Main earth bond

[maltaron]

Is it OK to use 2 x 6mm cables in parallel in place of 1 x 10mm.

TIA

 

[riveralt]

Is it OK to use 2 x 6mm cables in parallel in place of 1 x 10mm.

TIA

Assuming you mean Main Protective Bonding conductor - I would say Yes
The regulations talk about minimum sizes of conductors not the number of conductors.
The critical point is that using test method 2 you get a very low continuity resistance of 0.05 Ω or less.

 

[EFLImpudence]

I would also say it is acceptable but I would not do it.
Can you not obtain 10mm²?

What earthing method is it?
Do you need more than 6mm².

 

[Bluehair]

Is this 0.05 ohm limit a reg or just guidance?

 

[EFLImpudence]

It is just a figure that is generally regarded as 'negligible impedance'.

After all, no measurement is going to be 0.00Ω.

 

[Bluehair]

But is the idea not to keep the touch voltage down to less than 50V and not blow a 100A fuse?

0.05Ω max cannot be a reg.

 

[Spark123]

The figure isn't quoted in the regs but yes, it is to keep touch voltage down whilst a 100A service fuse blows.

 

[EFLImpudence]

But is the idea not to keep the touch voltage down to less than 50V

Not really. Your thinking of Supplementary Bonding.

That does not apply to Main Bonding although it will be the result.

Main Bonding is sized to be able to carry, without damage, the currents caused by a fault to earth or a fault on the network.

and not blow a 100A fuse?

I don't know what you mean.

0.05Ω max cannot be a reg.

No, it's not a reg.

 

[JohnW2]

But is the idea not to keep the touch voltage down to less than 50V

Main Bonding is sized to be able to carry, without damage, the currents caused by a fault to earth or a fault on the network.

That clearly should be the case, but is there not a problem in calculating the required MPB conductor CSA unless one knows the resistance to earth from the (bonded) extraneous-c-p? Do you simply consider the 'worst possible scenario'? I would imagine that such a 'worst possible scenario' would have to assume total loss of the installation's earth combined with a negligible impedance L-MET/CPC fault in the installation (at a point where it was only protected by the service fuse), with an assumed resistance from extraneous-c-p to earth of zero. In that situation, I presume that the potential fault current would presumably approach double the PSCC. Is that how you would do it?

Kind Regards, John

 

[EFLImpudence]

But is the idea not to keep the touch voltage down to less than 50V

Main Bonding is sized to be able to carry, without damage, the currents caused by a fault to earth or a fault on the network.

That clearly should be the case, but is there not a problem in calculating the required MPB conductor CSA unless one knows the resistance to earth from the (bonded) extraneous-c-p?

I'm sure you know but -

We only have to calculate from the (actual) PEFC or fit the stated csa.

Do you simply consider the 'worst possible scenario'?

No, not specifically.

I would imagine that such a 'worst possible scenario' would have to assume total loss of the installation's earth combined with a negligible impedance L-MET/CPC fault in the installation (at a point where it was only protected by the service fuse), with an assumed resistance from extraneous-c-p to earth of zero.

I suppose it would be but is that possible, not to mention likely?

A fault from L tail to MET/CPC within CU while EC disconnected AND zeroΩ to earth via e-c-p.
(Who have you been talking to?)

In that situation, I presume that the potential fault current would presumably approach double the PSCC.

You mean - double because of the e-c-p to earth of zeroΩ?

In other words a 'perfect TT'.

 

[EFLImpudence]

Of course, with such a fault the bonding conductor would not have to be very big, would it?

A lesser and even more unlikely fault would need to occur to cause it damage.

 

[JohnW2]

I'm sure you know but - We only have to calculate from the (actual) PEFC or fit the stated csa.

Is it really as simple as that? I would have thought that 'PEFC' is, in itself, not directly relevant to the current which could flow along an MPB conductor, since it is determined on the basis of fault current flowing through the installation's 'normal' earth connection. The calculation you describe would be appropriate for the installation's main earthing conductor (from MET to earth) but does not directly tell us what could flow through the bonding conductor (particularly if the main earth was 'lost') - that will depend upon the 'Ze' of the extraneous-c-p, not of the normal earthing arrangement. Also, when you talk of 'PEFC' do you mean with or without MPB connected?

I would imagine that such a 'worst possible scenario' would have to assume ... I suppose it would be but is that possible, not to mention likely? A fault from L tail to MET/CPC within CU while EC disconnected AND zeroΩ to earth via e-c-p. (Who have you been talking to?)

Incredibly unlikely, I would say, but I'm not clear how unlikely a (theoretically) 'possible' scenario we have to design for. Quite frankly, I would suggest that it's probably also incredibly unlikely that one would ever see a current in an MPB conductor which couldn't safely be carried by a 2.5mm² or smaller conductor - but we are clearly expected to design for worse scenarios than that.

In that situation, I presume that the potential fault current would presumably approach double the PSCC.

You mean - double because of the e-c-p to earth of zeroΩ?

Yep, a return path of zero impedance, rather than (per PSCC) the impedance of the N conductor back to the transformer.

