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Topic Title: Earth Cable
Topic Summary: Earth cable from switchboard to MET
Created On: 24 July 2012 03:30 PM
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 24 July 2012 03:30 PM
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bspinks1

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Joined: 24 July 2012

I need help, I'm confused over earthing.

I have an industrial premise where the supply cable is 2x240 4 core swa cable run to a new switchboard [run from the main building about 150 meters away.]

My question is what size of earth cable do I install from the new switchboard to the main earth terminal [800x50x6mm] in the new premises?
 24 July 2012 03:36 PM
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AJJewsbury

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Is the new switchboard to provide a connection for main bonding (BMET) for the out building?

What's the earthing system? (PME?)

- Andy.
 24 July 2012 03:39 PM
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bspinks1

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t's a new build with a TNCS supply
 24 July 2012 03:49 PM
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jsa986

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You could use the CSA of the armour of 4core SWA. It will comply for use as a CPC for cables up to 240mm. (70 degree and 90 degree PVC and XLPE ) Above wont comply as "k" value is insufficient.

If your running a separate earth 120mm should do it

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Edited: 24 July 2012 at 04:03 PM by jsa986
 24 July 2012 03:57 PM
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AJJewsbury

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TNCS supply

And how big is the N of the DNO's supply?

- Andy.
 24 July 2012 04:04 PM
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bspinks1

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I've the CSA of the armour of 4core SWA to work, it's between the Switchboard main earth terminal and the MET.

As the MET is only a small copper bar, do I take a 240mm earth to the MET or can I use Table 54.8 - 150mm²≥ 50mm² as there is a difference between the NICEIC and BS 7671 views on the subject.
 24 July 2012 04:10 PM
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jsa986

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Ok i think i know what you mean now, sorry as Andy said what is size of the N of Supply?

BTW - NICEIC dont make the regs!

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 24 July 2012 04:13 PM
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AJJewsbury

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Table 54.8 - 150mm²≥ 50mm²

That's the direction I was heading in - but note that the 50mm2 is for copper, not steel. Equivalent conductance for steel is about 1/9th that of copper - so on the face of it you'd need 450mm2 of steel armour to satisfy the bonding requirements. (I don't have my SWA tables to hand at the moment).

It'll also have to satisfy your requirements for use as a c.p.c. (adiabatic (or table 54.7 if you prefer) and Zs etc).

Some caution against using SWA as a bonding conductor in PME systems as the diverted-N current in normal service can heat the cable.

- Andy.
 24 July 2012 04:17 PM
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bspinks1

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Using a single core earth cable would be a better option ?
 24 July 2012 04:31 PM
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OMS

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back to the beginning chaps - given the load on the second building, where is the supply transformer for the main intake - if it's just for the single consumer it won't be PME will it.

If you now have PNB, and presumably the N-E bond is at the client main switchboard then all you have in reality is a TN -S system.

the armour would need to meet an adiabatic limit and the earth marshalling bar in the second building would only need connection to the MET by something equivalent to a 25mm2 of copper - (about 200mm2 of steel roughly) - 240mm2 XLPE has a steel CSA of about 290mm2 so no drama.

Of course, if it really is PME then just ignore me and chuck a few LPS tapes in the trench with the armoureds - it'll be fine

regards

OMS

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 24 July 2012 05:01 PM
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jcm256

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2x240 4 core swa - take that to mean 2X 240mm 4core SWA in parallel.

Even the supply people do it that is use the blue on one parallel feeder for the neutral and the other parallel feeder blue bent over and lugged/bolted on to the earth bar. Cost of green-yellow tape 25P or 50mm x 12mm diam heat shrink £1, cost of separate 50mm green-yellow earth cable? Your choice
 24 July 2012 05:06 PM
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UKPN

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--well, at the risk of a topic running into days--

for this level of installation, we are looking at 50mmsq bonding
conductors and 70mmsq earthing lead.

so called "inspection councils" and "regulatory" bodies have
no bearing on the supply regulations.

Regards
 24 July 2012 05:08 PM
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AJJewsbury

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Even the supply people do it that is use the blue on one parallel feeder for the neutral and the other parallel feeder blue bent over and lugged/bolted on to the earth bar.

Effectively a half-sized N then?

Wouldn't that require overcurrent protection in the N? (431.2.1 and 524.2.3 (ii))

- Andy.
 24 July 2012 05:52 PM
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John Peckham

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OMS

"If you now have PNB, and presumably the N-E bond is at the client main switchboard then all you have in reality is a TN -S system. ".

Do you want to debate that (again)?

It might be TN-C_S but is it PME?

You will also need to know the Zs at the supply end board and the type and rating of the protective device so that you can meet the disconnection time for the cable when sizing the CPC.

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John Peckham

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 24 July 2012 06:12 PM
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OMS

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Originally posted by: John Peckham

OMS



"If you now have PNB, and presumably the N-E bond is at the client main switchboard then all you have in reality is a TN -S system. ".



Do you want to debate that (again)?



It might be TN-C_S but is it PME?



You will also need to know the Zs at the supply end board and the type and rating of the protective device so that you can meet the disconnection time for the cable when sizing the CPC.


