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Topic Title: Cross bonding
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Created On: 08 January 2013 09:11 PM
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 11 January 2013 12:12 AM
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geoffsd

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Originally posted by: MrOtherA final consideration: let's say a bathroom doesn't have supp bonding but needs it or that your nieghbourhood friendly plumber has removed it -- under a fault conditions it may turnout that supp bonding elsewhere may still offer decent low resistive path back to the MET.

Its purpose is not to "offer decent low resistive path back to the MET."

Its purpose is to equalise potential between simultaneously accessible conductive parts in the event of a fault.

Providing a "decent low resistive path back to the MET" where NONE existed before, e.g. isolated radiators, could prove fatal.

Conductive parts should be tested to find out if they are extraneous and require supplementary bonding.
Not just done because it is metal.
 11 January 2013 08:03 AM
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ebee

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Any "cross bonding" is just some supplementary bonding.
I always supplementary bond even if our new regs allows us not to.
Reliance on an RCD with possible 7% failure rate and expected to save 95% (therefore not saving 5%) of the population , if working, does not fit well with me.

That 12ish percent still at risk could be mitigated by supp bonding

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 11 January 2013 09:11 PM
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MrOther

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Originally posted by: geoffsd

Originally posted by: MrOtherA final consideration: let's say a bathroom doesn't have supp bonding but needs it or that your nieghbourhood friendly plumber has removed it -- under a fault conditions it may turnout that supp bonding elsewhere may still offer decent low resistive path back to the MET.


Its purpose is not to "offer decent low resistive path back to the MET."



Its purpose is to equalise potential between simultaneously accessible conductive parts in the event of a fault.



Providing a "decent low resistive path back to the MET" where NONE existed before, e.g. isolated radiators, could prove fatal.



Conductive parts should be tested to find out if they are extraneous and require supplementary bonding.

Not just done because it is metal.


So knowing that that rad pipe is already bonded in the bathroom wouldn't make a difference then?
 11 January 2013 10:13 PM
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geoffsd

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Nevertheless, your reasoning is not correct.

Originally posted by: MrOtherJust done my Mum's bedroom up. Supp/Cross bonded both pipes on the room's only rad and bonded an old/possibly/probably discontinued pipe.[/quote]
 11 January 2013 11:25 PM
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MrOther

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I don't see why it is.

Questions to self:

1) Is it metal work? Yes.
2) Is it extraneous. Yes.
3) Is there potential, therefore, for it to become live under fault conditions? Possibly, domestic premises almost no seperation of services under floorboards.
4) Is there simultaneously "touchable" metal work? Yes -- sockets, etc etc. most of which will be protected via. cpc.
5) Is the pipes equipotentially protected? Yes (only confirmed by previous test cert. as this moment in time.)
6) Is the circuit protected by RCD? No, but will be.
7) Is the metal work's resistence less then 23KOhms? Not confirmed, willing to bet isn't though.

Think my reasoning is pretty sound.

Supplementary bonding, Cpc, Equipotential bonding, ultimatley all lead back to the MET, that is the point, that is how we create the equipotential.

In reality a metal pipe should be equipotentially bonded, so the "isolated radiator" isn't going to happen is it? It's still going to have an earth path, by not supp bonding it is your actually adding resistence to the fault path.

(In this way, I always like to think of Supp bonding as a short cut or a fail safe, so the more of it the better. Radiator gets changed in room A and the plumber removes BS951 clamp, no probs theres one in room B, and so on, makes even more sense in the modern domestic house post Changing Rooms -- places get done up now non-stop.)

Geoff, if I'm still incorrect then please feel free to comment because I'd rather be educated then ignorant but I feel that you're contradicting me on case of definitions, which if that's the case fair enough, as that's OP's origianl point in the thread.
MrOther.
 12 January 2013 01:09 PM
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MrP

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You guys must have lots of Dosh and plenty of time on your hands doing something that is not required


MrPnot so hot at the mo ice on the car this morning
 12 January 2013 04:50 PM
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leckie

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Read Parleys post.
It sets out the requirements for supplementary bonding and the circumstances when it can be omitted.

If it is required, it is to create a equipotential zone in a local room containing a bath or shower. No need to run a supplementary bond cable back to the met. You bond all items parley's post mentioned. That's it. The circuits in that area if they are compliant, will provide a path back to there distribution boards that should be low enough to operate the protective device. You are not seeking to change this value, the circuits must comply in there own rights. You are just seeking to create a local equipotential zone. All in parleys post.

If you wish to install local supplementary bonding even if you don't actually have to, that's not a problem, that's your option.