In other words a 'perfect TT'.

Sort-of - but if one is not going to assume the worst (obviously impossible) case of a zero 'Ze' for the extraneous-c-p, what alternative figure do you think we should assume?

I'm not trying to be awkward, nor even am I playing Devil's Advocate. I'm genuinely trying to understand the basis on which one is meant to calculate the required CSA for a MPB conductor (if one choses not to use the 'default' {'stated'} CSA), and the reasoning behind that basis. Are you saying that you would calculate it on the basis of the PEFC - i.e. the L-E fault current that would flow through the 'real earth' (if intact) in the absence of any bonding? Do you not entertain the possibility that certainly with TN-S, and obviously with TT) that the extraneous-c-p could have a lower resistance to earth than the installation's 'earth', such that higher currents than the 'PEFC' might flow in the MPB conductor?

Kind Regards, John

 

[JohnW2]

Of course, with such a fault the bonding conductor would not have to be very big, would it? A lesser and even more unlikely fault would need to occur to cause it damage.

I'm not sure I understand that - maybe I'm missing something.

Consider (easy numbers for illustraton) ... PSCC = 920A, hence impedance of supply L = impedance of supply N = 0.125Ω. MPB conductor 0.05Ω, assumed 'Ze' of extraneous CP zero. Real earth 'lost'. Negligible impedance L-CPC/MET fault. Current in MPB conductor = 230/(0.125+0.05) = 1314A. Would a 'not very big' conductor suffice?

Even if, more realistically, one assumed that the 'Ze' of the extraneous-c-p were 0.25Ω (which is what my water supply pipe IS), the MPB current would be 230/(0.125+0.05+0.25) = 541A, which might still not do too well with a 'not very big' MPB conductor.

Am I missing something?

Kind Regards, John

 

[Bluehair]

Of course, with such a fault the bonding conductor would not have to be very big, would it? A lesser and even more unlikely fault would need to occur to cause it damage.

I'm not sure I understand that - maybe I'm missing something.

Consider (easy numbers for illustraton) ... PSCC = 920A, hence impedance of supply L = impedance of supply N = 0.125Ω. MPB conductor 0.05Ω, assumed 'Ze' of extraneous CP zero. Real earth 'lost'. Negligible impedance L-CPC/MET fault. Current in MPB conductor = 230/(0.125+0.05) = 1314A. Would a 'not very big' conductor suffice?

Even if, more realistically, one assumed that the 'Ze' of the extraneous-c-p were 0.25Ω (which is what my water supply pipe IS), the MPB current would be 230/(0.125+0.05+0.25) = 541A, which might still not do too well with a 'not very big' MPB conductor.

Am I missing something?

Kind Regards, John

You measured the "Ze" of your water pipe? I thought that I was the only person that had done that. I have a Zs of zero at my cutout.

 

[EFLImpudence]

We only have to calculate from the (actual) PEFC or fit the stated csa.

Is it really as simple as that?

Yes.

I would have thought that 'PEFC' is, in itself, not directly relevant to the current which could flow along an MPB conductor,

It may not be but the EC csa is calculated from the PEFC (without bonding) and the MEB is half of that or as stated for PME.

since it is determined on the basis of fault current flowing through the installation's 'normal' earth connection. The calculation you describe would be appropriate for the installation's main earthing conductor (from MET to earth)

It is half of that calculation except PME.

but does not directly tell us what could flow through the bonding conductor (particularly if the main earth was 'lost') - that will depend upon the 'Ze' of the extraneous-c-p, not of the normal earthing arrangement. Also, when you talk of 'PEFC' do you mean with or without MPB connected?

As above - without.
I don't really think that 'losing' the EC is one of the 'faults' considered in the bonding requirements.

Yep, a return path of zero impedance, rather than (per PSCC) the impedance of the N conductor back to the transformer...
... but if one is not going to assume the worst (obviously impossible) case of a zero 'Ze' for the extraneous-c-p, what alternative figure do you think we should assume?

I don't see how that can happen so I suppose the PEFC is adequate for use.

I'm not trying to be awkward, nor even am I playing Devil's Advocate. I'm genuinely trying to understand the basis on which one is meant to calculate the required CSA for a MPB conductor (if one choses not to use the 'default' {'stated'} CSA), and the reasoning behind that basis.

As above, but I'm surprised that you are asking these questions.

Are you saying that you would calculate it on the basis of the PEFC - i.e. the L-E fault current that would flow through the 'real earth' (if intact) in the absence of any bonding?

Yes, that's how it's done.

Do you not entertain the possibility that certainly with TN-S, and obviously with TT) that the extraneous-c-p could have a lower resistance to earth than the installation's 'earth', such that higher currents than the 'PEFC' might flow in the MPB conductor?

With TT obviously, TN-S is usually comparable with TNC-S as far as Ze goes.

I suppose if encountering an installation like yours (TT Ze with TN-S Zs) I would probably fit a comparable MEB but that need only be calculated by using (Uo/Zs)/2 as are normal installations or would you think this should be treated as the earth.
I can't remember if you have said in the past if you have or would want fitted a proper EC to the water service.