PME & Broken neutral

Knobhead · Jul 29, 2012

I know some countries use protection to guard against a broken neutral in PME systems.
Anyone know how they work?

Engineer54 · Jul 30, 2012

ackbarthestar said:
I can't help suspecting that there is a conspiracy of denial going on here.
Every electrician known to me, who's been in the game for a while has come across at least one failed neutral, that's not to say that it was open circuit, but more high resistance. As soon as the resistance rises beyond a few ohms it will begin to cause problems and will get progressively worse unless it is recognised and dealt with pdq.
I would certainly agree that spiking the MET at every supply intake will alleviate certain problems but will also hide them as well

I think it more a case of PME never being fully accepted in the UK by the older electricians, having been brought up, as it were with TN-S and TT systems. A single incident somehow turns into multiple incidents, and many electricians are simply telling of someone else's experience as their own, ...it soon spirals out of all recognition then.... Not so in the vast majority of Europe where you rarely, if ever hear any scaremongery about PME/TNC-S installations, that and TT systems is all they have ever known...

If as you say, every electrician of any age, has had experience of a PME failure of any description, then there would by now be a public outcry. Yes it does happen, but no-where near to the extent that some would have us believe. All the DNO's are very aware of the potential dangers of their PEN conductors and connections and go to great lengths to ensure the integrity of these systems. You only have to ring a DNO with just a suspicion of a problem involving PME/TNC-S supply and they will be generally there within the hour...

The way i see it, the conspiracy is from those perpetuating the myths and scaremongery that surrounds UK PME distribution systems...

johnboy6083 · Jul 30, 2012

Chr!s said:
You on the :54:

Its the touch voltage you need to consider.

Fair point about the touch voltage, but that is the point of CPC's and equipotential/supplementary bonding, so that even if the touch voltage is high, then there is no potential between any extraneous or eposed conductive parts.
in my inmd there is little difference between the physics of current flowing down a rod, or down a pipe to earth. Eitrher way, if the bonding cable is sized correctly, it shouldnt pose a danger to the end user of the installation.

No alcohol by the way, just rubbish at typing fluidly/accurately at the same time!

Chr!s · Jul 30, 2012

johnboy6083 said:
Fair point about the touch voltage, but that is the point of CPC's and equipotential/supplementary bonding, so that even if the touch voltage is high, then there is no potential between any extraneous or eposed conductive parts.
in my inmd there is little difference between the physics of current flowing down a rod, or down a pipe to earth. Eitrher way, if the bonding cable is sized correctly, it shouldnt pose a danger to the end user of the installation.

No alcohol by the way, just rubbish at typing fluidly/accurately at the same time!

Several problems,

1, voltages within the equipotential zone maybe quite high.
2, it's sometime's difficult to maintain that equipotential zone.
3, high voltage's may appear outside.

A low Ra will limit the touch voltage, 200 ohm's no good, more like 2

johnboy6083 · Jul 30, 2012

i agree with the need for low Ra's. If the Ra is relativley high compared to the extraneous metalwork, then theres really no point in it at all
I also agree that its difficult to maintain the equipontential zone. how many DIYers do away with bonding, because they have no idea, or misunderstand its purpose.

so can we come to an agreement that an earth rod on TNC-S supplies is good, as long as it has an Ra thats better than Ze with bonding connected/main earth disconnected?

Edd · Jul 30, 2012

Engineer54 said:
I take with a large pinch of salt, those that state they have seen numerous instances of broken neutrals on PME systems, it just doesn't happen, fortunately!!! I have a recently retired friend (DNO regional manager) that has only known of 3 such instances since joining, as was then, Eastern Electricity through to the time of his retirement in 2011, approx 35 years in total!!!

One of the easiest and cost effective ways of mitigating such problems to some extent, is to have a local earth rod at ''Each'' and every PME/TNC-S supply head, as they do in many European countries. But not at a numbty 200 ohm level, that would be a complete and total waste of time!

Strange that 25 of them years I would have been a sparks, and had meetings at rayleigh EEB main office. (sadly a meter changer for a year) And my father was a chair man Of the ECA and was well up there as to SAY!(knew everyone) and your friend only told you of 3 such incedences!! yep its been well covered

as for 200 ohms, I totally 100% agree with you there.
((was looking for a thumbs up but couldnt find it!))

Chr!s · Jul 30, 2012

Tony said:
It would seem some countries use a RCD with a built in over voltage trip to protect the consumers equipment.

This vector diagram shows what happens with a floating neutral.

The Tx would be solid earthed, if the PEN was broke the star point is still maintained , under-voltage would be the way to go, although the only way to limit the diverted currents is with a low impedance.

So one of these

http://www.hager.com.sg/energy-distribution/protection-devices/permanent-over-under-voltage-auxiliary-mz209-mz21/4844.htm

And an electrode me thinks.

Edd · Jul 30, 2012

Just another note on PME, about 2 months ago i was doing a CU change, I upgraded the bonding to gas and water. but when i was testing the Ze i got 11 Ohms! (TNS) ok i should have checked before. So I get the client to phone the DNO (ive tryed before but they only wana speek to the bill payer) and I parrot talk him through the fault.
Im fitting an alarm system as well in the property at the time so was around for a while, so in an hour the DNO turn up and check the Ze and he says yep it was probelly like this from when they did the joint in the road, using wooden joint boxes.
Two options TT the house or get the occupier to apply for TNCS,the 3rd option from the chap who turned up was I PME the head and he would re seal if needed! I went with option 3 but re sealed it myself! There was no care of what the condition of the undrground joints were in. so to say...

