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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 11:33 AM
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leckie

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I asked a question recently regarding exporting PME and this is a follow up question because I have been thinking and confused myself again! I must stop doing that.

So we have a 50sq.mm main protective bonding conductors in the source building because we have a large incoming supply. We also have a detached building with a 100A TPN supply.

Why does the exported protective bonding conductor need to be 50 sq.mm? If the supply is limited to 100A then how could any diverted neutral currents be large enough to require as large a cable as that?

I know I am missing something here but I'm not quite sure what is is
 08 July 2014 11:40 AM
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Parsley

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Think about where the N-E link is in the source building. The diverted N currents aren't necessarily from the outbuilding and they don't care how they return to the Tx star point. if the detached building has a lot of steel going into the ground the currents may prefer that route to the suppliers cable.

Regards
 08 July 2014 11:42 AM
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Legh

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It maybe that your primary system has the capacity to supply a current greater than 300A (>150mm2). So under normal imbalances and possible PEN problems the MPB needs to be sized to cope with these circulating currents.

Legh

-------------------------
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 08 July 2014 11:42 AM
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daveparry1

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Probably due to the size if the suppliers fuse in the source building i's say Leckie?
 08 July 2014 11:45 AM
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OMS

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The extent of diverted neutral current isn't a function of the supply rating (and there is no fuse in the neutral anyway)

The bonding size is based on just how much current will flow which is set by the probable resistance of that metal building or it's services acting as an electrode

Equally the value of the electrode resistance sets the effective touch voltage which is a function of load.

Basically it needs to be "as big as" because you can't control what goes down that cable - it's set by the network configuration ajnd relevant position of the broken PEN and other "multiple earths" - it's not there to deal with diverted neutral currents - that's just a by product

Regards

OMS

-------------------------
Failure is always an option
 08 July 2014 12:01 PM
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leckie

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Ah right, see I have told you all before I am a moron! so now bear with me while I try to understand a bit more fully.

So I can now see that any stray currents from say a broken PEN conductor will go for the least line of resistance and that if the building was a giant steel electrode there is a good chance that where it will find its way. So that's the bonding conductor reason explained. Until I think of something else.

Now I just need to understand a bit more fully what where and why diverted neutral currents occur. Obviously if you have an open PEN I can see how they would occur. But how else? If you had a low IR neutral to earth on a final circuit then that would result in current that should be going down the neutral that ends up going down the earth. Ditto computer gear leaking to earth. So are they causes of what are classed as diverted neutral currents, or am I missing something still
 08 July 2014 12:13 PM
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AJJewsbury

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or am I missing something still

I think you're missing resistors in parallel theory. Current does NOT 'go for the least line of resistance', but splits in proportion to conductance (inverse of resistance) of each path. If you had a 1-Ohm and 2-Ohm resistor in parallel, then you wouldn't find that all the current goes though the 1-Ohm resistor and none through the 2-Ohm, but it would split 2:1 - i.e. two-thirds of the current would go down the 1-Ohm resistor and one third though the 2-Ohm one.

So with PME, the supplier's N has a certain resistance back to the star point, but so do extraneous-conductive-parts - which might be quite low, especially if there in contact with something that's bonded to someone else's earth teminal (PME or TNS) on the same distribution network.

- Andy.
 08 July 2014 12:15 PM
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Parsley

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In TN especially TNC-S PME systems diverted neutral currents are everywhere even in healthy installations. They can flow even when the main switch for the installation is switched off. Some DNO's have now stopped supplying steel framed multi occupancy buildings with individual PME supplies due to the neutral currents from adjoining installations returning via the steelwork and overloading the service cable of the supply closet to Tx.

Regards
 08 July 2014 12:19 PM
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leckie

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Sorry Andy, a bit of a clumsy description, I do understand about parallel paths.

What I am asking is if there is not a broken PEN, but there is an N-E low IR, ie a parallel path, is this what is being described as a diverted neutral current? I have always just called them earth faults?
 08 July 2014 12:29 PM
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AJJewsbury

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Hi leckie - no, it's nothing to do with low IR. Think about current having done its work starts its journey home back towards the star point, gets to the supplier's cut out and PME N-PE link. It now splits - some goes back via the normal supply N for the building - but some can go back via bonding and extraneous conductive parts to the star point (perhaps via true earth, perhaps via metal all the way). It's this second route that provides a parallel path (no broken CNE, no insulation fault) and carries diverted N current.
- Andy.
 08 July 2014 12:36 PM
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leckie

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Sorry Parsley I was writing when you posted. So I see what you are saying now, the diverted current can come from outside the building.

So in my previous post with a PME query I was talking about a detached steel framed building with a 25mm SWA TPN supply. The supply is probably 35-40 years old. The SWA is being used as both the CPC and the bonding conductor. So it is undersized as a bonding conductor, with the SWA being the equivalent of about
8 - 9 sq.mm.

