Earthing vs. (Supplementary) Bonding

[JohnW2]

In an ancient thread which was recently hijacked....

...earthed equipotential bonding does not imply that there is a such thing as "earth bonding". There isn't any such thing, nor was there ever any such thing. Earthing and bonding are completely different concepts.

The concepts are, indeed, totally different, but for those who are already confused/uncertain about the difference, I can see them having a particular problem with supplementary bonding.

We can hopefully agree that (in both cases under fault conditions) ‘earthing’ is used to facilitate ADS and that ‘bonding’ is used to minimise potential differences, essentially ‘touch voltages’. Bonding should minimise potential ‘touch voltages’, whilst (via ADS) earthing should limit (to a few seconds at most) the duration of potentially dangerous touch voltages, should they arise (as well as offer protection against fire etc.).

With Main Bonding, we (and the regs) rely on the presence of an adequate bonding conductor to minimise the potential differences that can exist between exposed conductors within the premises (i.e. to create an 'equipotential zone’).

With Supplementary Bonding, we (and the regs) presumably could again rely on the presence of the bonding conductor to prevent significant potential differences between simultaneously touchable parts - but the regs don’t seem to take that approach. Instead, they specify (in 415.2.2) a test of “the effectiveness of the supplementary bonding” which is based on the requirement that a voltage difference of 50V or greater between simultaneously touchable parts would result in disconnection of the supply by a protective device. Even though the motivation to is limit the duration of persistence dangerous touch voltages, that sounds to me like ADS - in fact, more demanding than the usual ADS (which only requires disconnection if the potential of exposed-c-ps rises to full mains voltage).

Since the regs indicate that the adequacy of supplementary bonding is to be judged in relation to 'ADS', I can well see that some of those who have a degree of understanding of the difference between the concepts of earthing and bonding may well find themselves more than a little confused!

ADS obviously doesn’t come into Main Bonding (even though I’m sure it would be easy enough to produce a protective device which disconnected the supply if current in the MPB conductor rose above some threshold) but I am a little surprised that the regs don’t seem to impose any requirement/test in relation to the “effectiveness” of MPB (in the way they do for supplementary bonding), other than to impose a minimum CSA for the bonding conductor (which is probably mainly to ensure that it doesn't melt). With supplementary bonding, even if the CSA of the bonding conductor is compliant with the regs, they still specify a ‘test of its effectiveness’. In the case of main bonding, although there is guidance (elsewhere) suggesting a maximum MPB conductor resistance of 0.05 Ω, AFAIAA, the regs themselves have no ‘requirements’ beyond that of CSA, and specify no 'tests'.

Any thoughts/comments?

Kind Regards, John

 

[Risteard]

In an ancient thread which was recently hijacked....

Apologies for that. I only noticed the date of the post I had responded to after I had posted. I'm not entirely certain why the thread was resurrected.

 

[Risteard]

In the case of main bonding, although there is guidance (elsewhere) suggesting a maximum MPB conductor resistance of 0.05 Ω, AFAIAA, the regs themselves have no ‘requirements’ beyond that of CSA, and specify no 'tests'.

Incidentally, the information in Guidance Note 3 was not suggesting that the maximum resistance of the conductor was 0.05 Ohms (although this has often been misinterpreted as such), that it should be confirmed that there was a sound connection between the conductor and the extraneous conductive part. A resistance of 0.05 Ohms or less might suggest a sound connection.

 

[securespark]

0.05 is the figure I have worked with and which has been recognised by my peers for many a year.

 

[JohnW2]

In an ancient thread which was recently hijacked....

Apologies for that. I only noticed the date of the post I had responded to after I had posted. I'm not entirely certain why the thread was resurrected.

No need for you to apologise - 'twasn't you wot did the hijacking

Kind Regards, John

 

[JohnW2]

0.05 is the figure I have worked with and which has been recognised by my peers for many a year.

Indeed. Despite what Risteard (correctly, IIRC) says about the precise wording of GN3, my understanding is that, as you say, most people seem to have regarded 0.05Ω as being the figure they should aim to not exceed (for total resistance from MET to the extraneous-c-p). That figure allows for about 23m of 10mm², which ought to be plenty in most domestic properties.

