Main earth bond

[JohnW2]

You measured the "Ze" of your water pipe? I thought that I was the only person that had done that.

Only indirectly. My TT rod usually has a resistance of about 70Ω, but the Ze of my installation with MPB (and other incidental paths) connected is about 0.25Ω.

I have a Zs of zero at my cutout.

I think you may need to explain that (is your cutout mounted on the substation transformer?!!)! ... and what are you doing making measurements at the cutout, anyway?

Kind Regards, John

 

[JohnW2]

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.

But are you seriously saying that one can apply that to a TT installation? The PEFC (without bonding) of my installation is about 3.3A. Are you suggesting that I can size my MPB conductor accordingly? (I think their are probably some minimum csa's specified somewhere, but I can't recall exactly where).

It is half of that calculation except PME.

As above, half of something exceedingly small with TT! (subject to whatever minimum may exist)

I don't really think that 'losing' the EC is one of the 'faults' considered in the bonding requirements.

OK, but if you don't lose the earth connection (I wasn't thinking of losing the Earthing Conductor but, rather, something like the infamous loss of a TN-C-S neutral), then a fault current will be shared between the earth connection and the extraneous-c-p in inverse proportion to their respective 'Ze' values. The 'half' calculation seems to imply an assumption that more current will go down the Earthing Conductor than down the MPB conductor.

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.

Primarily because they are questions which appear to become important with a TT installation.

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.

That's my very point. My situation might be extreme, but I'm sure that there are many TT installations out there with pretty low resistance paths to earth via bonded extraneous-c-ps. In the event of an L-E fault in such an installation (particularly mine), virtually all of the fault current would flow through the MPB conductor. Logically speaking, therefore, the conductor probably should have a CSA that would be required for an Earthing Conductor of an installtion with Ze equal to the 'Ze' of the extraneous CP, not half of it (the 'half' calculation presumably assuming that more fault currently would go to the 'proper' earth than to the bonded extraneous-c-p).

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.

When you say 'EC', I presume you mean MPB conductor? If so, yes, I have 10mm² main bonding - even if your PEFC-based calculations might suggest that 0.5mm² might be enough

Kind Regards, John

 

[Bluehair]

You measured the "Ze" of your water pipe? I thought that I was the only person that had done that.

Only indirectly. My TT rod usually has a resistance of about 70Ω, but the Ze of my installation with MPB (and other incidental paths) connected is about 0.25Ω.

I have a Zs of zero at my cutout.

I think you may need to explain that (is your cutout mounted on the substation transformer?!!)! ... and what are you doing making measurements at the cutout, anyway?

Kind Regards, John

The substation is about 5m from my cutout. My CU is next to the cut out.

 

[Spark123]

What are you using to measure it? Zero would actually mean fault current is infinity! Could be <0.01 ohms

 

[EFLImpudence]

Do you need to zero the leads? The best meters require this.

 

[Bluehair]

What are you using to measure it? Zero would actually mean fault current is infinity! Could be <0.01 ohms

Two calibrated Metrels.

 

[JohnW2]

What are you using to measure it? Zero would actually mean fault current is infinity! Could be <0.01 ohms

If his Zs were <0.01&#937;, then the PFC would be >23kA, which would probably have some interesting implications for some of the components in his installation

EFLI, what CSA would you recommend for the MPB conductor(s)?

Kind Regards, John

 

[EFLImpudence]

Actually, if you consider a disconnection time of 0.01s it works out to exactly 10mm².
Is that spooky or what?




Sorry for the delay.
I have been to do battle with suddenly walk backwards people.
Supermarket.

But are you seriously saying that one can apply that to a TT installation? The PEFC (without bonding) of my installation is about 3.3A. Are you suggesting that I can size my MPB conductor accordingly? (I think their are probably some minimum csa's specified somewhere, but I can't recall exactly where).

