Main incoming earth bonding

[EFLImpudence]

Part 2 may have separate entries but the actual definitions are not dissimilar.

I don't understand why anyone would want to use 54.7 - it is merely a cover-all figure ...

Agreed, and I only mentioned it because you, in turn, had mentioned it as an alternative to calculaion.

No. I was replying to Daftpunk who had stated that 16mm² was required for PME.

It's people (including a good few electricians), not Table 54.7, that have "the magic 16mm²" in their heads.

Yes, true - or quite.

I suppose I shouldn't second guess the behaviour of DNOs, but I was assuming that 'S' (for incoming supply) would not normally be less than 16mm² - in which case the minimum CSA for an earthing conductor would, per 54.7, be 16mm² for any incoming cable size up to 35mm².

... but you said it was primarily for CPCs so could be 1mm².

For incoming supplies the DNO will have used 16mm² for 100A supplies, so yes but if you calculate it will be less.
Also, should you have a 60A fuse then 10mm² is all that would be required regardless of that actually fitted.

 

[JohnW2]

Part 2 may have separate entries but the actual definitions are not dissimilar.

I'm afraid that don't agree - the definitions in Part 2 are different, and incompatible. Per those defintions, a CPC connects exposed-c-ps to the MET, whereas an Earthing Conductor connects the MET to "an earth electrode or other mans of earthing". Those are clearly different parts of the earth path, aren't they?

I don't understand why anyone would want to use 54.7 - it is merely a cover-all figure ...

Agreed, and I only mentioned it because you, in turn, had mentioned it as an alternative to calculaion.

No. I was replying to Daftpunk who had stated that 16mm² was required for PME.

Yes, that's how it started but then...

Table 54.7 is only for use , as it states, "When it is desired NOT TO CALCULATE... ...in accordance with 544.1.3" (the equation).

I suppose I shouldn't second guess the behaviour of DNOs, but I was assuming that 'S' (for incoming supply) would not normally be less than 16mm² - in which case the minimum CSA for an earthing conductor would, per 54.7, be 16mm² for any incoming cable size up to 35mm².

... but you said it was primarily for CPCs so could be 1mm²

I was talking about an Earthing Conductor with TT - and obviously not if one chose to use Table 54.7.

For incoming supplies the DNO will have used 16mm² for 100A supplies, so yes but if you calculate it will be less. ... Also, should you have a 60A fuse then 10mm² is all that would be required regardless of that actually fitted.

Yes, but if one chooses to use Table 54.7, one surely has to go on the basis of what cable size the DNO has used, not what they could have used (but didn't)??

Kind Regards, John

 

[EFLImpudence]

Part 2 may have separate entries but the actual definitions are not dissimilar.

I'm afraid that don't agree - the definitions in Part 2 are different, and incompatible. Per those defintions, a CPC connects exposed-c-ps to the MET, whereas an Earthing Conductor connects the MET to "an earth electrode or other means of earthing". Those are clearly different parts of the earth path, aren't they?

Well, yes different parts of the earth path but the same purpose.
CPCs and THE earthing conductor are both (see) protective conductors.

Also, should you have a 60A fuse then 10mm² is all that would be required regardless of that actually fitted.

Yes, but if one chooses to use Table 54.7, one surely has to go on the basis of what cable size the DNO has used, not what they could have used (but didn't)??

No.

 

[westie101]

At the end of the day, answer this!

The conductor from a DNO earth terminal or cable sheath to the MET/CU as well as providing a bond to the DNO earth for the installation is also there to provide a path for current to flow to ensure the DNO's cut-out fuse will operate in 5 secs for an L-E fault prior to the protective devices in the CU.

It is up to the DNO to provide that connection (unless it is a terminal provided for use on the cut-out)

The DNO is not governed by BS7671

So the size of that cable is the responsibility of the DNO, not the customer, not the electrician, not building control.
For a service cable size up to 50mm2 DNOs consider 10mm" sufficient for a PME supply (and also a cable sheath earth supply) which complies with ESQCR & the Electricity Act.

