Earthing vs. (Supplementary) Bonding

[EFLImpudence]

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'.

There is - the effectivenes of the connections are checked ensuring there is minimal pd between the two earths.
What else is there?

YI 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?

Plus the effectiveness of the connections.
What else is there?

(you described it as 'negligible' later in yoiur post)

O.O5Ω is regarded as negligible.

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.

Is that not already met? 580x0.05=29V.

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.

It's 27m

There is debate over the 0.05&#937; but, anyway, it is rarely exceeded.

 

[JohnW2]

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 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.

Quite - and I don't think there is anything particularly 'wrong' with the approach they have decided to take. My only real point (which I think is likely to confuse some people regarding the 'bonding vs. earthing' distinction) is that, by choosing their approach, they are (at least to my mind) talking about something which is not 'true' (at least, not 'pure') 'bonding'.

Please correct me if I'm wrong, but I have been brought up to understand that true 'bonding' consists of establishing (if not already present) an electrical connection of sufficiently low resistance/impedance between two things such as to prevent a dangerous potential difference being able to arise between those two things.

Supplementary bonding, as described in the regs, does not necessarily achieve that. It potentially (sorry!) allows a 'dangerous potential difference' to arise between the parts, but requires that the situation is such that a protective device will bring that situation to a halt within ~5 secs. I am not saying that I do not regard that as a satisfactory way of protecting people from dangerous touch voltages, but it is not true/pure 'bonding' in the sense that I have been brought up to understand - which is why I think that some people may get confused.

Kind Regards, John

 

[JohnW2]

(you described it as 'negligible' later in yoiur post)

O.O5&#937; is regarded as negligible.

In many senses it is, but if more than 1000A flows through it, the potential difference between its ends (hence voltage difference between MET and extraneous-c-p) will be greater than 50V.

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.

Is that not already met? 580x0.05=29V.

As above, it will be met for currents <1000A (i.e. fault loop impedance >0.23&#937; at 230V), but not for higher currents than that - where does your implied 580A figure come from?

0.05 &#937; (about 23m of 10mm²).

It's 27m

My tables say that a 10mm² conductor has a resistance of 0.0022 &#937; per metre. 0.050/0.0022 = 22.72&#937; (hence 'about 23&#937;'). What are you basing your calculation on?

There is debate over the 0.05&#937; but, anyway, it is rarely exceeded.

Agreed on both counts.

Kind Regards, John

 

[EFLImpudence]

where does your implied 580A figure come from?

It is the required Ia of 100A BS88-2 although looking in old book I see it's 630A for 100A BS1361.

My tables say that a 10mm² conductor has a resistance of 0.0022 &#937; per metre. 0.050/0.0022 = 22.72&#937; (hence 'about 23&#937;'). What are you basing your calculation on?

It is 0.00183&#937; Yours is 80% for 70°.

 

[JohnW2]

where does your implied 580A figure come from?

It is the required Ia of 100A BS88-2 although looking in old book I see it's 630A for 100A BS1361.

Hmmm. That's all very well, but what the infamous situation in which very high currents are flowing through the MPB conductor because of a 'lost neutral' in a PME installation - that current will not be going through the cutout fuse!

My tables say that a 10mm² conductor has a resistance of 0.0022 &#937; per metre. 0.050/0.0022 = 22.72&#937; (hence 'about 23&#937;'). What are you basing your calculation on?

It is 0.00183&#937; Yours is 80% for 70°.

That is, indeed, what my figure is, but that was deliberate - don't you think that an MPB conductor with 'hundreds of amps' flowing through it would be more likely to be closer to 70 °C than 20 °C? Indeed, my figure may well not be very conservative, since even your 580A would, I imagine, get a 10mm² conductor up to above 70 °C pretty quickly, wouldn't it?

Kind Regards, John

 

[bernardgreen]

It is to prevent a person touching more than 50V

Putting a spanner in the works.

