing 'earthing' or 'bonding'?

Kind Regards, John.

 

[Electrifying]

There is a difference and it's really quite simple - ... etc. etc.

As I wrote yesterday, everyone (other than the op) involved in this thread fully understands all that.

Judging by a lot of the posts in this thread, very few people understand what I have posted above.


...I have a cable which, under L-E fault conditions within my installation would carry more than 99% of the fault current to earth, and hence more than 99% of the current that would result in the operation of a protective device. If you didn't know (or guess!) the background to the question, would you describe that cable as providing 'earthing' or 'bonding'?

The cable is a 'Main Protective Bonding Conductor' - as that is the purpose for which it was installed.
The fact that it will carry current under fault conditions is irrelevant - all 'main protective bonding conductors' will carry current under fault conditions due to the nature of what they are connected to ....i.e. Extraneous Conductive Parts.

 

[JohnW2]

If you didn't know (or guess!) the background to the question, would you describe that cable as providing 'earthing' or 'bonding'?

The cable is a 'Main Protective Bonding Conductor' - as that is the purpose for which it was installed.

Indeed it is, but you only know that because I told you. You defined earthing as:

EARTHING limits the duration (time) that a voltage will exist on exposed metalwork during an earth fault (by Automatic Disconnection of Supply)

... which is precisely the function that this 'MPB' conductor serves in my installation.

I'm not trying to be awkward, but am merely pointing out that a conductor which serves the purpose which fulfills your (totally correct) definition of providing 'earthing' is, in fact, a cable that was installed as a bonding conductor - hence illustrating that the vocabulary is not, in practice, as straightforward as some may think or suggest.

Kind Regards, John.

 

[ricicle]

There is a difference and it's really quite simple -

EARTHING limits the duration (time) that a voltage will exist on exposed metalwork during an earth fault (by Automatic Disconnection of Supply)

This is why 'Boiler Casings' etc are 'earthed' - if the line conductor touches the casing causing it to go 'live', then the CPC (earth) connection will carry the fault back to source (the Earth Fault Loop Path), activating the MCB/RCD.

BONDING limits the level of the Potential Difference (voltage) between two simultaneously-touchable conductive parts (pieces of metal) during the 'fault', thereby reducing the risk to a person of serious shock before the protective device trips.

Now there may be overlaps in the real world - i.e. combined bonding/earthing conductors etc etc.
BUT, I think it's important to understand the difference between the two - even if it's just to know what the hell it is you are actually doing!!!

Hallelujah !!!!

 

[Electrifying]

EARTHING limits the duration (time) that a voltage will exist on exposed metalwork during an earth fault (by Automatic Disconnection of Supply)

... which is precisely the function that this 'MPB' conductor serves in my installation.

No it's not.
The fact that the 'Bonding Conductor' is part of the 'Earth Fault Loop Path' doesn't mean it's performing the function of a CPC (or 'earthing' if you must)

The 'Main Protective Bonding Conductor' is connected between the MET and your Gas Incommer (or it should be).
So, unless the fault occurs on the MET or on the Gas incommer, then the Protective Conductor connecting these two is not performing the job of 'earthing', it is 'Bonding' - the current flow is , again, irrelevant.

Yes, connection to the Gas incommer, in this instance, reduces any Zs measurement nicely, but it doesn't reduce the Ze (or Ra if you like), so it has no bearing on fault clearance.

If the plumber disconnected the 'Bonding Conductor', your 'Earth Fault' would still clear in the required time - or it certainly should do.

Your TT install will have a front end RCD for 'earth fault protection' - this is based on the high Ze, not the low Zs that your Gas Incommer will give you - otherwise you would just rely on your MCBs for ADS under earth fault - only you're not allowed to rely on a Gas pipe for 'earthing',..........but it still has to be bonded because it's introducing a different potential to the building.