Edited: 12 January 2013 at 09:43 PM by leckie
 12 January 2013 05:09 PM
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geoffsd

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Originally posted by: MrOtherI don't see why it is.

It seemed so.

1) Is it metal work? Yes.

Undoubtedly.

2) Is it extraneous. Yes.

Have to take your word for that. Did you test it? No plastic pies anywhere?

3) Is there potential, therefore, for it to become live under fault conditions? Possibly, domestic premises almost no seperation of services under floorboards.

OK.

4) Is there simultaneously "touchable" metal work? Yes -- sockets, etc etc. most of which will be protected via. cpc.

But you have not bonded to CPCs or exposed parts - just two pipes which are connected by the radiator and a disused pipe which may now be isolated.

5) Is the pipes equipotentially protected? Yes (only confirmed by previous test cert. as this moment in time.)

There is no requirement nor need for supplementary bonding (even done properly) in a bedroom.

6) Is the circuit protected by RCD? No, but will be.

Not relevant at the moment.

7) Is the metal work's resistence less then 23KOhms? Not confirmed, willing to bet isn't though.

I think you have that the wrong way round.

You did not mention any of the above in the post on which I commented but gave the impression you were bonding pipes merely because they were there.

Think my reasoning is pretty sound.

I disagree.

Supplementary bonding, Cpc, Equipotential bonding, ultimatley all lead back to the MET, that is the point, that is how we create the equipotential.

If you are trying to make the whole installation so.
Normally supplementary bonding is 'in a location' i.e. one room to simultaneously accessible parts in which case it does not require connecting to anything outside the 'location'.

In reality a metal pipe should be equipotentially bonded, so the "isolated radiator" isn't going to happen is it?

You have stated not in this case but it could be - and the redundant pipe?

It's still going to have an earth path, by not supp bonding it is your actually adding resistence to the fault path.

Well, not adding, just not reducing.
A high resistance to elsewhere could be advantageous - as long as all simultaneously accessible parts are at the same potential.

(In this way, I always like to think of Supp bonding as a short cut or a fail safe, so the more of it the better. Radiator gets changed in room A and the plumber removes BS951 clamp, no probs theres one in room B, and so on, makes even more sense in the modern domestic house post Changing Rooms -- places get done up now non-stop.)

I think you are demonstrating a lack of understanding of the purpose and implementation of supplementary bonding.

Geoff, if I'm still incorrect then please feel free to comment because I'd rather be educated then ignorant but I feel that you're contradicting me on case of definitions, which if that's the case fair enough, as that's OP's origianl point in the thread.

I have tried but it is rather long.

Original question -
"In a domestic all with rcd and main bonding in place, there is no need for supplementary bonding.

Disconnection times (without RCD) must also be met

What about cross bonding ?? At boilers heating systems etc ???

No such thing and pointless if not connected to the boiler as well but for some reason favoured by plumbers who refuse certification for boiler without it.
 12 January 2013 05:09 PM
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MrOther

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Are we arguing over semantics here people?
 12 January 2013 05:15 PM
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daveparry1

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Cross bonding is a term introduced by plumbers, cos they can't spell supplementary or equipotential!

Please regard this as a joke!

Dave.
 12 January 2013 05:33 PM
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weirdbeard

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Originally posted by: ebee


Reliance on an RCD with possible 7% failure rate


I keep seeing this 7% figure mentioned, but where does it come from?

If anything i have replaced more rcds as they seem to become oversensitive, rather than having to replace them as they have failed the prescribed tests -- so whilst in my experience some rcds fail, mostly it is on the good side of fail!
 12 January 2013 06:26 PM
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ebee

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Dear Weird.
I have found RCDs to either not work at all - complete failure
or
Work but outside the Standard times - still a failure

I have not found any that I recall to be over-sensitive unless they were connected to a circuit.

Connection to a circuit , due to capacitance, inductance and insulation resistance may alter (or appear to alter under test) the trip times of an RCD and should be discounted.

Research by different bodies in different countries have given some differing failure rates , possibly as high as 7%. I am not saying this is conclusive but if enough research was done on a big enough scale we might well see a figure approaching that number, I suspect.

I am not saying RCDs are never over-sensitive under proper testing, I`m sure there will be some, but I suspect not many (someone might correct me on that).

I think the idea of either pressing test button first or trying a 5 x test or educating everyone to press the test button regularly might help to mitigate the failure rate (whatever it is) substantially by reducing/eliminating "stiction" (thanks GB )

As I said, this failure rate (whatever it is) plus the %age of people not saved by a fully functioning RCD might well lead us to an alarming conclusion.