An electric shock requires two points of contact with the body and that these points are at different potentials. I that difference is less than 50 volts then the shock is considered to be "harmless"

Bonding is to ensure that all touchable points are at the same potential. It doesn't matter what the potentials above ground are providing the difference between them is less than 50 volts.

Two items at 230 volts above ground if bonded together could be the two points of contact that could be simultaneously touched without danger provided there was no other point of contact

DO NOT TRY IT. sneak paths to ground are everywhere, like damp walls.

https://www.youtube.com/watch?v=SkFH8lLvKZ0

 

[JohnW2]

It is to prevent a person touching more than 50V

Putting a spanner in the works. ... An electric shock requires two points of contact with the body and that these points are at different potentials. I that difference is less than 50 volts then the shock is considered to be "harmless" ... Bonding is to ensure that all touchable points are at the same potential. It doesn't matter what the potentials above ground are providing the difference between them is less than 50 volts. ... Two items at 230 volts above ground if bonded together could be the two points of contact that could be simultaneously touched without danger ....

I don't think that's really a 'spanner', and I think you're probably being a bit unfair to pick up on EFLI's (uncharacteristically) potentially misleading wording that you quote above. If you look through this thread (and the regs) you'll see that it is very clear that both EFLI and myself have (per the regs) been talking throughout about potential differences >50V between simultaneously touchable exposed-c-ps and extraneous-c-ps in a bathroom.

Kind Regards, John

 

[EFLImpudence]

That's all very well, but what the infamous situation in which very high currents are flowing through the MPB conductor because of a 'lost neutral' in a PME installation - that current will not be going through the cutout fuse!

That is why it has to be of a certain csa but not what is being discussed.

That is, indeed, what my figure is, but that was deliberate - don't you think that an MPB conductor with 'hundreds of amps' flowing through it would be more likely to be closer to 70 °C than 20 °C? Indeed, my figure may well not be very conservative, since even your 580A would, I imagine, get a 10mm² conductor up to above 70 °C pretty quickly, wouldn't it?

It would but an earth fault between the cut-out and CU is extremely unlikely and the fault disconnection likely to be instantaneous.
The 580A quoted was just a worst case for touch voltage.


This doesn't seem to be going anywhere.
I know you like a good discussion but...



I shall take solace in the fact that the only thing I have been picked up on by anyone else is saying a person could touch 50V without mentioning that it would have to be two touches.

 

[JohnW2]

That's all very well, but what the infamous situation in which very high currents are flowing through the MPB conductor because of a 'lost neutral' in a PME installation - that current will not be going through the cutout fuse!

That is why it has to be of a certain csa but not what is being discussed.

Well, it's 'off topic' in the sense that I started this thread to talk about SB, but it has slid partially onto MPB. However, in terms of MBP, it is very much relevant, if you accept that (as with any true bonding) the purpose of MPB is to prevent dangerous pds existing between the MET (hence exposed-c-ps throughout the installation) and extraneous-c-ps. Such a dangerous pd can theoretically exist (without any OPD being in circuit' to time-limit the situation) because of a supply-side fault, even with a compliant MBP csa.

That is, indeed, what my figure is, but that was deliberate - don't you think that an MPB conductor with 'hundreds of amps' flowing through it would be more likely to be closer to 70 °C than 20 °C? ....?

It would but an earth fault between the cut-out and CU is extremely unlikely and the fault disconnection likely to be instantaneous.

If that's where the fault was, I agree. However, as above, if the fault current were due to a supply-side fault, then there would be no OPD in the fault path, other than any that might exist at the transformer/ substation, and that would probably never have a reason to operate, anyway (the DNO-side would not be seeing any higher L current than 'normal' {indeed, it would almost certainly be less than 'normal'}- the problem would be that much/most of it was returning via MPBs and extraneous-c-ps, rather than via their N conductor). An RCD at the transformer would, of course, pick that up - but I don't think DNOs generally do such things

Kind Regards, John

 

[THRIPSTER]

Just to add at a tangent to this topic.