It's not PME - it's not TN-C-S - it's PNB and has the characteristics of TN-S

The "MEL" is effectively the supply incoming neutral conductors - they will comply in every respect as S = S in terms of line conductors. It's TN-S as all that's happened is the neutral earth point has moved from the transformer spill box to the switchboard

If, and we don't know this yet, if that's the set up to the main building then the two lots of armouring will comply in every respect as a CPC and as a TN-S bond to the outbuilding marshalling terminal.

regards

OMS

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 24 July 2012 06:38 PM
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jcm256

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Wouldn't that require overcurrent protection in the N? (431.2.1 and 524.2.3 (ii))

- Andy.

Nice one, quoting a new regulation,(431.2.1) So, you would need one big intelligent MCCB for adjustable ratings about 125 to 1600A.
(What will they think of next).
In lots of quarries parallel feeders to sub-switchboards are done like that core in one cable (paralleled up with the SWA of course) and one core in the other cable used as the neutral. Nowadays mostly in new installations in quarries 4 core is run to motors on quarry belts etc with one core the earth instead of relying only on the SWA. Interesting results for R2 as with quarry regulations the conductance of the earthing conductor should be at least 50% of line conductors, the results sometimes work out at over 300% .
524.2.3 , there lies the cop out . The load carried by the circuit under normal service conditions should be practically equal distributed between the lines. Therefore, it is up to a good designer to abide with that.
Regards
jcm

Edited: 25 July 2012 at 08:15 AM by jcm256
 24 July 2012 08:59 PM
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John Peckham

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OMS

But the star point is connected to the earth electrode at the transformer! Therefore the neutral conductor has 2 functions so the earth and neutral is combined in part of the installation. I like the explanation of PNB in BS7430!

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John Peckham

http://www.astutetechnicalservices.co.uk/
 24 July 2012 11:59 PM
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AJJewsbury

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But the star point is connected to the earth electrode at the transformer!

I don't have it to hand, but isn't the transformer electrode marked as 'optional' (or similar)? (and in practice isn't done).

Weren't you both at the same meeting as me when I tried to get some GN wording altered on the basis that TN-C-S wasn't exactly equivalent to PME and it was explained very patiently to me that I was wrong, TN-C-S did mean exactly PME, that the PNB diagram that was in the DPC (derrived from the BS 7430 one) was pulled for that reason and the BS 7430 committee were to 'clarify' things in the next edition? (Or did I dream that one too?)

- Andy.
 25 July 2012 09:23 AM
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OMS

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Originally posted by: John Peckham

OMS

But the star point is connected to the earth electrode at the transformer! Therefore the neutral conductor has 2 functions so the earth and neutral is combined in part of the installation. I like the explanation of PNB in BS7430!


It most certainly isn't John - it may be at the transformer, it may be along the conductor or it maybe at the switchboard - but there is only one connection, and invariably that's at the switchboard as it holds down the neutral voltage in the consumers installation (where the transformer is private). From a DNO perspective, that link will be at the transformer or at the substation fuse rack if more convenient and the distributors as seen from the consumer side are effectively TN-S.

From the transformer spill box you have 4 conductors - L1, L2, L3 and N. At the switchboard you connect the neutral to the switchboard earth bar - connected to that switchboard earth bar will be the electrode, bonding to the transformer, the installation main earth bar and any equipotential bonds plus the LPS if present.

The transformer star point is expressed via copper strap within the tank to the LV spill box - ie it is one conductor inside the transformer. Simply extending it several metres via a single or mulkticore cable doesn't change that - electrically it is still a single connection to the star point - in fact it has to be because if you want unrestricted (and possibly multistage) earth fault protection, that neutral conductor needs to be enclosed by the protection CT before it connects to the means of earthing such that neutral current in and out of the CT is equal in normal operation , any difference must be going to earth so you reach the protection trip point (say 30% of FLC for 1 second and a correspondingly higher current for a shorter time) and drop out the transformer (probably on both HV and LV sides) - this is your final backstop on critical installations as it can "see" all of the installation rather than the zone of protection offered by restricted earth fault protection (which usually also contains the transformer overtemperature/pressure protection as well)

As Andy pointed out, the seeds of confusion sown by the BS 7671 committee and the BS 7430 committee resulted in significant changes following the DPC - and correctly in my opinion because it was patently incorrect.

PNB only has a single point of connection to earth, and no conductor undertakes more than one function.

You can try to convince me that I'm wrong John, but I've been doing this for a long time now, and I trained with old skool engineers who had also been doing it for years - I don't belive that the collective knowledge of some pretty bright electrical engineers right acrosss the UK and beyond has been fundamentally incorrect for many decades just because a single committee managed to get its collective knickers in a twist

regards

OMS

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 25 July 2012 08:29 PM
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paulskyrme

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OMS,
I am interested in the PNB vs TN-C-S (PME) discussion.
I'm going to start a new thread, please remember though I don't "do" HV, or DNO type stuff, just the LV side, & machine stuff!
Would you please be so kind as to join the discussion, I'm sure John & Andy will be along from their posts above?
IET » Wiring and the regulations » Earth Cable

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