Earthing and bonding

[Rocboni]

Copied this from trainee section to see if I can get an answer, it is talked about a lot on the forum but reading through the old posts I can't determine whether I'm right or wrong.

What I'm looking to understand is sub main earthing conductors. Is what I am saying right?

So a sub main to an installation not requiring bonding the earthing conductor can be determined using the Adiabatic equation or table 54G

However a sub main which requires a protective equipotential bonding conductor, to say a steel framed shed, this is determined by the size of the suppliers neutral conductor using tab 54.8 if PME or half the size of the main earthing conductor min 6mm not exceeding 25mm.

In this situation would your earthing conductor be the armour or a core of the supply cable and run the separate bonding conductor straight to the structural steel, water or gas? Or would you take it to the Sub main distribution board and them from there to the structural steel? Does this make the the earth terminal in the DB a EMT?

So if you had a PME supply with 95mm incoming neutral and the sub main requiring bonding to structural steel was a 5 core 16mm you would run a separate 25mm earth wire to the structural steel.

What if you had a installation with PME 35mm DNO supply and your sub main was a 10mm 5 core could the earthing conductor within the cable be used as a main bonding conductor aswell as the circuit protective conductor?

Thanks in advance.

 

[Wilko]

Hi R, all good questions. I'll have a go but I shouldn't be driving :
- Yes
- Yes
- The armour of SWA sub main is unlikely to be sufficiently sized to work for bonding due to steel's resistivity (about 1/8 of copper) and then if PME there's the DNO neutral size to factor in.
- Yes. Afaik there is only one MET in an installation and that's at the origin. Any others are EMT. Bonding conductor is taken from MET to the outbuilding EMT and then connection made from gas/water to that EMT.
- PME with 95mm DNO neutral needs a 50mm earthing conductor from cutout to MET (table 54.7) and 25mm bonding conductor from MET to whatever wants bonding (table 54.8). This will be the same for further bonding conductors on site using the DNO means of earthing, so 25mm required for your outbuilding. Tick. I don't have 5 core data, but 4 core 16mm has 72mm of steel, or about 9mm copper bonding equivalent, so it's no good. Likewise the conductors are too small. So it's a 25mm single. Tick. But I'd run that to a local EMT and connect that to whatever needs bonding with 25mm again.
- PME with DNO 35mm neutral, so that's 16mm, then 10mm ... so yes could use a conductor from your 10mm 5 core SWA submain. PME currents could be circulating in the bonding conductor, so all cores need to be considered as heat generating.

Gee I hope that's right or it's humble pie on the breakfast menu

 

[Rocboni]

Thanks Wilko, so it's ok for the bonding and main circuit protective conductors to be connected e.g I'f the 25mm boning was taken into the DB and the DB earth terminal became the EMT then out to the structural steel and also your final circuits?

So if you did have a 16mm 5 core and a separate 25mm earth for bonding was required can you reduce this to a 4 core and use the 25mm earth only as both CPC and bonding conductor?

 

[Wilko]

The GN8 covers this area better than me
See section 5.6 and figure 5.15 specifically. EMT is normally shown separate from DB, but I don't know of a reg making that compulsory.
I would use the armour as CPC for the cable, provided it was ok (which it usually is). No problem (in theory) with using the 25mm bonding conductor for the cpc as it will exceed 543.1, if it is run together with the SWA. But we have to connect SWA armour anyway and the two will work nicely, exceeding the minimum requirements.

 

[Rocboni]

GN8 is £25 ive managed to get an older copy for £2 hopefully not much has changed.

 

[westward10]

We now use green/yellow to identify the cpc/bond/earthing conductor, for £2 yours may show them as solid green, SORRY tis Sunday.

 

[Rocboni]

I don't think it's that old! Includes 2004 amendment, for £2 if it's no good I'm not too worried.

 

[cliffed]

Great to see this question,& to know that all skilled sparkies know what's the difference between earth & bonding conductors.Just did a Eicr which results in probs with Swa's supplying remote buildings.
Still trying to explain it to my gaffer who ain't got a clue,will steer him into this direction .cheers

 

[westward10]

Only jesting it should show all you need to know, can probably pick up a cheap 17th Ed when the 18th comes out.