In determining what needs to be bonded, you need to work out whether a given part comes under the definition 'extraneous conductive part'. Generally accepted advice (and which would probably suffice for a domestic installation) is that a resistance <22,000 ohms between the ExtCP and the MET means that the part in question comes under the definition of Extraneous Conductive Part and needs to be bonded (though each installation needs to be evaluated under its own merits).

As to supplementary bonding, as permitted by 544.2.4, a portion of extraneous conductor can be used as supplementary bonding as long as it meets the requirements of 543.2.6.

I do not claim originality for any of this information.

Regards

 

[EFLImpudence]

Well, yes ?

However, other parts of 543 and 544 are relevant, too.

Why have you mentioned only these two regulations?

I quite like 543.2.3.

 

[JohnW2]

Just to add at a tangent to this topic. ... In determining what needs to be bonded, you need to work out whether a given part comes under the definition 'extraneous conductive part'. Generally accepted advice (and which would probably suffice for a domestic installation) is that a resistance <22,000 ohms between the ExtCP and the MET means that the part in question comes under the definition of Extraneous Conductive Part and needs to be bonded ...

Indeed that's the very point that EFLI frequently makes, although I think usually 23k&#937; (rather than the 22k&#937; you mention), being the resistance through which 1mA would flow if 230V appeared across it. Many also say that there is a lower limit of that resistance, below which bonding is also not required - along the lines of the SB test mentioned below.

I really don't know where people think resistances between conductive parts and MET of thousands, or tens of thousands, of ohms would come from (if they ever do) - in practice, the resistance is either going to be very low (no more than a handful of ohms, at most) or 'near infinite' (megohms)

As to supplementary bonding, as permitted by 544.2.4, a portion of extraneous conductor can be used as supplementary bonding as long as it meets the requirements of 543.2.6.

Indeed - that is, as you will be aware, one of the issues which has been discussed in this thread.

I think there is potential confusion about the definition of an extraneous-c-p when it comes to SB in bathrooms. It's fair enough when one is talking about conductors which are extraneous-c-ps as far as the building, which require Main Bonding. However, when it comes to SB, people talk in terms of conductors which are extraneous 'to the location' (i.e. the bathroom). If satisfactory main bonding is in place, it's quite difficult to see how any conductor can be 'liable to introduce a potential' significantly different from that of the MET into any part of the building (aka equipotential zone).

EFLI's test of whether SB is required (aka BS7671's test of the effectiveness of SB) not unreasonably relates specifically to the resistance between exposed-c-p's and extraneous-c-ps within the bathroom (not resistance between the extraneous-c-p and MET) - and (important when one is thinking about the upper limit) that, of course, will often/usually include the resistance of the CPC (i.e. 'R2') of the circuit concerned. The effectiveness of SB is said (by regs) to be satisfactory if that resistance is less than 50/Ia (Ia being the 'trip threshold' of the circuit's protective device) ... and that takes us back to how/why this thread started - if the test of the adequacy of SB is based on it being adequate to ensure that a protective device to operate (with touch voltages of 50V or above), that sounds more like 'earthing' than 'bonding' to me

Kind Regards, John

 

[EFLImpudence]

if the test of the adequacy of SB is based on it being adequate to ensure that a protective device to operate (with touch voltages of 50V or above),

I don't want to start again, but you are looking at it the wrong way round again and, I think, missing something fundamental.

I realise that the figures are reciprocal but there is a difference in that the SB conductor is not involved in the ADS, (other than a small parallel path).
This (ADS) will occur in the event of a fault to earth at an exposed-c-p (not the extraneous which are bonded) and would occur whether the SB were in place or not.
The SB limits the touch voltage between exposed and extraneous parts during this occurrence until disconnection.

that sounds more like 'earthing' than 'bonding' to me

It's not.
I still feel that you are thinking that SB is for helping when a Live touches the extraneous part (pipe). It's not.