I cant tell if the SWA is getting significantly hot, etc, so how do I know how much diverted neutral current may be going through it, and if it poses a danger? There are no indications of overheating cables, terminations, etc, and its been in place for a long time. In the previous post there was suggestions of both a Code 2 and a Code 3, that's why I have been trying to understand the potential problem a little better as I want to advise correctly.
 08 July 2014 12:49 PM
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AJJewsbury

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how do I know how much diverted neutral current may be going through it,

Can you get a clamp meter around the whole cable? L/N/PE current due to the load should be in balance so any residual should be diverted N. Not total proof of anything as the network conditions (e.g. loads both in your installation and others) may not produce worst-case conditions, but might give you an indication.
- Andy.
 08 July 2014 01:21 PM
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leckie

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Well I could get my clamp meter around the SWA.

I've just measured the supply into my own building. First I clamped the tails around the three lines and the neutral and measured about 10mA. So this would give a rough check on the IR of the building.

Then I measured around the earthing conductor and got 6.4A. I have a metal A frame type industrial building on an industrial estate of the same types. So I presume I have just measured stray neutral currents on my earthing conductor?

Edited: 08 July 2014 at 01:38 PM by leckie
 08 July 2014 01:39 PM
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OMS

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Or you've just measured earth leakage on the building ?

Depends on the ammeter, but worth clamping it both ways around - that may give you an indication of what direction the current is "flowing"

I've measured quite a few 10's of amps flowing into a building and exiting via the LPS system electrodes


Regards

OMS

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Failure is always an option
 08 July 2014 02:07 PM
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leckie

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Oms
I thought when I measured around all the live conductors that the 10.4mA reading was the earth leakage of the building, and so when I measured just the earthing conductor and got 6.4 Amps that was the earth leakage of the building plus the diverted neutral current in that conductor? Wouldn't all the building earth leakage show in that first test?
 08 July 2014 04:01 PM
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mapj1

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However, and this is only what is the ultimate limit of neutral current could be in the worst possible case, when the PEN is interrupted immediately downstream of the first bond to a metallic service, the current that service pipe might be asked to carry will never be more than the fraction of the substation output current that would have gone down the interrupted neutral branch.
Which sadly is far higher than the supplier fuse in a small outbuilding, but because there is a connection between service pipe to neutral in the head, it may be asked to carry all that load - and in many cases there will be no ADS, well not straight way, until the service cable melts/...

-------------------------
regards Mike
 08 July 2014 04:35 PM
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leckie

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Well any diverted neutral currents in this case would either go through the steels of the building or through the SWA, there are no pipes, etc.

Anyone confirm my thoughts on my clamp test results?

Also, we used to have 6sqmm equipotential bonding conductors many years ago on PME 100A SPN supplies. This is still considered adequate providing there's no sign of thermal damage, etc to the bonding conductors. Does anyone know if smaller equipotential bonding conductors were order of the day for larger supplies? What I am getting at is, when this job I have been discussing was installed about 30 years + ago, what were the regs on the larger bonding cable sizes for PME? I knew I should have kept my 14th edition
 08 July 2014 04:48 PM
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John Peckham

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I saw a situation last year where the 3 phase supply was not connected, supply fuses in polythene bag next to the head, but the bonding had been connected to the suppliers means of earthing. There was a loss of the neutral on the supply transformer and the diverted neutral currents from the surrounding consumers premises found there way back to the transformer via the foil on the pipework lagging with lots of smoke and sparking all over the site which scared all the trades including the sparks on site. The sparks on site spent a lot of time looking for problems on their separate TBS to the site temps. thinking that as the supply fuses were out of the new main supply that could not be the problem.

So aside from any leakage due to kit and neutral earth faults from your own installation you will have current flowing in your earthing conductor and bonding from nearby other installations under normal and fault conditions. very unlucky if you have diverted neutral currents flowing under fault conditions as you may well have a fire. I have seen 6 in the last 3 years.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 08 July 2014 05:03 PM
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leckie

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Blimey, I presume that that would only have been a small part of the diverted neutral current, i.e. there would have been lots of other lower impedance parallel paths. How can you protect against that? You cant go around bonding foil can you really!

You are giving an example where if the bonding had been disconnected it might have been safer aren't you?
 08 July 2014 05:14 PM
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mapj1

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Well,
Arguably, from the fire protection perspective, the PEN needs to be disconnected too - and this ties in with other threads in recent times on this PME not really safe discussion.

Currently to interrupt the CPC after the PEN> N& E split is not permitted, neither is it acceptable to interrupt the PEN before the split. Though 3 pole LN & E isolation for car chargers will be recommended it seems.

Come back fused neutrals, all is forgiven ;-) (well maybe not, but perhaps a 100A phase fuse and a 200A PEN one would not look so silly....)

Really of course if it was actually dangerous we'd be recommending TNS or TT, but that would represent
a significant change of direction.

-------------------------
regards Mike
IET » Wiring and the regulations » A PME follow up question

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