However, the fact remains (and the point I was making) is that the regs themselves impose no maximum resistance for an MBP conductor (and, just for the record, nor do they say that it cannot contain joins ).

Kind Regards, John

 

[EFLImpudence]

We can hopefully agree that (in both cases under fault conditions) ‘earthing’ is used to facilitate ADS and that ‘bonding’ is used to minimise potential differences, essentially ‘touch voltages’. Bonding should minimise potential ‘touch voltages’, whilst (via ADS) earthing should limit (to a few seconds at most) the duration of potentially dangerous touch voltages, should they arise (as well as offer protection against fire etc.).

I would say ensure rather than facilitate but yes.

With Main Bonding, we (and the regs) rely on the presence of an adequate bonding conductor to minimise the potential differences that can exist between exposed conductors within the premises (i.e. to create an 'equipotential zone’).

Yes.

With Supplementary Bonding, we (and the regs) presumably could again rely on the presence of the bonding conductor to prevent significant potential differences between simultaneously touchable parts - but the regs don’t seem to take that approach.

If you mean rely on the main bonding conductor they do except where standing in water is involved when supplementary (additional) bonding MAY be required to limit touch voltage to 50V.

Instead, they specify (in 415.2.2) a test of “the effectiveness of the supplementary bonding” which is based on the requirement that a voltage difference of 50V or greater between simultaneously touchable parts would result in disconnection of the supply by a protective device.

Nothing to do with a greater than 50v causing ADS; it limits the touch voltage to 50V until ADS occurs.

Even though the motivation is to limit the duration of persistence dangerous touch voltages, that sounds to me like ADS - in fact, more demanding than the usual ADS (which only requires disconnection if the potential of exposed-c-ps rises to full mains voltage).

Is this where you are going wrong?
It's not for 'persistent' dangerous touch voltage. It is for fault conditions until ADS occurs; this could be 5s but these days probably far less.

Since the regs indicate that the adequacy of supplementary bonding is to be judged in relation to 'ADS',

Yes, if you mean the relevant current of the OPD involved.

I can well see that some of those who have a degree of understanding of the difference between the concepts of earthing and bonding may well find themselves more than a little confused!

Why?

ADS obviously doesn’t come into Main Bonding (even though I’m sure it would be easy enough to produce a protective device which disconnected the supply if current in the MPB conductor rose above some threshold) but I am a little surprised that the regs don’t seem to impose any requirement/test in relation to the “effectiveness” of MPB (in the way they do for supplementary bonding),

Yes they do.
It must be of suitable c.s.a. for likely current and must effectively connect the parts to the MET.

other than to impose a minimum CSA for the bonding conductor (which is probably mainly to ensure that it doesn't melt).

Yes.

With supplementary bonding, even if the CSA of the bonding conductor is compliant with the regs, they still specify a ‘test of its effectiveness’.

Ah. The tests aren't carried out between the ends of the SB conductor. They are done between parts of simultaneous accessibility to verify that the remotely applied SB has acceptably reduced the PD between those accessible parts.
To ensure a maximum of 50V because it does not have to be connected at the relevant (danger) points otherwise the impedance between the parts would be negligible.

In the case of main bonding, although there is guidance (elsewhere) suggesting a maximum MPB conductor resistance of 0.05 Ω,

0.05Ω is merely an acceptable figure which represent negligible impedance.
The effectiveness of the main bonding has to be checked.

AFAIAA, the regs themselves have no ‘requirements’ beyond that of CSA, and specify no 'tests'.

But you have just quoted some tests which must be done.


27m.

 

[JohnW2]

We can hopefully agree that (in both cases under fault conditions) ‘earthing’ is used to facilitate ADS and that ‘bonding’ is used to minimise potential differences, essentially ‘touch voltages’. ...

I would say ensure rather than facilitate but yes.

OK. I agree 'ensure' would be a better word.

With Main Bonding, we (and the regs) rely on the presence of an adequate bonding conductor to minimise the potential differences that can exist between exposed conductors within the premises (i.e. to create an 'equipotential zone’).

Yes.

So far, so good

With Supplementary Bonding, we (and the regs) presumably could again rely on the presence of the bonding conductor to prevent significant potential differences between simultaneously touchable parts - but the regs don’t seem to take that approach.