If anyone is worried that a pipe may come into contact with a live conductor, in a shower for example, then this pipe would be an exposed-c-p and should then therefore be earthed and the conditions for ADS met.

 

[THRIPSTER]

22K between ExtCP and MET (to be measured). 1K assumed for body resistance.

Regsrds

 

[JohnW2]

I don't want to start again, but you are looking at it the wrong way round again and, I think, missing something fundamental. ... I realise that the figures are reciprocal but there is a difference in that the SB conductor is not involved in the ADS, (other than a small parallel path). This (ADS) will occur in the event of a fault to earth at an exposed-c-p (not the extraneous which are bonded) and would occur whether the SB were in place or not.

As I think you realise, I understand all that - but i think, in turn, that you may be missing something, as well - namely the different requirements for protective device operation in the case of 'normal ADS' and in relation to SB. Mind you, as you say, it's all very 'reciprocal', in that one can look at things 'either way around'.

Can we perhaps start by agreeing that, in practice, the extraneous-c-p will have a (usually very) low impedance path to 'earth'/MET?

If so, then the touch voltage (as a result of a fault) (between exposed-c-p and extraneous c-p) to which you refer will be essentially the same as the p.d. between exposed-c-p and earth/MET. Agreed?

The requirements for 'normal ADS' are that disconnection times should be met (i.e. that fault current should be equal to at least the Ia of device) in the event of a direct short ('of negligible impedance') between L and the exposed CP. Particularly when Ze is very low, R2 could easily be more than 21.7% (50/230) of the total Zs, so that the p.d. between the exposed-c-p and the MET could easily be greater than 50V (until disconnection occurs) in an ADS-compliant circuit. Put another way, 'normal ADS' requires that disconnection times be met when the voltage between exposed-c-p and MET is above some value, but that value may well be >50V. For the purpose of the subsequent discussion, let's just guess that, in a particular situation/installation, this 'some value' is 75V.

On that basis, if you accept the initial point I made (that the touch voltage will essentially be the same as the p.d. between exposed-c-p and earth/MET), this means that 'normal ADS' disconnection time requirements would not necessarily be satisfied (i.e. fault current would be less than Ia) when the 'touch voltage' was only 50V (it might require, say, 75V). However, the regs require that SB achieves a situation in which the disconnection time requirements are met with a touch voltage (hence roughly the exposed-cp to MET p.d.) of 50V. This is a more exacting requirement than for 'normal ADS', and one way of looking at the effect of 'effective SB' is that it has to reduce the impedance of the path from exposed-cp to MET to the extent that this 'more demanding than usual' disconnection time requirement will be met.

Since, in the situation I have described, the required disconnection with a touch voltage of 50V would not occur the absence of the SB, one can look at the SB as (partially) facilitating the required disconnection (at 50V touch voltage), which is functionality we would usually call 'earthing'.

The SB limits the touch voltage between exposed and extraneous parts during this occurrence until disconnection.

This is where the reciprocity comes in. If one installs SB to reduce the exposed-cp to MET impedance such that Ia will be achieved (hence satisfactory disconnection) when the 'exposed-cp to MET' PD is only 50V (which sounds like earthing), then it will follow that the touch voltage will also be limited to 50V during the period until disconnection occurs (which sounds like bonding)...

... so I guess you can really 'pay your money and take your choice'. If you take steps to make ADS happen when exposed-cp/MET pd is only 50V (earthing) (although 'normal ADS' might only require this to happen with a pd of, say, 75V) you will automatically have also limited the touch voltage to 50V (bonding). If you take steps to limit touch voltage to 50V (bonding), you will also have caused disconnection to occur when the exposed-cp/MET pd is lower than 'usual' (75V or whatever) for ADS (earthing)

Kind Regards, John