Quote E54 (not sure how to do this so i copy and paste)

All the DNO's are very aware of the potential dangers of their PEN conductors and connections and go to great lengths to ensure the integrity of these systems. You only have to ring a DNO with just a suspicion of a problem involving PME/TNC-S supply and they will be generally there within the hour...

Crock of Sh it..

Oh yer they were there in an hour but had no intrest in the joints, just wanted me to sort it out!

PS I have no issues with PME if done correctly. And not taken to outside sources, Like hot tubs, (Shudders)

Marvo · Jul 30, 2012

Jimmy Boy said:
<snip> It is not a simple matter to retrofit TN-C-S distribution systems with such a facility. This is true for
TN-C-S systems (such as those in South Africa) that do not require the installation of a foundation
earth at the service entrance. Unless every consumer each have their own service entrance
foundation earth facilities, in the event of a break in the PEN conductor, the load current of all
other consumers may find a return path through those foundation earth connections that happen
to be present.
An extreme example would be the case of only one consumer having a foundation earth. In such
a situation, all the load currents of all consumers connected to that distribution system would try
to flow through the single foundation earth present. The consequential overheating together with
the related possibilities of incendiary ignition will be obvious.

In Cape Town nearly all the domestic installations I've seen are TN-C-S and look like this.

[ElectriciansForums.net] PME & Broken neutral

Every house has at least one earth spike.

There's two very different tiers of electrical distribution though, in the informal areas they use a maypole type system supplying multiple dwellings overhead from a single pole. I'm not sure what earthing system this system uses.

Engineer54 · Jul 31, 2012

so in an hour the DNO turn up and check the Ze and he says yep it was probelly like this from when they did the joint in the road, using wooden joint boxes.
Two options TT the house or get the occupier to apply for TNCS,the 3rd option from the chap who turned up was I PME the head and he would re seal if needed! I went with option 3 but re sealed it myself! There was no care of what the condition of the undrground joints were in. so to say...

I've never seen or heard of buried wooden cable joint boxes. In the early day's virtually all underground cable joints would have been heavy gage cast iron with plumed lead joints!!!

And if the DNO electrician came to any job i was involved with and spouted off that sort of crap, i'd have been on to his manager, asking a few pertinent questions. One of which, would have been allowing a jobbing electrician to undertake connection alterations to a service cut-out!!!

Edd · Jul 31, 2012

Engineer54 said:
I've never seen or heard of buried wooden cable joint boxes. In the early day's virtually all underground cable joints would have been heavy gage cast iron with plumed lead joints!!!

And if the DNO electrician came to any job i was involved with and spouted off that sort of crap, i'd have been on to his manager, asking a few pertinent questions. One of which, would have been allowing a jobbing electrician to undertake connection alterations to a service cut-out!!!

Well aparently they have to apply for PME with who ever they pay their bills too.
The option was to dig up the street! but in his words its probelly best to PME the head, but as the meter was supplyed by someone else, it needs to be applyed for by the occupier. he even volentered to return and reseal, so he wasnt dodging the job, took me like 2 minuits. And as ive mentioned I worked for Eastern Electricity so it wasnt a big deal.
Looking back I could have kicked up a fuss, Have you ever tryed talking to the DNO!!! I bet NOT,, theres only so much Barry Manalow you can take....

Deleted member 9648 · Jul 31, 2012

I can only recall one instance of a broken neutral causing a shock hazard in my time,over 30 yrs,that in a butchers shop in a village.
Regarding the 200 ohms value of an earth rod....it is ridiculous to mockingly quote this figure in conjunction with a TNCS where such a rod may be used to limit the danger of a lost N....the 200 ohms is intended for a TT system designed to operate at high Zs values....not a system designed to utilize a low value of Zs.

Engineer54 · Jul 31, 2012

Edd said:
Well aparently they have to apply for PME with who ever they pay their bills too.
The option was to dig up the street! but in his words its probelly best to PME the head, but as the meter was supplyed by someone else, it needs to be applyed for by the occupier. he even volentered to return and reseal, so he wasnt dodging the job, took me like 2 minuits. And as ive mentioned I worked for Eastern Electricity so it wasnt a big deal.
Looking back I could have kicked up a fuss, Have you ever tryed talking to the DNO!!! I bet NOT,, theres only so much Barry Manalow you can take....

You don't just PME when there is a problem with a TN-S supply, not even the DNO's would be foolish enough to do that!! The distribution supply cable in question needs to meet certain criteria before N-E connections can be made to that supply cable.

Not saying that the DNO's these day's are anything like the old Electricity Boards, but they DO take problems with PME supplies very seriously, and generally WILL be on site within the hour when a problem has been brought to their notice. More than they dare do, is allow a fault on PME'd system persist for any length of time....

Wirepuller

My mentioning the 200 ohm Ra was made tongue in cheek, but by the same token, how many of these newer or Electrical Trainee's electricians would know any different?? Unless of course they are members here and had the good sense to ask, before banging in one of those twigs...

bigspark17 · Sep 22, 2012

An rcd wouldn't detect this and would stay in the functional position.. Some believe the bet way to protect against this rare but terrifying fault would be a contactor after the main switch.. Thus cutting the supply to this point at least.

PME & Broken neutral

Knobhead

Engineer54

johnboy6083

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Chr!s

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johnboy6083

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Edd

Chr!s

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Edd

Marvo

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Engineer54

Edd

Deleted member 9648

Engineer54

bigspark17

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