If you mean rely on the main bonding conductor they do except where standing in water is involved when supplementary (additional) bonding MAY be required to limit touch voltage to 50V.

No, I don't mean that at all. With main bonding, the mere presence of the bonding conductor of adequate CSA appears to be regarded as adequate to ensure that a dangerous pd cannot arise between the two parts it is connecting. However, when supplementary bonding is required, its 'mere presence' is apparently not regarded as enough to ensure that a dangerous potential will not arise between the parts connected by the bonding conductor; in this case, as well as the requirement for a bonding conductor joining the parts, there is also a requirement that, even though the parts are bonded together, conditions are such that a device would disconnect the supply should a touch voltage >50V arise. In other words, whilst with MPB the bonding conductor alone is regarded as adequate to ensure that there are no dangerous 'touch voltages', with SB the bonding conductor alone is not trusted to be adequate.

Instead, they specify (in 415.2.2) a test of “the effectiveness of the supplementary bonding” which is based on the requirement that a voltage difference of 50V or greater between simultaneously touchable parts would result in disconnection of the supply by a protective device.

Nothing to do with a greater than 50v causing ADS; it limits the touch voltage to 50V until ADS occurs.

As you actually know as well as I do, just as a 30mA RCD does not limit the current through a person to 30mA, nor does what we are talking about limit the touch voltage to 50V. As you know, in both cases, all the protective device does is limitvthe duration of the current through the person (which could be hundreds of mA) or the touch voltage (which could be 230V).

Whatever, my point was that, even though the two parts have been joined by a bonding conductor, so as to minimise the pd that could exist between them, unlike the situation with MPB (where that 'connection by a bonding conductor' alone is deemed adequate to minimise that pd), there is also a requirement that a protective device should operate should a pd >50V between the parts arise.

Even though the motivation is to limit the duration of persistence dangerous touch voltages, that sounds to me like ADS - in fact, more demanding than the usual ADS (which only requires disconnection if the potential of exposed-c-ps rises to full mains voltage).

Is this where you are going wrong? It's not for 'persistent' dangerous touch voltage. It is for fault conditions until ADS occurs; this could be 5s but these days probably far less.

I didn't say 'persistent dangerous touch voltage'. I said that there was a requirement for "the duration of persistence of dangerous touch voltages" to be limited (as you to, to 5 secs or less). Maybe my unnecessarily 'wordy' statement confused you - "duration of persistence" is just a long-winded way of saying "duration"!

Since the regs indicate that the adequacy of supplementary bonding is to be judged in relation to 'ADS', I can well see that some of those who have a degree of understanding of the difference between the concepts of earthing and bonding may well find themselves more than a little confused!

Why?

Because we see it being 'drummed into people' that earthing is to ensure that protective devices operate under fault conditions (and nothing to do with minimising potential differences between things) and that bonding is to minimise pds between things (and nothing to do with ensuring that protective devices operate).

I am a little surprised that the regs don’t seem to impose any requirement/test in relation to the “effectiveness” of MPB (in the way they do for supplementary bonding),

Yes they do. It must be of suitable c.s.a. for likely current and must effectively connect the parts to the MET.

Perhaps a slightly odd response - given that, as you go on to quote, the rest of my sentence was:

other than to impose a minimum CSA for the bonding conductor (which is probably mainly to ensure that it doesn't melt).

With supplementary bonding, even if the CSA of the bonding conductor is compliant with the regs, they still specify a ‘test of its effectiveness’.

Ah. The tests aren't carried out between the ends of the SB conductor. They are done between parts of simultaneous accessibility to verify that the remotely applied SB has acceptably reduced the PD between those accessible parts.

I realise that. However, my point is that there is no apparent requirement for a corresponding test (which would be just as easy to implement) for MPB. Having said that, you go on to say...

The effectiveness of the main bonding has to be checked.

You appear to be proposing some "test of effectiveness" of MPB which I have not noticed the regs requiring (although it does require such a test for SB). What have I missed?

Kind Regards, John

 

[EFLImpudence]

I didn't say 'persistent dangerous touch voltage'. I said that there was a requirement for "the duration of persistence of dangerous touch voltages" to be limited (as you to, to 5 secs or less). Maybe my unnecessarily 'wordy' statement confused you - "duration of persistence" is just a long-winded way of saying "duration"!

