Lost neutral on PME consequences

[Steve T]

Hi all,
I was reading about what can happen if the neutral on a PME system is lost at some point before the cutout and I have a few questions that I'm hoping someone could help me with.
1) Am I correct in thinking that for a properly earthed and bonded system, if the neutral is disconnected before the cutout, the current that would normally return via the neutral would instead return through the earth and potentially through a gas or water pipe that is bonded? Would all earthed metal work in the house also become live and present a shock risk if anyone was to touch it?
2) No mcbs or rcds would trip would they, because as far as they can tell the system is working normally, as the current is flowing through the rcd neutral normally, but then afterwards returns through the earth instead of the neutral? The rcd still wouldn't trip if someone touched an earthed metal object that was actually live would it as the person would just be another path to earth, just like any bonded pipes?
3) What would be the safest way to immediately rectify the situation, if you noticed it? I'm thinking turn off main switch first (obviously) but would it be a good idea to disconnect the main earth as well to prevent any current from neighbouring houses returning via your bonding (and making any earthed metal work live), or just leave it connected and leave the house and don't touch anything metal? Goes without saying that you would also call the emergency DNO number ASAP.
4) What, if anything, is in place to protect against these sorts of dangerous situations occurring? Some of the stuff I've mentioned above would surely depend on where exactly the broken neutral is (ie is it just after your property before any joints or anything or is it at a substation)
Apologies if I've used any incorrect terminology there or made any of my questions confusing, hopefully you'll understand them! I'm not really familiar with how the distribution network operates so I may well have some misconceptions, but would greatly appreciate any answers/explanations to my questions above, I'm just trying to improve my understanding!
Thanks for any replies

 

[Wilko]

1) Am I correct in thinking that for a properly earthed and bonded system, if the neutral is disconnected before the cutout, the current that would normally return via the neutral would instead return through the earth and potentially through a gas or water pipe that is bonded?

Yes, the current would try to return. As an example : If the fridge was idle and this fault occurred, no problem if you touch it (maybe, I won’t try it ). But when you grab the handle and open the door the light gets connected to L and current will flow through the filament and whatever path is available to Earth. That could be via bonding and/or your-good-self...

 

[pc1966]

In a properly installed system where the gas/water/etc extraneous paths have been bonded to the supply earth then in the event of an open PME neutral you are safe from shock inside the building, as you won't see any voltage difference.

However, any metalwork attached to the system 'earth' is now part of the path the neutral current will attempt to follow and depending on the current and impedance that could be anywhere from tens of volts to more or less the full 230V supply. So if you had, say, a metal fence with an earthed floodlamp on it, or the more obvious problems of an electric car being charged, you might get a possibly fatal shock by touching the metalwork while in contact with the true Earth outside.

Normal RCD will not protect you as they look at the difference between L & N to detect lekage, and do not monitor the E current as such.

Car charging (EV) or caravans are seen as a particular risk because you might be wet it at the time (rain, washing car, etc), so there are various regulations on means of making that situation safe. Some use a separate earth rod and created a TT supply so they don't use the PME "earth" (standard practice for caravan parks for years), whereas some EV charges use TT, and others have variations on a RCD and voltage-monitoring to detect an open PME and disconnect both the live and earth conductors.

In terms of your home safety, there is not much you could do except switch off the supply. If it is only your home that has lost the PME neutral then that de-energises it, but it won't make much difference if the fault is further away and you are seeing the imbalance neutral from multiple homes on a segment of the distribution network.

What it would help with though is if the fault causes an over or under voltage that could damage your home's electrics, though in most cases by time you know about the fault your home has been subject to it for a very long time in electronic terms!

 

[Steve T]

In a properly installed system where the gas/water/etc extraneous paths have been bonded to the supply earth then in the event of an open PME neutral you are safe from shock inside the building, as you won't see any voltage difference.

However, any metalwork attached to the system 'earth' is now part of the path the neutral current will attempt to follow and depending on the current and impedance that could be anywhere from tens of volts to more or less the full 230V supply. So if you had, say, a metal fence with an earthed floodlamp on it, or the more obvious problems of an electric car being charged, you might get a possibly fatal shock by touching the metalwork while in contact with the true Earth outside.

Normal RCD will not protect you as they look at the difference between L & N to detect lekage, and do not monitor the E current as such.

