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Topic Title: 200mA main bonding current on TN-S
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Created On: 05 March 2019 10:21 pm
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 05 March 2019 10:21 pm
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wallywombat

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Joined: 19 October 2018

At a house that is (almost certainly) TN-S, there is consistently about 200mA flowing on the earthing conductor, with about 100mA each on the water and gas bonding conductors. Opening the main switch and disconnecting CU and water from the MET (so only the EC and gas are still connected), there's about 200mA on the gas bond. The water pipe is lead and the gas pipe is steel (to the property anyway).

Is this excessive? And can anyone think of a fault (presumably with the network or at a neighbour's) which would provide such a current (for at least 2 weeks now)?

The house is urban, and about 60m from the substation. The supply cable was formerly lead-sheathed and ran into the cellar, but about 15 years ago an external meter cabinet was installed and the lead cable under the small front garden was severed and spliced to a new cable going to a new cut out in the cabinet - which has an earthing terminal and has has no "PME" label or similar.
 05 March 2019 11:06 pm
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broadgage

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It might be normal.
On a PME supply, metallic gas and water supply pipes are in effect a duplicate neutral, and a small proportion of the normal neutral current will go via such pipes, with the majority of the neutral current flowing in the suppliers cables.

Whilst this supply is believed to be TN-S, do not count on this ! DNOs seem to have a policy of stealth conversion to PME without telling anyone.
Extension or cable jointing often result in the neutral core and the earthed sheath of existing cables being BOTH connected to the CNE of a new cable.
Jointed or otherwise altered service cables may have a combined neutral and earth until just outside the property.

If the neutral and earth are the same conductor up to the service cut-out, then this is called PME/ TNC-S.
If the neutral and earth are the same conductor up to a buried joint a few meters away, and then separated thereafter, then this is called TN-S.

From a practical point of view it makes little difference if the neutral and earth are combined up to the cut out, or up to a joint a few meters away.
 05 March 2019 11:19 pm
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mapj1

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Seconded - this property may be presenting TNS, but lets assume that there are at least 2 neighbours with TNC-s (PME) on the same substation, perhaps as an addition or a repair.
Each of the neighbours also has proper earth bonding to further along the same set of gas and water pipes.
Now we have lives and neutral running along the street, but at these points, there is in effect, an extra connection between the neutral and the water pipes and gas pipes. But in normal use there is a modest voltage drop along the neutral, as there is along the lives for that matter, because the wires have small but finite resistance. This modest voltage drives a fraction of the load current along the pipes as an alternate neutral path.

Now, if you have TNS, your earth is at the potential of the neutral back at the substation, so looks like another obvious short-cut relative to the neutral voltage..

So probably there is no fault, just someone on the same substation with PME. (it may not be a house it could just as easily be a lamp-post.)

As pipes often have a far larger metal cross-section than the supply cables, it is not unknown for these diverted currents to be as high as several amps, and more if there is a high resistance joint in the supply neutral.

With the right equipment you can look at the phase of this diverted current and hazard an educated guess as to which of the 3 phases is carrying the greatest load, and also see the levels of 3rd harmonic.

-------------------------
regards Mike
 06 March 2019 10:06 am
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wallywombat

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Thanks for the info guys. So, some further questions.

When referring to the DNO quietly upgrading things to PME, would that just be branches to properties, or could parts of the main street cable be CNE too? i.e. with the risk of the loss of N+E to many properties rather than just one?

Given the difficulty in distinguishing between TN-S and TN-C-S supplies, is it always safe to assume -C-S? I.e. are the rules for -C-S stricter in every respect than -S? (Of course, there may be cost implications of assuming the worst).
 06 March 2019 12:09 pm
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AJJewsbury

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When referring to the DNO quietly upgrading things to PME, would that just be branches to properties, or could parts of the main street cable be CNE too? i.e. with the risk of the loss of N+E to many properties rather than just one?

They seem to have a habit of combining N and PE at every opportunity - so the link could by anywhere.

Given the difficulty in distinguishing between TN-S and TN-C-S supplies, is it always safe to assume -C-S? I.e. are the rules for -C-S stricter in every respect than -S? (Of course, there may be cost implications of assuming the worst).

Strictly speaking, it's safest to assume PME (as distinct from TN-C-S - technically there are non-PME arrangements that some official standards also refer to as TN-C-S). If you assume PME then yes all the rules are stricter - larger main bonding (and possibly earthing conductor) and no supplier's earth connection to caravans or boats (or a few more things of the DNO's choosing).

- Andy.
 06 March 2019 12:38 pm
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wallywombat

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And what size should one use for main bonding / EC if assuming PME on a hybrid network, since this is supposed to be a function of the CNE conductor size?
 06 March 2019 01:20 pm
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AJJewsbury

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And what size should one use for main bonding / EC if assuming PME on a hybrid network, since this is supposed to be a function of the CNE conductor size?

