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Topic Title: PME, no bonding, and no RCD
Topic Summary: C1 or C2?
Created On: 06 October 2018 07:59 PM
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 10 October 2018 06:34 PM
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Zoomup

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Joined: 20 February 2014

Originally posted by: geoffsd

Yes, that - and the instruction that the actual extraneous-c-p be left unbonded.


Hello Geoff,
I have been moving extension ladders about today and working on my roof, and am a bit tired. My brain is slow this evening. Will you please explain further as I can't quite understand your comment.

Thanks,

Z.
 10 October 2018 06:55 PM
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geoffsd

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Hello Z, Yes -

544.1.2 has for years stated that where there is an insulating section in the supply pipe the MPB shall be connected to the consumer's side (of the meter within 600mm.).

That is - bond the internal pipe which is not an extraneous-c-p and leave unbonded the supply pipe which (probably) is an e-c-p.

This has now, at last, been rectified.
 10 October 2018 07:52 PM
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chrispearson

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Well folks, the water supply where I am staying at the moment comes in a metal pipe. It is impracticable to get an MPBC anywhere near the entry point, but I have considered whether it would be possible to insert an insulating section. Sadly, the incoming pipe is not sufficiently accessible. The only option is to bond the water pipes further downstream. Hey ho!
 10 October 2018 08:13 PM
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geoffsd

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That's why it says "connect where practicable".
 11 October 2018 10:07 PM
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chrispearson

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It's all in the past now, 'cos the water pipes have been bonded. Under the bathroom floor was not a pretty sight. Copper pipes, plastic pipes, chewed up insulation (of the CH pipes) and mouse droppings; also a couple of unidentified cables with bare conductors.

The earthing arrangements are another matter. More to come in a few days after I get home.
 11 October 2018 10:29 PM
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UKPN

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"Except where PME conditions apply"

Regards, UKPN.
 11 October 2018 10:53 PM
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geoffsd

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"Except for highway power supplies and street furniture, where PME conditions apply..."
 12 October 2018 07:35 AM
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Zoomup

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Originally posted by: chrispearson

It's all in the past now, 'cos the water pipes have been bonded. Under the bathroom floor was not a pretty sight. Copper pipes, plastic pipes, chewed up insulation (of the CH pipes) and mouse droppings; also a couple of unidentified cables with bare conductors.

I wonder if plastic pipes are now a house's Achilles' Heel with sharped toothed rodents about?

Z.
 12 October 2018 07:40 AM
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Zoomup

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Originally posted by: geoffsd

Hello Z, Yes -



544.1.2 has for years stated that where there is an insulating section in the supply pipe the MPB shall be connected to the consumer's side (of the meter within 600mm.).



That is - bond the internal pipe which is not an extraneous-c-p and leave unbonded the supply pipe which (probably) is an e-c-p.



This has now, at last, been rectified.


Thanks Geoff, I now understand what you were saying. Perhaps it was considered that a small length of 600 mm or less of incoming external extraneous metal service pipe offered little risk.

Bye,

Z.
 12 October 2018 12:54 PM
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AJJewsbury

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544.1.2 has for years stated that where there is an insulating section in the supply pipe the MPB shall be connected to the consumer's side (of the meter within 600mm.).

That is - bond the internal pipe which is not an extraneous-c-p and leave unbonded the supply pipe which (probably) is an e-c-p.

This has now, at last, been rectified.

But what about the possibility that internal metallic pipework can still be an extraneous-conductive-part even though the supply pipe is insulating? E.g. because the metallic pipework then exits the building (to supply an outbuilding or garden tap) or even picks up a true earth potential due to being chased into a damp wall or floor internally?

- Andy.
 12 October 2018 01:47 PM
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geoffsd

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What about it?
 13 October 2018 11:10 AM
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AJJewsbury

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What about it?

Ah, sorry, that wasn't well explained (probably because it wasn't well thought out!)