Yes, the minimum is 6mm² - 544.1.1

then a fault current will be shared between the earth connection and the extraneous-c-p in inverse proportion to their respective 'Ze' values. The 'half' calculation seems to imply an assumption that more current will go down the Earthing Conductor than down the MPB conductor.

Perhaps that is why the MEB has to be half of the EC. (but min.6mm²)

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?

It is possible but your water service is about as good as it could be and is not really.

That's my very point. My situation might be extreme, but I'm sure that there are many TT installations out there with pretty low resistance paths to earth via bonded extraneous-c-ps. In the event of an L-E fault in such an installation (particularly mine), virtually all of the fault current would flow through the MPB conductor. Logically speaking, therefore, the conductor probably should have a CSA that would be required for an Earthing Conductor of an installtion with Ze equal to the 'Ze' of the extraneous CP, not half of it (the 'half' calculation presumably assuming that more fault currently would go to the 'proper' earth than to the bonded extraneous-c-p).

Isn't that what I said?

When you say 'EC', I presume you mean MPB conductor?

I meant should your MEB be considered an EC (Earthing Conductor).

If so, yes, I have 10mm² main bonding - even if your PEFC-based calculations might suggest that 5mm² might be enough

 

[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.

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 = 0.125&#937;. MPB conductor 0.05&#937;, 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?

At 920A a 100A BS1361 (BS88-2 are quicker) would disconnect in 1.2s the EC required is 8.8mm² (k=115) so
the MEB required is 4.4mm². Therefore the TN-S 6mm² minimum will be adequate. PME will be 10mm²

Even if, more realistically, one assumed that the 'Ze' of the extraneous-c-p were 0.25&#937; (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.

Haven't you counted something twice?

Anyway - (0.125+0.05+0.25) = 0.38, 230/0.38 = 605 which gives a disconnection time of 6s. but
still the EC required is 12.9mm² so
the MEB required is 6.45mm² so probably still adequate.

Don't forget that this is all calculated for your extremely unlike L to MET/CPC/MEB fault upsteam of the CU OPDs.

 

[JohnW2]

Actually, if you consider a disconnection time of 0.01s it works out to exactly 10mm². Is that spooky or what?

Spooky things happen I suppose it's probably reasonable to assume 0.01s, even though the curves usually stop at 0.1s. I just wonder, however, whether the curves perhaps stop there because there is a minimum time (maybe around 0.1s??) in which any fuse will operate at any current (that's certainly true of MCBs, although it's obviously a totally different situation).

(I think their are probably some minimum csa's specified somewhere, but I can't recall exactly where).

Yes, the minimum is 6mm² - 544.1.1

Right. Thanks for reminding me. I guess that short-circuits much of the discussion, since it means that the answer is going to be either 6mm² or 10mm² in the vast majority of domestic installations.

then a fault current will be shared between the earth connection and the extraneous-c-p in inverse proportion to their respective 'Ze' values. The 'half' calculation seems to imply an assumption that more current will go down the Earthing Conductor than down the MPB conductor.

Perhaps that is why the MEB has to be half of the EC. (but min.6mm²)

Isn't that what I said (and you quoted), at least qualitatively? Whether is should be exactly 'a half' obviouslsy depends upon what assumptions one makes about current-sharing (i.e. the respective 'Ze' figures).

Logically speaking, therefore, the conductor probably should have a CSA that would be required for an Earthing Conductor of an installtion with Ze equal to the 'Ze' of the extraneous CP, not half of it (the 'half' calculation presumably assuming that more fault currently would go to the 'proper' earth than to the bonded extraneous-c-p).

Isn't that what I said?

When you say 'EC', I presume you mean MPB conductor?

I meant should your MEB be considered an EC (Earthing Conductor).

Ah, I see. I misunderstood you, apologies. So, yes, we are saying the same thing. However, although this 'same thing' that we're both saying makes electrical sense, I'm not sure that someone simply 'following the regs' would say it, would they? Wouldn't they just carry on regarding the MPB conductor as an MPB conductor?