Car charging (EV) or caravans are seen as a particular risk because you might be wet it at the time (rain, washing car, etc), so there are various regulations on means of making that situation safe. Some use a separate earth rod and created a TT supply so they don't use the PME "earth" (standard practice for caravan parks for years), whereas some EV charges use TT, and others have variations on a RCD and voltage-monitoring to detect an open PME and disconnect both the live and earth conductors.

In terms of your home safety, there is not much you could do except switch off the supply. If it is only your home that has lost the PME neutral then that de-energises it, but it won't make much difference if the fault is further away and you are seeing the imbalance neutral from multiple homes on a segment of the distribution network.

What it would help with though is if the fault causes an over or under voltage that could damage your home's electrics, though in most cases by time you know about the fault your home has been subject to it for a very long time in electronic terms!

Thanks, that makes sense to me I think apart from one point, so as you say if you are in contact with true earth (outside) and live metal work (due to neutral fault) you could get a fatal shock. I understand that, but would that not also be the case inside if the floor happened to be more conductive than usual such as a flagstone floor? Or even if the floor was not particularly conductive would you not at least get a slight tingle?

 

[pc1966]

Thanks, that makes sense to me I think apart from one point, so as you say if you are in contact with true earth (outside) and live metal work (due to neutral fault) you could get a fatal shock. I understand that, but would that not also be the case inside if the floor happened to be more conductive than usual such as a flagstone floor? Or even if the floor was not particularly conductive would you not at least get a slight tingle?

If you happened to have a very conductive floor inside then yes, it is just possible. But such floors tend to be rather cold hence the common practice of a suspended wooden floor above the underlying concrete/soil, or having carpeting down, and usually are not wet, so that risk is I guess pretty small.

I guess a shower on ground floor might just come in to that sort of a category and probably would not be designed to be OK, where as swimming pools are recognised risks for this so would usually have an earthing mat below, and/or the rebar in the concrete bonded to keep the potential down if a PME supply is considered likely.

 

[marcuswareham]

If you happened to have a very conductive floor inside then yes, it is just possible. But such floors tend to be rather cold hence the common practice of a suspended wooden floor above the underlying concrete/soil, or having carpeting down, and usually are not wet, so that risk is I guess pretty small.

I guess a shower on ground floor might just come in to that sort of a category and probably would not be designed to be OK, where as swimming pools are recognised risks for this so would usually have an earthing mat below, and/or the rebar in the concrete bonded to keep the potential down if a PME supply is considered likely.

I would say this type of cold floor is very common practice in kitchens and conservatories and the like, out of the 7 places iv lived only 2 of them have not had a cold probably more conductive floor in parts. Although the hope is the DPC will reduce this conductivity to the soil, if there even is one with older houses, My partner's dads house you can lift the terracotta tiles in the kitchen and you are presented with some sand and then soil under that, rubbish hey! at least his supply is TT ?

I am also sceptical as to how well insulated you are on timber floor boards etc, as most people who would say there is no risk of shock because you are insulated from true earth on a wooden floor wouldn't volunteer to touch a live conductor whilst standing on one !!, and I have heard of plenty of people receiving shocks upstairs in a house on timber floorboards and joists.

 

[marcuswareham]

Hi all,
I was reading about what can happen if the neutral on a PME system is lost at some point before the cutout and I have a few questions that I'm hoping someone could help me with.
1) Am I correct in thinking that for a properly earthed and bonded system, if the neutral is disconnected before the cutout, the current that would normally return via the neutral would instead return through the earth and potentially through a gas or water pipe that is bonded? Would all earthed metal work in the house also become live and present a shock risk if anyone was to touch it?
2) No mcbs or rcds would trip would they, because as far as they can tell the system is working normally, as the current is flowing through the rcd neutral normally, but then afterwards returns through the earth instead of the neutral? The rcd still wouldn't trip if someone touched an earthed metal object that was actually live would it as the person would just be another path to earth, just like any bonded pipes?
3) What would be the safest way to immediately rectify the situation, if you noticed it? I'm thinking turn off main switch first (obviously) but would it be a good idea to disconnect the main earth as well to prevent any current from neighbouring houses returning via your bonding (and making any earthed metal work live), or just leave it connected and leave the house and don't touch anything metal? Goes without saying that you would also call the emergency DNO number ASAP.
4) What, if anything, is in place to protect against these sorts of dangerous situations occurring? Some of the stuff I've mentioned above would surely depend on where exactly the broken neutral is (ie is it just after your property before any joints or anything or is it at a substation)
Apologies if I've used any incorrect terminology there or made any of my questions confusing, hopefully you'll understand them! I'm not really familiar with how the distribution network operates so I may well have some misconceptions, but would greatly appreciate any answers/explanations to my questions above, I'm just trying to improve my understanding!
Thanks for any replies

