PME & Broken neutral

[Knobhead]

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

 

[snowhead]

There was a post recently on here where someone posted a drawing showing it.

RCD before the N to E connection??

 

[Knobhead]

I thought along those lines but it’s prone to imported faults

 

[Edd]

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

Acording to E54 it never happens, so why bother.

 

[Knobhead]

It happened to my house years back.

 

[Edd]

seen it 3-4 times now, last time it had taken out the flexy hoses under the kitchen sink, when they turned the shower on! scarry stuff. but hey they had a good Ze

As snowhead says wonder why they dont fit a 100A 500ma RCD before the NE join.

 

[plugsandsparks]

Doesn't the white wire on the RCD help with this?

 

[Jimmy Boy]

Speak to the DNO ask them if you can fit an Earth Rod as a back up they will probably say no but do it anyway lol
J

 

[Sintra]

Speak to the DNO ask them if you can fit an Earth Rod as a back up they will probably say no but do it anyway lol
J

That is the way it is done in some countries an external electrode is connected to the N / E juncture within the service head.

 

[plugsandsparks]

Just checked the schneider site and they do an under-voltage module for their C60 din stuff, that may do it

 

[somersetsparks]

That is the way it is done in some countries an external electrode is connected to the N / E juncture within the service head.

Most of the underground PME joints to supply buildings are staked by the dno

 

[Jimmy Boy]

That is the way it is done in some countries an external electrode is connected to the N / E juncture within the service head.

As the N/E are shared at the head can you see any problem with taking a rod from MET ? I am struggling to find a reason how this wouldn't improve the safety of the installation if you lost the Neutral
J

 

[MarkieSparkie]

Link to the thread Snowhead referred to earlier, Tony:
http://www.electriciansforums.net/e...3-protection-against-lost-neutral-tn-c-s.html

 

[Jimmy Boy]

I don't quite get where he's saying it should be positioned in relation to it's effectiveness ? My CU has a 30mA non delayed RCD as a main switch will it trip out if the N drops out ? it's the same as his diagram isn't it ?
J

 

[snowhead]

I don't quite get where he's saying it should be positioned in relation to it's effectiveness ? My CU has a 30mA non delayed RCD as a main switch will it trip out if the N drops out ? it's the same as his diagram isn't it ?
J

This is looking at loosing the incoming neutral, before the Neutral / Earth connection, the normal C.U RCD can't detect that.
Current flow thro' the C.U rcd won't be any different.
The Green / Yellow ("Earth") now becomes a return waiting for someone to complete the path.

 

[Edd]

Doesn't the white wire on the RCD help with this?

The old MEM RCD did have a functional earth but that was years back. the only time Ive seen the functional earth now is on RCBO's. With any normal front end RCD if the neutral is lost before its split then the RCD wont become unballenced.
Supply comes down the live through all your equipment, back up the neutral/earth and as theres no neutral back to the substation then its only return path is your bonding conductors, or what ever the lowest resistance it can find to earth!

 

[dean.diy]

Afternoon Snowhead , interesting thread you introduced. One of the contributors mentions this "pre" rcd as unsuitable for UK'S tncs system.This must be due to DNO's
not wanting sparks fitting an rcd to the(concentric) supply cable.

 

[Jimmy Boy]

Found an interesting PDF on this now I am thinking if 10 houses are on the same phase as me and the N drops off will I get all their fault load as well because my nice shiny new earth rod will provide a lower path of resistance than all their bonded extraneous water and gas ?
J
<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.

View attachment Loss of Neutral.pdf

 

[DPG]

It happened to my house years back.

Happened to mine last year. Broken neutral in the joint box under pavement at end of drive. Guy who fixed it reckons there's loads of them happening. Daz

 

[Knobhead]

I am thinking if 10 houses are on the same phase as me and the N drops off will I get all their fault load as well because my nice shiny new earth rod will provide a lower path of resistance than all their bonded extraneous water and gas ?

This is the problem I had years back. I had 40A+ going down to the rod.
Will have a read of the PDF later
Thanks

 

[ackbarthestar]

Supply neutrals do fail and either go high resistance or open circuit.

I can't understand why everybody says its a rare occurrence since everybody I know says they have had PME problems. So its not as rare as some people make out.

Farms are your best bet for fireworks. I had one a few years ago where all the showers went live, the lights all blew and we had glowing 'stars' in the corrugated metal shed where the current was using the metalwork of the shed as a return earth path.

Now the IET, in one of their periodicals recommend that you should stake the pme supply at the MET. There is one small problem with this and that is everybody and his dog down stream of the fault will use your earth stake as part of their neutral return which could result in quite a few amps flowing through your green and yellow wires.
However, its wise to make sure your MPB is of a suitable size for the neutral and connected to the incoming services

 

[Jimmy Boy]

And if you have plastic gas/water coming in it gets worse.
J

 

[SPARTYKUS]

But as we all know, the laws of physics changed when coincidentally it became a tad expensive using TNS, so it'll be fine to use the N as an earth path.

 

[ackbarthestar]

And if you have plastic gas/water coming in it gets worse.
J

As long as your final circuits do not have a parallel earth paths via extraneous parts then MPB is not required. Gas pipes will require MPB if they exist in the property!

 

[Knobhead]

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.

 

[Engineer54]

You are far more likely (if ever) to come across a high resistance supply neutral fault than a completely broken neutral, and that's a fact like it or not... You can check all the IEC and ITE and anyone else's reports/studies you like. They will all say the same thing, ''the incidence/occurrence of neutral breaks in low voltage supply distribution systems is so low, as not to even deserve consideration''!!! That's not to say that you should not protect against such an occurrence....

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!!

Can't see why the combined over voltage/RCD devices that's been mentioned in this thread can't also be utilised in the UK either, that sounds like a reasonably solid idea, one that should be actively promoted.....

 

[ackbarthestar]

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

 

[SPARTYKUS]

Bring back TNS

 

[johnboy6083]

i cant see how it would be a problem if all of the neutrakl curent was trying to go down one electrodwe. the laws of a certain mr ohm would take care of that. Aint you supposed to size the cable to the rod by best case resistance anyway? So in tother words, itys very unlikely that you would have a sub 10 ohms electrode. so the current that would flow would be 2.3 ohms. a 10mm cable for example would take that indefinately.
thy problem comeswhen people have undersized the cable to the rod, and you get a low current, long duration fault.

 

[Chr!s]

i cant see how it would be a problem if all of the neutrakl curent was trying to go down one electrodwe. the laws of a certain mr ohm would take care of that. Aint you supposed to size the cable to the rod by best case resistance anyway? So in tother words, itys very unlikely that you would have a sub 10 ohms electrode. so the current that would flow would be 2.3 ohms. a 10mm cable for example would take that indefinately.
thy problem comeswhen people have undersized the cable to the rod, and you get a low current, long duration fault.

You on the :54:


Its the touch voltage you need to consider.

 

[Engineer54]

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]

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]

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]

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]

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]

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]

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]

<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.





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]

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]

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]

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]

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]

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.