 

[JohnW2]

EARTHING limits the duration (time) that a voltage will exist on exposed metalwork during an earth fault (by Automatic Disconnection of Supply)

... which is precisely the function that this 'MPB' conductor serves in my installation.

No it's not. The fact that the 'Bonding Conductor' is part of the 'Earth Fault Loop Path' doesn't mean it's performing the function of a CPC (or 'earthing' if you must)

Well, to be fair, it was you who (as above) chose to define 'EARTHING', rather than 'the function of a CPC' In any event, in BS7671-speak, it would normally be called an 'Earthing Conductor'. However, you seem to be denying the reality, because it differs from 'intent'. Sure, the cable in question was installed for an entirely different (and very legitimate) purpose (which it still serves), and was not 'intended' to fulfill the function of providing a path to earth, but the inescapable reality is that, with things as they are, it does actually serve that function, whether this fits neatly with what people like to think or not; disconnect my earthing conductor/rod and the electrical installation would be hard-pressed to even notice the difference.

[BTW, just for the record, you write throughout about my 'gas incomer', but it's actually the incoming water supply we're talking about]

If the plumber disconnected the 'Bonding Conductor', your 'Earth Fault' would still clear in the required time - or it certainly should do.

Indeed so. By the same token, if an electrician disconnected the 'Earthing Conductor' and forgot to replace it, the earth fault would still clear in the required time - illustrating that, in practical reality, with things as they are the MPB (together with extraneous-conductive-parts) can (and normally does) provide an effective path to earth ('earthing', if you like), and hence the required automatic disconnection, just as can my Earthing Conductor and Earth Electrode. I obviously know that I'm not allowed to rely upon this, but (given the need for main protective bonding, which I certainly don't deny) nor is there any way I can stop it being the case.

I really don't think there is any difference between us in terms of the electrical concepts or requirements, and I'm amazed that there can be so much discussion just about words!

Kind Regards, John.

 

[Electrifying]

In any event, in BS7671-speak, it would normally be called an 'Earthing Conductor'.

Incorrect.

The 'Earthing Conductor' connects the MET to the 'Means of Earthing' - your water incommer is not the 'means of earthing'......it's not allowed to be.

The correct term for the cable is 'Main Protective Bonding Conductor' - it is 'sized' and 'installed' as a 'bonding conductor', to provide 'equipotential bonding' between the installation earthing system and the 'Extraneous Conductive Part' (your water pipe).

It cannot be an 'Earthing Conductor' as I described above.

It cannot be a CPC, as the water pipe does not require 'earthing'..........it cannot become live under fault conditions.

If it fulfills some other duties as well, then so be it, but it is 'bonding'.

I really don't see the point to your argument.

 

[JohnW2]

In any event, in BS7671-speak, it would normally be called an 'Earthing Conductor'.

Incorrect. The 'Earthing Conductor' connects the MET to the 'Means of Earthing' ...

I know that, but this is getting a bit silly. When you wrote:

The fact that the 'Bonding Conductor' is part of the 'Earth Fault Loop Path' doesn't mean it's performing the function of a CPC (or 'earthing' if you must)

.. I assumed you meant that the fact the conductor is part of the loop doesn't mean that it is performing the function of a connection to a means of earthing (i.e. Earthing Conductor)... but I really don't care what we call the conductor, provided that we know what it is intended to do, and what it actually does do.

It cannot be a CPC, as the water pipe does not require 'earthing'..........it cannot become live under fault conditions.

Nothing much to do with this discussion but, as a general point, there are all sorts of ways in which pipework (most/all of which is in electrical continuity in an all-metal-pipe house) can become live under various fault conditions - so it usually will need earthing as well as bonding (and, of course, the same cable might theoretically do both ).

Kind Regards, John.

 

[dingbat]

... I'm amazed that there can be so much discussion just about words!

Amazed?

Have you had a read through some of your posts?

You seem obsessed with continuing discussions way beyond their normal conclusions, mostly based on your distortions of the words used.