Well, it wasn't wordy enough as you had omitted the second 'of' leading me to infer it was a spelling mistake.

Because we see it being 'drummed into people' that earthing is to ensure that protective devices operate under fault conditions (and nothing to do with minimising potential differences between things) and that bonding is to minimise pds between things (and nothing to do with ensuring that protective devices operate).

That is clear. Do you disagree?

I am a little surprised that the regs don’t seem to impose any requirement/test in relation to the “effectiveness” of MPB (in the way they do for supplementary bonding),

But they don't impose any test with relation to the actual conductor but between the accessible parts which have been remotely bonded.
If the tests are satisfactory before the bonding conductor is applied then it is not needed.

I realise that. However, my point is that there is no apparent requirement for a corresponding test (which would be just as easy to implement) for MPB. Having said that, you go on to say...

The effectiveness of the main bonding has to be checked.

You appear to be proposing some "test of effectiveness" of MPB which I have not noticed the regs requiring (although it does require such a test for SB). What have I missed?

As above.
Plus there is no need as the Main bond is applied at the farthest point.

 

[JohnW2]

Well, it wasn't wordy enough as you had omitted the second 'of' leading me to infer it was a spelling mistake.

Oh, sorry - I hadn't even noticed that. As you will have seen, I 'automatically' included that second 'of' when I re-typed it in my post, because my mind knew it was meant to be there!

Because we see it being 'drummed into people' that earthing is to ensure that protective devices operate under fault conditions (and nothing to do with minimising potential differences between things) and that bonding is to minimise pds between things (and nothing to do with ensuring that protective devices operate).

That is clear. Do you disagree?

Not at all - I totally agree. So, if we agree that it is appropriate to stress to people that "bonding ... has nothing to do with ensuring that protective devices operate", can't you see why I feel they might get pretty confused when they are told that the prescribed test of "the effectiveness of bonding" is based whether or not it was adequate to ensure that a protective device would operate under certain circumstances?

I am a little surprised that the regs don’t seem to impose any requirement/test in relation to the “effectiveness” of MPB (in the way they do for supplementary bonding),

But they don't impose any test with relation to the actual conductor but between the accessible parts which have been remotely bonded.

As I said before, I realise that (except that the regs don't actually say anything about "...which have been remotely bonded"). However, as I also said, that doesn't explain why they feel this is necessary for SB, but that the equivalent is not necessary for MPB. I suppose you are giving your answer to that when you go on to say:

You appear to be proposing some "test of effectiveness" of MPB which I have not noticed the regs requiring (although it does require such a test for SB). What have I missed?

As above. Plus there is no need as the Main bond is applied at the farthest point.

I'm not sure what the 'as above' refers to, but I accept that your second point may be part of the explanation. Does that mean that you wouldn't consider a test necessary if you could see both ends of a SB conductor and one end was connected to the extraneous-c-p fairly close (on either side of wall) to where it entered the location (say within about 600mm)?

{in relation to SB} If the tests are satisfactory before the bonding conductor is applied then it is not needed.

Yes, I realise from the other current thread that such is your view and, as I have said, I agree that it makes very good electrical sense. I presume that you don't have a corresponding view in relation to Main Bonding? Assuming that you don't, why not - is it because finding an 'adequately low' resistance (per whatever test were prescribed) would not tell you whether or not the CCC through the measured resistance would be adequate - or something else?

Kind Regards, John

 

[JohnW2]

P.S. I should have added that it was not my intention for this to become a repeat or extension of 'the other thread'. Rather, I just wanted to point out why I think those who are confused/uncertain about the difference between earthing and bonding might become particularly confused in relation to supplementary bonding when they see the test of its effectiveness which is specified in the regs.

Kind Regards, John

 

[EFLImpudence]

Not at all - I totally agree. So, if we agree that it is appropriate to stress to people that "bonding ... has nothing to do with ensuring that protective devices operate", can't you see why I feel they might get pretty confused when they are told that the prescribed test of "the effectiveness of bonding" is based whether or not it was adequate to ensure that a protective device would operate under certain circumstances?

You keep saying this.

It is nothing to do with it being adequate to ensure an opd will operate.

It is to prevent a person touching more than 50V while the opd operates because of a fault by limiting volt drop.