Fair question - I'd not really thought about it before. I'd probably just assume that any CNE would normally be the same size as the supplier's N on a TN-S presented system (e.g. assume <= 35mm2 for a normal domestic) and ignore the possible extra c.s.a. provided by the lead sheath or other separate PE conductor.

- Andy.
 06 March 2019 04:07 pm
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mapj1

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well, BS7671 woud have you reaching for 10mm cable for a 100A supply, assuing 35mm supply cable in the street, or or 16mm for 50mmsq.
35mmsq is a safe bet for a branch that feeds one or two houses.

-------------------------
regards Mike
 06 March 2019 04:36 pm
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perspicacious

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And what size should one use for main bonding / EC if assuming PME on a hybrid network, since this is supposed to be a function of the CNE conductor size?

Cue UKPN with the answer.......

Regards

BOD
 07 March 2019 01:39 pm
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broadgage

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As an aside, I consider it probable that a future edition of the regulations might remove the distinction between TNC-S and TNC and simply introduce requirements for earthing and bonding without the present distinction between the two systems.

TT would still be subject to different requirements. of course.
 07 March 2019 05:44 pm
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Weirdbeard2

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Originally posted by: wallywombat

At a house that is (almost certainly) TN-S, there is consistently about 200mA flowing on the earthing conductor, with about 100mA each on the water and gas bonding conductors. Opening the main switch and disconnecting CU and water from the MET (so only the EC and gas are still connected), there's about 200mA on the gas bond. The water pipe is lead and the gas pipe is steel (to the property anyway).



Is this excessive?




Hi wally, are the existing conductors large enough to handle the sustained 200 mA?

Why are you taking these readings?
 07 March 2019 10:57 pm
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AJJewsbury

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As an aside, I consider it probable that a future edition of the regulations might remove the distinction between TNC-S and TNC and simply introduce requirements for earthing and bonding without the present distinction between the two systems.


(presuming you meant TN-S, rather than TNC)

It'd surprise me a little I think. True TN-S systems don't have many of the disadvantages of PME - and there will always be reliably TN-S systems (e.g. from private generation or private transformers) - applying full PME rules to all them would just be a waste of resources. It might potentially contradict some methods of side-stepping PME, such as employing an isolating transformer to create a local TN-S system).

A NOTE to the effect that earthing facilities provided by the public LV network within the UK should be regarded as potentially being PME even if it's not presented as such, I would welcome though.

- Andy.
 07 March 2019 11:56 pm
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wallywombat

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Originally posted by: Weirdbeard2

Hi wally, are the existing conductors large enough to handle the sustained 200 mA?


Well, the EC and bonding are 16mm/10mm, so I should hope so!

Why are you taking these readings?


I had just bought a new earth leakage clamp meter and wanted to put it through its paces.
 08 March 2019 09:30 am
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mapj1

Posts: 12039
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And this is why when trouble shooting RCD and RCBO type problems, it is far more sense to measure the leakage on a circuit as the RCD would, namely by enclosing L and N conductors in the eye of the meter, rather than clamping the CPC, which for this sort of reason tends not to add up to the same, and can be downright confusing, as electrons arrive and escape down the cable TV, telephones, plumbing and all sorts.

You can also pass a thin wire a few times through the eye to increase the sensitivity - each turn adds another dose of magnetic field so a meter showing a reading 10m/A with 5 turns is really passing the same 2mA five times through the eye, for example. (I hope it is obvious that the turns have to go the same way to add and opposite flows, like L/N cancel)
If this is a new toy (sorry, piece of vital test equipment) then getting to know its strengths and weaknesses quietly and well away from a nosy customer as you are doing, is to be recommended.

-------------------------
regards Mike
 08 March 2019 06:08 pm
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Weirdbeard2

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Originally posted by: wallywombat

Originally posted by: Weirdbeard2

Hi wally, are the existing conductors large enough to handle the sustained 200 mA?


Well, the EC and bonding are 16mm/10mm, so I should hope so!

Why are you taking these readings?


I had just bought a new earth leakage clamp meter and wanted to put it through its paces.


Thanks for the reply Wally, i would suggest not worrying too much about the supply side of things, BS7671 is on the side of the installation, TNC-S and TN-S share the same requirements, so bonding sizes etc are the same, it is only where the supply is declared PME with a sticker that there are slightly more onerous requirements regards main protective bonding minimum sizing, which your installation sounds like it probably meets anyway. it is not within the remit of a bs7671 electrician to declare that PME conditions apply on the supply side as there is no way of knowing.
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