So why argue what BS7671 says, it does not apply?
In terms of compliance with ESQCR it is even described as an Equipotential Bonding Conductor in our documentation as it is bonding the DNO earth to all other metal that needs bonding.
So in those terms is an earthing conductor not a connection to an earth rod?

(the same applies for Zs, it is there to ensure the DNO fuse operates and will be at whatever level the DNO decides)

 

[JohnW2]

Well, yes different parts of the earth path but the same purpose. CPCs and THE earthing conductor are both (see) protective conductors.

Yes, they are both protective conductors - that's why 543.1 etc applies to both of them

Also, should you have a 60A fuse then 10mm² is all that would be required regardless of that actually fitted.

Yes, but if one chooses to use Table 54.7, one surely has to go on the basis of what cable size the DNO has used, not what they could have used (but didn't)??

No.

You've lost me. If one chooses to use Table 54.7 (rather than calculating) then that table indicates the minimum CSA of a protective conductor in terms of the CSA of the associated line conductor. That has surely got to be the line conductor which actually exists, not a hypothetical one of a different size which might have been installed (but wasn't).

Don't forget that if one uses the (lazy) Table 54.7 method, the size/nature of the OPD is irrelevant - the required minimum CSA of the protective conductor depends on only the CSA of the 'associated line conductor'.

Kind Regards, John

 

[JohnW2]

So the size of that cable is the responsibility of the DNO, not the customer, not the electrician, not building control. ... For a service cable size up to 50mm2 DNOs consider 10mm" sufficient for a PME supply (and also a cable sheath earth supply) which complies with ESQCR & the Electricity Act. ... So why argue what BS7671 says, it does not apply?

I wouldn't say that people are arguing about it, but you can't really blame them for talking about Table 54.8 of BS7671 - since, for whatever reason, it indicates the minimum CSAs for Earthing Conductors in PME installations. That table indicates a miniumm of 10mm² for neutral supply conductors up to 35mm², 16mm² for neutrals 35-50mm², 25mm² for neutrals 50-95mm² etc.

Since that is part of BS7671, and even though BS7671 does not apply to your work, I presume that an electrician undertaking an EICR would have to code a non-compliance if (s)he found an Earthing Conductor which did not comply with Table 54.8 of BS7671.

... and don't forget, as you mentioned, modern PME cutouts often just provide an earth terminal, to which an electrician (necessarily working to BS7671) will connect a conductor going to the MET - so would have to comply with BS7671-stated requirements for that conductor.

Kind Regards, John

 

[EFLImpudence]

I wouldn't say that people are arguing about it, but you can't really blame them for talking about Table 54.8 of BS7671 - since, for whatever reason, it indicates the minimum CSAs for Earthing Conductors in PME installations. That table indicates a miniumm of 10mm² for neutral supply conductors up to 35mm², 16mm² for neutrals 35-50mm², 25mm² for neutrals 50-95mm² etc.

Bonding conductors, not earthing.



54.8 does indeed relate to bonding conductors for PME.

However, I have been talking about 54.7 which (along with 543.1.4 of which it is part and people miss) relates to CPCs and earthing conductors for all earthing methods and therefore bonding conductors, other than PME.

 

[westie101]

table indicates a miniumm of 10mm² for neutral supply conductors up to 35mm²

So as the largest copper equivalent single phase service neutral/earth generally being used by DNOs is 25mm2
Why are folk insisting on 16mm2 earths?

(note any reference to 35mm2 by DNOs is to the Aluminium phase conductor, which the copper equivalent of is 25mm2, the CNE is copper and of that size!)

I'm even more bemused now!

 

[EFLImpudence]

Why are folk insisting on 16mm2 earths?

Because they are mistaken or -

following 54.7 which is the one size fits all method - similar to the OSG stating tails must be 25mm².

 

[JohnW2]

I wouldn't say that people are arguing about it, but you can't really blame them for talking about Table 54.8 of BS7671 - since, for whatever reason, it indicates the minimum CSAs for Earthing Conductors in PME installations. That table indicates a miniumm of 10mm² for neutral supply conductors up to 35mm², 16mm² for neutrals 35-50mm², 25mm² for neutrals 50-95mm² etc.