As I said before, I realise that (except that the regs don't actually say anything about "...which have been remotely bonded"). However, as I also said, that doesn't explain why they feel this is necessary for SB, but that the equivalent is not necessary for MPB. I suppose you are giving your answer to that when you go on to say:

Because I am explaining something, it is unfair to keep retorting that the regs. don't actually say anything about {my wording}.
I am explaining how it works.

You have accessible parts in a bathroom.
Because there is a 40A circuit not RCD protected the resistance between the accessible parts must be less than 0.25Ω; 200x0.25=50V
You measure between the sink pipes 0.3Ω,
between these pipes and shower CPC 0.4Ω; 200x0.4=80V

You bond between shower CPC and shower pipe and
between bath pipes.

You now measure between all pipes and between pipes and shower CPC.
All measurements are now very low and satisfactory even though only two short pieces of SB have been used; 200x0.1=20V
The SB could have been applied outside of the room with similar satisfactory results albeit longer SB conductors.

Or in the case of my very small flat with the services all on the other side of the bathroom wall and no CPC or pipe longer than 3m the measurements would have been acceptably low without any SB so there is no point adding a bonding conductor.

You appear to be proposing some "test of effectiveness" of MPB which I have not noticed the regs requiring (although it does require such a test for SB). What have I missed?

That there is a 10mm² conductor satisfactorily connected between the farthest point of the e-c-p and the Met.
All you would be measuring is the resistance of the conductor.
There is no part of the e-c-p farther away to measure.

Does that mean that you wouldn't consider a test necessary if you could see both ends of a SB conductor and one end was connected to the extraneous-c-p fairly close (on either side of wall) to where it entered the location (say within about 600mm)?

You are not measuring at both ends of the SB conductor but between parts in the room which are touchable.

{in relation to SB} If the tests are satisfactory before the bonding conductor is applied then it is not needed.

Yes, I realise from the other current thread that such is your view and, as I have said, I agree that it makes very good electrical sense.

I really don't think it is just 'my view'.

I presume that you don't have a corresponding view in relation to Main Bonding? Assuming that you don't, why not -

I have explained.

is it because finding an 'adequately low' resistance (per whatever test were prescribed) would not tell you whether or not the CCC through the measured resistance would be adequate - or something else?

The resistance will be negligible and the CCC adequate because of the csa as, as I keep saying, you will be measuring just the conductor at its joints and the pipe within the equipotential zone; not a path through the pipes with bonding connected elsewhere.

 

[JohnW2]

I'm going to split this response, to avoid (even more!) excessively lengthy posts! Firstly ....

Not at all - I totally agree. So, if we agree that it is appropriate to stress to people that "bonding ... has nothing to do with ensuring that protective devices operate", can't you see why I feel they might get pretty confused when they are told that the prescribed test of "the effectiveness of bonding" is based whether or not it was adequate to ensure that a protective device would operate under certain circumstances?

You keep saying this. It is nothing to do with it being adequate to ensure an opd will operate. It is to prevent a person touching more than 50V while the opd operates because of a fault by limiting volt drop.

They would be incredibly unlucky if they were to be touching the two parts in question during the very brief period (no more than 5 secs) between when a fault arose which resulted in a pd >50V between the parts and when the OPD disconnected the supply. I think a much more realistic way of looking at it is that the OPD will operate if that pd ever does rise to >50V, so that there is no chance of someone subsequently touching parts which had a voltage >50V between them (i.e. 'pre-emptive').

I'm not sure why you are having so much difficulty in understanding my point. Yes, as above, the point of SB (when required) is to create a situation in which (except for a period of ≤5secs when the fault first arises) there cannot be a voltage between parts >50V which people could 'experience'. However, the mechanism for achieving this, and the test of whether the SB is effective enough to achieve this, is based on the operation of an OPD.

They could have (but have not) required a situation (and a test for it) which was not OPD-dependent and which I would personally regard as 'true bonding'. They could have specified that the resistance between extraneous-c-p and exposed-c-p had to be no greater than 0.22 times the (Ze+R1) of the circuit in question (0.22 being 50/230). That (which I imagine would not be difficult to achieve with a fairly short SB conductor) would mean that, even in the absence of an OPD, in the worst case of all the fault current flowing through the electrical connection between parts (whether that connection be through an SB conductor or whatever), the voltage between those parts could never be >50V. That would, to my mind, have been 'true bonding' (as with MPB). However, they chose not to do this but, instead, to rely on the supplementary 'bonding' being adequate to ensure operation of an OPD (within ~5s) should a a touch voltage >50 ever appear - which, to my mind, is deviating somewhat from the true concept of 'bonding'.

