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Topic Title: A PME follow up question
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Created On: 08 July 2014 11:33 AM
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 08 July 2014 05:23 PM
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Avatar for Parsley.
Parsley

Posts: 1090
Joined: 04 November 2004

In John's example it sounds like someone had been a bit naughty and unknowingly interlinked the TT and TNC-S. The protective bonding conductors shouldn't have been connected to the suppliers PME terminal until the TBS supply was removed.

Regards
 08 July 2014 05:31 PM
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OMS

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In essense, the UK is basically all PME regardless of how it's labeled.

It's not a problem, PEN conductors rarely give up the ghost and if they do, there are network earths, connections between different distributor cables at the neutral and you also have other consumer loads as a shunt.

Not having bonding is far, far worse than having it - as I once pointed out, I'd rather have that diverted current going down a 10mm2 bond than the 1.5mm2 CPC to my immersion heater. Equally, the bonding will improve safety due to building faults in limiting touch voltages

Going back to the OP - yes, there is a slight risk in only having the SWA to act as both CPC and bonding conductor - but it's a slight risk - it hasn't been realised yet and the EICR should simply comment on that.

Personally speaking, I wouldn' t have just run the armoured across between buildings without dropping in a big chunk of copper alongside it - but you are where you are - so basically you need to comment, then engage with the client over mitigation (or not) - I don't get the sense you'll be digging up the ground anytime soon as aside from the broken PEN, there is no evidence that you actually experience significant diverted neutral currents anyway.

Keep in mind that whilst you don't have a "copper equivalent" you do have a lot of metal in that armour and it wil take some real high current to blow both the armour and the neutral core due to current flow being diverted.

Regards

OMS

-------------------------
Failure is always an option
 08 July 2014 08:42 PM
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leckie

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Now that OMS, is a superb reply that gives me a technical/theoretical and risk based approach that I can take into consideration.

I am I fact going to press the client to upgrade the bonding conductor but on the EICR I am edging towards a C3 with a comment. But I am going to make a couple more checks and inspections to put my mind at rest. Thanks.

Now I'm off to bed to read GN8
 09 July 2014 08:54 PM
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UKPN

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"the UK is PME regardless of labelling"

"its not a problem"

Not correct, this cable needs a 50mmsq bond before a problem does arise.

In this case the original installer believed a 25mmsq SWA was ok because 6mmsq (copper equiv) was the min of the day, but he should have calculated om mains incomer.

Regards.

We blame consultants!
 09 July 2014 09:27 PM
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daveparry1

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Which is what I said quite early on in this thread UKPN, before people complicated it beyond belief, it seemed quite obvious to me!
 09 July 2014 09:44 PM
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Avatar for leckie.
leckie

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UKPN

if that is what's required then that is what I will do. I can give a Code 2 and a quote for about £1000 plus VAT for the work.

I just don't want to quote for work that is not strictly required. I can see that if the neutral went open circuit there would be some problems, but it hasn't happened in the last 30 plus years so I am a bit reluctant to give a client a big bill for something that has been ok for all that time.

I didn't understand what you meant with your last sentence. Are you saying that 6mm was the order of the day? Or are you saying that this was a common misconception and that the regs, even 30 years ago were based on the supply neutral? And therefore the 50sq.mm was a requirement even back then.
 09 July 2014 10:40 PM
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Avatar for UKPN.
UKPN

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what I am saying leckie is that in those days the min size earth bond was
6mmsq. (its 10mmsq at the moment)
A 25mmsq BS 6346 SWA 4 core copper equiv armour is 6mmsq.
so the installer took that to be ok, not realising that a main of 400amp
with (I assume, 185mmsq) needs 50mmsq bonds.

I hope this helps.

Regards
 09 July 2014 11:51 PM
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alancapon

Posts: 5844
Joined: 27 December 2005

leckie, you need to remember that with PME, the bonding is probably in parallel with the neutral conductor in the supply cable, via other metallic services. It will carry load current under normal circumstances, and may carry fault current. This could be a share of the current to operate a 500A fuse in the local substation, which could take a few seconds to clear. A "broken neutral" is just one of a myriad of faults, and probably one of the most unlikely to happen in the great scheme of things. It gets talked about because the results can be significant / spectacular.


Regards,

Alan.
IET » Wiring and the regulations » A PME follow up question

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