Bonding conductors, not earthing.

Literally, per that Table, yes - but as you know full well, 542.3.1 says that, in PME installations, Earthing Conductors have to comply with the same minimum CSA requirements as Main Bonding Conductors

However, I have been talking about 54.7 which (along with 543.1.4 of which it is part and people miss) relates to CPCs and earthing conductors for all earthing methods and therefore bonding conductors, other than PME.

No argument with that, but I must be missing your point. In fact, I think it also relates to PME - in as much as both 543.1 and 544.1.1 then have to be satisfied (although, in practice, the later is probably usually likely to be the more demanding).

Kind Regards, John

 

[JohnW2]

Why are folk insisting on 16mm2 earths?

Because they are mistaken or - following 54.7 which is the one size fits all method

I would have though the latter - as I said before, (given that the next size up from 10mm² is 16mm²) if the supply neutral is anything greater than 10mm², then Table 54.7 would call for an Earthing Conductor of 16mm².

Kind Regards, John

 

[westie101]

I wouldn't say that people are arguing about it, but you can't really blame them for talking about Table 54.8 of BS7671 - since, for whatever reason, it indicates the minimum CSAs for Earthing Conductors in PME installations. That table indicates a miniumm of 10mm² for neutral supply conductors up to 35mm², 16mm² for neutrals 35-50mm², 25mm² for neutrals 50-95mm² etc.

I would have though the latter - as I said before, (given that the next size up from 10mm² is 16mm²) if the supply neutral is anything greater than 10mm², then Table 54.7 would call for an Earthing Conductor of 16mm².

A bit of a contridiction, which is it?

 

[EFLImpudence]

Literally, per that Table, yes - but as you know full well, 542.3.1 says that, in PME installations, Earthing Conductors have to comply with the same minimum CSA requirements as Main Bonding Conductors

Ooh - I had forgotten about that bit.
Yes you are right. Sorry.

So, in other words (ignoring 54.7) the earthing conductor shall be sized according to the adiabatic equation but for PME must be no smaller than the bonding conductor.

 

[JohnW2]

I wouldn't say that people are arguing about it, but you can't really blame them for talking about Table 54.8 of BS7671 - since, for whatever reason, it indicates the minimum CSAs for Earthing Conductors in PME installations. That table indicates a miniumm of 10mm² for neutral supply conductors up to 35mm², 16mm² for neutrals 35-50mm², 25mm² for neutrals 50-95mm² etc.

I would have though the latter - as I said before, (given that the next size up from 10mm² is 16mm²) if the supply neutral is anything greater than 10mm², then Table 54.7 would call for an Earthing Conductor of 16mm².

A bit of a contridiction, which is it?

It's not a contradiction, it's a requirement to simultaneously satisfy two regulations - 543.1 (of which Table 54.7 is part, and which determines the minimum CSA for any protective conductor and 544.1.1 (which indicates the minimum CSA for Earthing & Main Bonding Conductors in PME systems). Hence, an Earthing Conductor in a PME installation has to satisfy both those regulations, so the higher of the two 'minimum' figures will obviously apply.

Kind Regards, John

 

[JohnW2]

Literally, per that Table, yes - but as you know full well, 542.3.1 says that, in PME installations, Earthing Conductors have to comply with the same minimum CSA requirements as Main Bonding Conductors

Ooh - I had forgotten about that bit. Yes you are right. Sorry.

No problem - glad you agree!

So, in other words (ignoring 54.7) the earthing conductor shall be sized according to the adiabatic equation but for PME must be no smaller than the bonding conductor.

Well, yes. As I've just written/implied to westies...

For non-PME, the minimum CSA of Earthing Conductor is given by (adiabatic calculation OR Table 54.7)

For PME, the minimum CSA of Earthing Conductor is given by the higher of (adiabatic calculation OR Table 54.7) AND (the CSA required for a PME main bonding conductor as given in 54.8 ).

Kind Regards, John