Kind Regards, John

 

[JohnW2]

Now 'Part 2' ....

As I said before, I realise that .... However, as I also said, that doesn't explain why they feel this is necessary for SB, but that the equivalent is not necessary for MPB. I suppose you are giving your answer to that when you go on to say:

Because I am explaining something, it is unfair to keep retorting that the regs. don't actually say anything about {my wording}. I am explaining how it works.

I'm a little confused - what you then go on to explain is how SB works, and I have no major disagreement with what you say - but my point/question was about why there is no required function test of MPB 'effectiveness'.

You appear to be proposing some "test of effectiveness" of MPB which I have not noticed the regs requiring (although it does require such a test for SB). What have I missed?

That there is a 10mm² conductor satisfactorily connected between the farthest point of the e-c-p and the Met. All you would be measuring is the resistance of the conductor. There is no part of the e-c-p farther away to measure.

I basically agree. However, I was responding to your having written:

The effectiveness of the main bonding has to be checked.

I took that to mean that you felt that some sort of 'test' was required - is the 'checking' you mentioned merely a matter of ensuring that an adequate CPC conductor is in place?

Whatever, I think my question remains. Although, as you say, if you undertook a test "all you would be measuring would be the resistance of the MPB conductor", it could be argued that this was an important measurement (in practice, it could really just be a calculation) - since that 'resistance of the MPB conductor', although 'low' (you described it as 'negligible' later in yoiur post) might actually be too high to limit the voltage between MET and the extraneous-c-p to 50V under worst-case conditions. In analogy with what I've just written about SB, there could be (but obviously isn't) a requirement for the resistance of an MPB conductor to be no greater than about 0.22 times Ze. As we know, the regs themselves don't currently appear to impose any maximum resistance for an MPB conductor, although there is 'guidance' which some interpret as suggesting a maximum of 0.05 &#937; (about 23m of 10mm²). If Ze is very low (which I imagine it sometimes is, particularly in places like London), 0.05 &#937; might not be enough to guarantee that 'touch voltages' would be <50V in worst case scenarios.

Kind Regards, John

 

[EFLImpudence]

They would be incredibly unlucky if they were to be touching the two parts in question during the very brief period (no more than 5 secs) between when a fault arose which resulted in a pd >50V between the parts and when the OPD disconnected the supply. I think a much more realistic way of looking at it is that the OPD will operate if that pd ever does rise to >50V, so that there is no chance of someone subsequently touching parts which had a voltage >50V between them (i.e. 'pre-emptive').

Conversely, that is so but as the primary purpose is to protect people that is the part which is defined.

I'm not sure why you are having so much difficulty in understanding my point. Yes, as above, the point of SB (when required) is to create a situation in which (except for a period of &#8804;5secs when the fault first arises) there cannot be a voltage between parts >50V which people could 'experience'. However, the mechanism for achieving this, and the test of whether the SB is effective enough to achieve this, is based on the operation of an OPD.

I understand the point but not why you keep making it.

They could have (but have not) required a situation (and a test for it) which was not OPD-dependent and which I would personally regard as 'true bonding'. They could have specified that the resistance between extraneous-c-p and exposed-c-p had to be no greater than 0.22 times the (Ze+R1) of the circuit in question (0.22 being 50/230). That (which I imagine would not be difficult to achieve with a fairly short SB conductor) would mean that, even in the absence of an OPD, in the worst case of all the fault current flowing through the electrical connection between parts (whether that connection be through an SB conductor or whatever), the voltage between those parts could never be >50V. That would, to my mind, have been 'true bonding' (as with MPB). However, they chose not to do this but, instead, to rely on the supplementary 'bonding' being adequate to ensure operation of an OPD (within ~5s) should a a touch voltage >50 ever appear - which, to my mind, is deviating somewhat from the true concept of 'bonding'.

They could have made it as complicated as you want or
they could have said all points must be bonded so that negligible impedance between them is achieved but
they didn't.