I actually had the change to 411.3.1.2 (insulating section at point of entry need not be bonded) in mind when I wrote that. I think both regs still read badly though in terms of always referring to the 'point of entry' - which I suspect most people read as being where the gas or water (or whatever) flows into the building - so neglecting points where the pipework exits the building, or indeed where it can pick up another potential within the physical limits of the building structure.

- Andy.
 13 October 2018 11:26 AM
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davezawadi

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I am sticking to exactly what I said, and I suggest UKPN provide written proof of his (her?) comments. The ESQCR is clear on what it says and ties in with BS7671 (as others have said). The 600mm distance for bonding is not a regulation as such, in many cases it may not be possible and the first accessible point in perfectly acceptable. In some ways this would be better if it read: MPB should be connected at the closest easily inspected point of entry..... because searching for it under a floor with laminate is impossibly unreasonable and very many seem to think this connection (which should be capable of inspection) can be hidden forever, and are sometimes loose or corroded, which could present a real danger.

Andy, you seem to be expecting that any pipework could introduce a potential, this is harking back to the 15th edition, and is unusually the case. A quick check between the MET and odd items will soon indicate whether bonding is required, anything greater than 23k is normally considered perfectly safe (in other words less than 10mA potential shock current). In fact I think that a name and definition for such items should be added to BS7671, because there is a tendency to refer to them as extraneous, which is of course incorrect, and implies that they need bonding.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 13 October 2018 11:34 AM
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AJJewsbury

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Andy, you seem to be expecting that any pipework could introduce a potential

Not at all - I've just seem enough installations with pipework disappearing back into the ground or into damp walls or floors to know it's not as simple as only considering the service entry points. Maybe I've just had more than my fair share of old buildings to deal with
- Andy.
 13 October 2018 02:59 PM
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mapj1

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I think there are plenty of houses with earthed things outside actually.

n fact I think that a name and definition for such items should be added to BS7671, because there is a tendency to refer to them as extraneous, which is of course incorrect, and implies that they need bonding.


If there is a chance to revisit the dictionary there is another distinction missing, that is germane to the PME and water main issue. That is the distinction between pipes that are actually also bonded to the supplier's earth at each of the neighbouring properties, and therefore are in in effect in parallel with the company PEN, and singly earthed items, like a metal lamp post in the garden or a private oil tank. The former can potentially find itself carrying hundreds of amps, even if the mains is switched off, while in the second case the largest current that might reasonably flow is set by an electrode-like resistance, and is likely to be single figure amps. Conflation of the two cases leads to wasteful practices, such as a dedicated 10mm bonding conductor running to a shed with an earthed steel frame, when a cross section of couple of mm would have done.

-------------------------
regards Mike
 13 October 2018 03:38 PM
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geoffsd

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Originally posted by: davezawadi

The 600mm distance for bonding is not a regulation as such, in many cases it may not be possible and the first accessible point in perfectly acceptable.

Surely it is - although subject to where practicable. It is in 544.1.2.
As it only applies to internal meters, it is difficult to envisage where it would not be practicable.

In some ways this would be better if it read: MPB should be connected at the closest easily inspected point of entry.....

Is that not what where practicable means?



Of course, now that 544.1.2 and 411.3.1.2 have been amended meters with insulating sections need not (should not) be main bonded.
 13 October 2018 04:23 PM
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davezawadi

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I don't think it is quite the same thing Geoff. I have a case today where the MPB is connected under the floor, but is 5m from the MET. It is not inspectable (which is the important bit) without somehow getting under the floor. I also agree with Mike in that it seems unreasonable to MPB (possibly with a large cable) items which may have a very high resistance to the supply neutral although they are extraneous. All these requirements are based on metallic services, which are becoming somewhat unusual as both water and gas service pipes are now fitted as plastic, and therefore non-conductive. Clearly structural steelwork may have a low earth resistance due to size and surface area, but the odd small diameter water pipe with plastic end connections, probably does not. Anything more than say 5 Ohms to the MET does not require significant bond conductors as the possible diverted current is small, and just a mechanical size (6mm) would be adequate. We will have to wait for Amdt. 1!

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
IET » Wiring and the regulations » PME, no bonding, and no RCD

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