If so, yes, I have 10mm² main bonding - even if your PEFC-based calculations might suggest that 5mm² might be enough

I thought you mentioned a 6mm² minimum?

Kind Regards, John

 

[EFLImpudence]

If so, yes, I have 10mm² main bonding - even if your PEFC-based calculations might suggest that 5mm² might be enough

I thought you mentioned a 6mm² minimum?

Yes.

The red is just a corrected typo. You wrote 0.5mm²

 

[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.

Consider ... Current in MPB conductor = 230/(0.125+0.05) = 1314A. Would a 'not very big' conductor suffice?

At 920A a 100A BS1361 (BS88-2 are quicker) would disconnect in 1.2s the EC required is 8.8mm² (k=115) so the MEB required is 4.4mm². Therefore the TN-S 6mm² minimum will be adequate. PME will be 10mm²

Fair enough. I guess I misunderstood what you meant by 'not very big' (before you reminded me about the 6mm² minimum).

Even if, more realistically, one assumed that the 'Ze' of the extraneous-c-p were 0.25&#937; (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.

Haven't you counted something twice?

Like what? ... L impedance back to transformer = 0.125&#937;, MPB conductor = 0.05&#937;, 'Ze' of extraneous-c-p = 0.25&#937;.

Anyway - (0.125+0.05+0.25) = 0.38, 230/0.38 = 605

I don't think so Per my figures above, (0.125 + 0.05 + 0.25) = 0.425, 230/0.425 = 541 - and least, using the rules of arithmetic I was taught

... which gives a disconnection time of 6s. but still the EC required is 12.9mm² so the MEB required is 6.45mm² so probably still adequate.

What about with the correct 541A? (I can't be bothered to do it myself, and doing the calculation can be your penance for your previous arithmetical error ).

Kind Regards, John

 

[JohnW2]

If so, yes, I have 10mm² main bonding - even if your PEFC-based calculations might suggest that 5mm² might be enough

I thought you mentioned a 6mm² minimum?

Yes. The red is just a corrected typo. You wrote 0.5mm²

Ah, you misunderstood my tongue-in-cheek comment It was not a typo - don't forget that it was written before you reminded me of the 6mm² minimum, and I was suggesting that 0.5mm² (perhaps fractionally larger) would probably be enough with a PEFC of 3.3A. Such a conductor could carry that PEFC indefinitely without harm, so disconnection times and the adiabatic calculation would become irrelevant!

Kind Regards, John

 

[EFLImpudence]

Even if, more realistically, one assumed that the 'Ze' of the extraneous-c-p were 0.25&#937; (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.

Haven't you counted something twice?

Like what? ... L impedance back to transformer = 0.125&#937;, MPB conductor = 0.05&#937;, 'Ze' of extraneous-c-p = 0.25&#937;.

Doesn't the Ze of 0.25 include the 0.125

Anyway - (0.125+0.05+0.25) = 0.38, 230/0.38 = 605

I don't think so Per my figures above, (0.125 + 0.05 + 0.25) = 0.425, 230/0.425 = 541 - and least, using the rules of arithmetic I was taught

Oh. yes. I put the 0.05 in the wrong column (in my head). Sorry.

What about with the correct 541A? (I can't be bothered to do it myself, and doing the calculation can be your penance for your previous arithmetical error ).

Lazy person

541A
disconnection time 9s ? I shall use 10s
EC 14.88mm²
MEB 7.44mm²

Still not too bad.

 

[EFLImpudence]

Ah, you misunderstood my tongue-in-cheek comment It was not a typo - don't forget that it was written before you reminded me of the 6mm² minimum, and I was suggesting that 0.5mm² (perhaps fractionally larger) would probably be enough with a PEFC of 3.3A. Such a conductor could carry that PEFC indefinitely without harm, so disconnection times and the adiabatic calculation would become irrelevant!

I beg your pardon.

I thought it referred to half of ten.