Supp bonding might actually help save a few of these by limiting touched voltages as intended.
Coincidentally it might also save a few by lowering earth fault paths in circuits connected, although it is not the intention - just a piece of good fortune.

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Edited: 12 January 2013 at 06:34 PM by ebee
 12 January 2013 09:49 PM
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MrOther

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Originally posted by: ebee


Coincidentally it might also save a few by lowering earth fault paths in circuits connected, although it is not the intention - just a piece of good fortune.


Whisper that.
 12 January 2013 10:11 PM
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leckie

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No such term as cross bonding is mentioned in the regs. As I said earlier, read parleys post, it's all there. The requirements, and when it can be omitted.

If you connect pipes, cpc's, etc together you may well lower the fault path. But as Ebee said this is not the intention of local equipotential bonding. It's just parallel paths. The circuits in the room need to disconnect the protective device within the prescribed time under fault conditions by means of a low enough earth fault path, i.e. a Zs within the maximum required value for the overload device. With supplementary bonding you are simply trying to create a local equipotential zone.
 12 January 2013 10:51 PM
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MrOther

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My argument isn't that it's the point of Supp bonding but merely a decent by product of such an endeavour.
 12 January 2013 10:54 PM
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leckie

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And I agree with that mr other.
 13 January 2013 10:41 PM
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AJJewsbury

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Supplementary bonding, Cpc, Equipotential bonding, ultimatley all lead back to the MET, that is the point, that is how we create the equipotential.


If you are trying to make the whole installation so.
Normally supplementary bonding is 'in a location' i.e. one room to simultaneously accessible parts in which case it does not require connecting to anything outside the 'location'.

I agree. The concept of "equipotential zone" is mis-sold. During a fault large currents flow along protective conductors, so large potential differences can be produced even along protective conductors with fairly low resistances. Typically in a TN system the voltage at the end of the c.p.c. at the fault is likely to be around half line voltage (the line and protective conductors act as a potential divider - if they have similar resistances to each other the voltage at the fault will be around half way between 230V and 0V - higher if reduced c.p.c.s are used, a bit lower if there are parallel paths) and as Ze is typically relatively low compared with Zs, the MET will remain fairly close to earth. 115V at the fault, 15V at the MET, so 100V p.d. within the installation, might be a reasonable example.

The point of supplementary bonding is small locations like bathrooms is to create a new "equipotential" zone in that area - so with a fault in the bathroom, and 115V at the fault, the bathroom metalwork might be kept closer to 115V, minimising the voltage difference within the location.

- Andy.
 14 January 2013 11:26 AM
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Parsley

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If the resistance of the extraneous-conductive-parts is less than 1667ohms to the MET than according to 7671 it's effectively connected.
If it's greater than 1667 ohms and less than 22 K ohms supp bonding should be installed and if it's greater than 22K you may decide it's not an extraneous-conductive part and decide not install the supp bonding.

As per my my previous post in an older installation with mixed disconnection times or some circuits outside the bathroom that are not 30ma RCD protected, (the latter part could also be a new installation) it may well be worth installing the supp bonding in the bathroom. As Andy pointed out Ut can be quite high in compliant installations especially if the Ze is below the normal TNC-S 0.35 ohm generic value.

Regards
 14 January 2013 11:46 AM
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AJJewsbury

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If the resistance of the extraneous-conductive-parts is less than 1667ohms to the MET than according to 7671 it's effectively connected.

Only if the circuit that could produce the p.d. is 30mA RCD protected. An immersion heater, located outside the bathroom, perhaps on 5s disconnection time, but able to introduce a potential into the bathroom via the hot water pipework, would need a much lower impedance to be safe.
- Andy.
 14 January 2013 12:25 PM
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Parsley

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Originally posted by: AJJewsbury

If the resistance of the extraneous-conductive-parts is less than 1667ohms to the MET than according to 7671 it's effectively connected.


Only if the circuit that could produce the p.d. is 30mA RCD protected. An immersion heater, located outside the bathroom, perhaps on 5s disconnection time, but able to introduce a potential into the bathroom via the hot water pipework, would need a much lower impedance to be safe.

- Andy.


I thought I covered mixed disconnection times and RCD's in my post Andy.
But yes possible in a new installation if there are circuits rated at over 32amps and MCB's haven't been used. the 701.415.2 only states that circuits supplying equipment in the location need RCD protection hence my comment it may well be worth installing the supp bonding in the bathroom.

Regards
Regards
IET » Wiring and the regulations » Cross bonding

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