 

[westward10]

Great to see this question,& to know that all skilled sparkies know what's the difference between earth & bonding conductors.Just did a Eicr which results in probs with Swa's supplying remote buildings.
Still trying to explain it to my gaffer who ain't got a clue,will steer him into this direction .cheers

I would guess I note this down at least twice a month on EICRs.

 

[Richard Burns]

An older version of GN8 should not have changed in its consideration of current flow through a steel conductor so I think you should be OK with it, just keep in mind any possible changes that may have occurred.
I believe that both you and wilko have the information correct so I will just add these comments.

In most cases using the armour of an SWA cable will not have sufficient csa to be suitable as a bonding conductor, in the cases you specify dividing the armour csa by 8.5 gives
16mm² five core csa of armour 72mm² (Cu 8.47mm²)
10mm² five core csa of armour 46mm² (Cu 5.41mm²)
neither of these is sufficient to use as a bonding conductor unless the csa required is at the minimum 6mm² for the 16mm².
The armour of a cable and an extra core cannot be added together to get your csa, one or the other must be suitable on its own.

Copied this from trainee section to see if I can get an answer, it is talked about a lot on the forum but reading through the old posts I can't determine whether I'm right or wrong.

What I'm looking to understand is sub main earthing conductors. Is what I am saying right?

So a sub main to an installation not requiring bonding the earthing conductor can be determined using the Adiabatic equation or table 54G.
Correct (now table 54.7)

However a sub main which requires a protective equipotential bonding conductor, to say a steel framed shed, this is determined by the size of the suppliers neutral conductor using tab 54.8 if PME or half the size of the main earthing conductor min 6mm not exceeding 25mm if not PME.
Correct

In this situation would your earthing conductor be the armour or a core of the supply cable and run the separate bonding conductor straight to the structural steel, water or gas? Or would you take it to the Sub main distribution board and them from there to the structural steel? Does this make the the earth terminal in the DB a EMT?
The cable run would be determined by practical circumstances.
A separate bonding conductor can be run from the MET to the extraneous conductive part or the sub main supply cable can be used as a combined earthing and bonding conductor from MET to the EMT and then a separate bonding conductor to the extraneous conductive part.

So if you had a PME supply with 95mm incoming neutral and the sub main requiring bonding to structural steel was a 5 core 16mm you would run a separate 25mm earth wire to the structural steel.
Correct or you could use a 25mm² SWA five core and use one of the cores as the earthing and bonding conductor, but normally more expensive.

What if you had a installation with PME 35mm DNO supply and your sub main was a 10mm 5 core could the earthing conductor within the cable be used as a main bonding conductor as well as the circuit protective conductor?
Yes
Thanks in advance.

 

[Rocboni]

This was never explained in college, it's becoming a case of the more you learn the more you realise how much you don't know.

 

[westward10]

I use a rule of thumb, if it is a four core cable below 50.0 it will be inadequate to support the minimum size of 10.0 bonding, no calculation required.

 

[westward10]

This is a page from an old UNEEDA publication showing where the armour can be used for bonding purposes, sizes below the dark line are suitable.

View attachment 35226

 

[Toneyz]

Sorry where did this table come from ? and Richard where did the formla come from

 

[westward10]

This is another sheet showing the csa of MI sheaths.

View attachment 35227

 

[westward10]

Sorry where did this table come from ? and Richard where did the formla come from

The sheet comes from an old UNEEDA guide on testing, it is an out of date publication but the table is still accurate. The formula can be found in GN8.

 

[Deleted member 26818]

From what I can recall, the copper equivalence in conductance for 10mm2 doesn't kick in until you reach 95mm2 SWA?
By that that time you'll be wanting a conductance equivalence of 25mm2.
I just take it as read, that the armour of SWA, will never be suitable for bonding.

 

[westward10]

From what I can recall, the copper equivalence in conductance for 10mm2 doesn't kick in until you reach 95mm2 SWA?
By that that time you'll be wanting a conductance equivalence of 25mm2.
I just take it as read, that the armour of SWA, will never be suitable for bonding.

95.0 is for 2 core, to say it will never be suitable is not correct.

 

[Deleted member 26818]

Umm, in an Ice cream parlour at the mo, so don't have access to any books.
So can you tell me at what CSA Line conductors does 10mm2 for bonding stop being acceptable?