Question 3 is an interesting one, as others have said all depends on where the fault is, perhaps we should have 3 pole isolators on TNCS supplies with L,N and PE being disconnected?

Question 4: The Protective Multiple Earthing (PME) is what is in place on a TN-C-S supply to mitigate the effects of a lost neutral from the transformer i.e. having many electrodes along the length of the PEN conductor. Some say this is rock solid and PEN faults never happen, I don't know the numbers for how many PEN faults happen per year for example although I would imagine a lot of near misses would never get reported and then not be recorded if there even is a record kept, To me it seems that PME is not a good enough mitigation measure otherwise we wouldn't be having all the hu-har about EV charging

Perhaps EVs have brought the risks to the forefront of peoples mind, although the outside tap risk has existed for years, metal tap and pipework installed by a Plummer or DIYer (without any thought to anything electrical (i.e no plastic section)) the pipe work is bonded to the PME MET and you often touch this tap while standing on wet true earth, ouch if there an open PEN!

There are many products to mitigate the PEN fault risk for EV chargers, in my eyes most are only 50% effective (as iv mentioned in other posts) but they are better than nothing if all you can use is a PME earth

 

[pc1966]

I am also sceptical as to how well insulated you are on timber floor boards etc, as most people who would say there is no risk of shock because you are insulated from true earth on a wooden floor wouldn't volunteer to touch a live conductor whilst standing on one !!, and I have heard of plenty of people receiving shocks upstairs in a house on timber floorboards and joists.

Certianly you can't guarentee you would feel nothing, but to get a significant shock the threshold is below around 20kOhm and I can't imagine seeing that low a resistance on most floors unless something is really rather damp.

 

[newfutile]

I think most perceived shocks from touching a live part (assuming shoes and clothes are worn) in not by direct conductance via a resistive path , but via capacitance presumably between flat plates of feet and the earth itself.

 

[pc1966]

Question 3 is an interesting one, as others have said all depends on where the fault is, perhaps we should have 3 pole isolators on TNCS supplies with L,N and PE being disconnected?

My concern with that is the implications if someone has other sources of power, such a PV panels or even a UPS for computer equipment. Some sort of earth rod after any isolation would be in order, I suspect.

 

[newfutile]

solar inverters shut down after loss of supply as they are synced to the 50HZ supply waveform.

 

[pc1966]

solar inverters shut down after loss of supply as they are synced to the 50HZ supply waveform.

After a short while...

 

[timhoward]

Interesting questions!
I've been in both situations (both years ago)- surprised to not get a jolt standing on soil when it had just rained after I messed up isolating an outside light, and surprised to get a jolt on a wooden step ladder on a dry concrete floor. So I wouldn't suggest banking on anything!
OP - Regarding Q3 - Other than switching off the isolator I wouldn't touch the installation at all. It's likely you would come into contact with return current path attempting to make things safer, and the current might just might prefer you to the bonding as a way to real earth. I could certainly construct arguments why it's unlikely but ultimately its not worth the risk.

 

[Steve T]

OP - Regarding Q3 - Other than switching off the isolator I wouldn't touch the installation at all. It's likely you would come into contact with return current path attempting to make things safer, and the current might just might prefer you to the bonding as a way to real earth. I could certainly construct arguments why it's unlikely but ultimately its not worth the risk.

I understand why you're saying that, I suppose the DNO response would be quick enough to not have to worry about doing anything other than switching the main switch off. I wonder though if some sort of switch for the main earth (interconnected with the normal main switch so you could not accidentally turn off the earth switch only) would be beneficial to pme installations. I think someone else mentioned the idea as well earlier in the thread.

 

[pc1966]

While you can easily get mulit-pole switches, it would require one pole to make first and break last for the earth. They are available (typically for the neutral in a 3-phase system) but less common.