 

[westward10]

Don't understand the question.

 

[Wilko]

If I've understood correctly...
For TNS and TT - line conductor over 35mm bonding goes to 16mm, from table 54.7 and reg 544.1.1
For TNCS - DNO neutral conductor over 35mm bonding goes to 16mm, from reg 544.1.1 and table 54.8.

 

[westward10]

I see now, don't see relevance of question though.

 

[Rocboni]

Not all TNCS supplies are PME but all PME supplies are TNCS so how do you know if it's PME, will the DNO always put a sticker on?

 

[Wilko]

Not all TNCS supplies are PME but all PME supplies are TNCS so how do you know if it's PME, will the DNO always put a sticker on?

I had not heard this ... I look in BYB section 312.2.1 fig 3.9 where TNCS and PME seem to be the same thing (?).

 

[Rocboni]

Just what I've read elsewhere, so not sure, I think you might be right though.

I suppose you could have a on site Tx which TNCS supply with no multible earthing. So if no tx on site then the TNCS supply will always be PME?

 

[westward10]

TN-C-S is generally considered PME with some exceptions, the example you give could be one.

 

[DPG]

Page 47/48/49 of GN8 explain (and show) the difference between PME and PNB.
Daz

 

[Deleted member 26818]

I see now, don't see relevance of question though.

Quite simple.
By the time the CSA of the armour is sufficient to offer the equivalent conductance of 10mm2 copper, you need the equivalence of 25mm2 copper.
As such SWA is never suitable for bonding.

 

[westward10]

Nope still don't know what you are getting at, armour can be used for supporting bonding.

 

[Deleted member 26818]

Don't understand why you're having trouble with this?
The conductance ratio between steel and copper is about 9:1.
By the time you've found an SWA cable where the armour has a sufficient CSA to be a bonding conductor, you will require a larger bonding conductor.

95mm2 SWA has armour which has the equivalent conductance of a 10mm2 copper conductor. Unfortunately a 95mm2 SWA cable, would where PME conditions apply require a bonding conductor with the equivalent conductance of a 35mm2 copper conductor.

 

[westward10]

These cables would be used for supplies to other buildings where bonding would be required, the size of main bonding has already been decided at the DNO source. We are not discussing DNO supply cables.

 

[sparksburnout]

These cables would be used for supplies to other buildings where bonding would be required, the size of main bonding has already been decided at the DNO source. We are not discussing DNO supply cables.

In this case then, what's to stop you using a 4C 10mm SWA (even if you don't need one), and using the 4th core for the Bonding conductor?

 

[westward10]

In this case then, what's to stop you using a 4C 10mm SWA (even if you don't need one), and using the 4th core for the Bonding conductor?

Nothing.

 

[Deleted member 26818]

These cables would be used for supplies to other buildings where bonding would be required, the size of main bonding has already been decided at the DNO source. We are not discussing DNO supply cables.

Lol.
What does that mean?
Are you suggesting that this supply to another building would be larger than the DNO supply at source?

 

[westward10]

Lol.
What does that mean?
Are you suggesting that this supply to another building would be larger than the DNO supply at source?

Are you trying to get a rise or are you just not understanding.

 

[Deleted member 26818]

I think it is you that is having a problem with understanding.
If the main incomer requires 10mm2 bonding conductors, that would suggest the supply conductors are no greater than 35mm2.
Why would you then use 95mm2 SWA for a sub main?

 

[westward10]

Getting a rise then.

 

[Toneyz]

I think it is you that is having a problem with understanding.
If the main incomer requires 10mm2 bonding conductors, that would suggest the supply conductors are no greater than 35mm2.
Why would you then use 95mm2 SWA for a sub main?

length of run ,volt drop earth loop imp. etc

 

[westward10]

I think it is you that is having a problem with understanding.
If the main incomer requires 10mm2 bonding conductors, that would suggest the supply conductors are no greater than 35mm2.
Why would you then use 95mm2 SWA for a sub main?

I have no idea why you keep referring to the supply conductor size, the point is can a sub main swa cable support bonding when the armour is used as the earth. This will nearly always apply to a three phase supply where the minimum size to support bonding would be 50.0. In this day and age people nearly always use three core cable utilising a core as the cpc for single phase supplies. Otherwise it would have to be 95.0 for a two core.