Lost neutral on PME consequences

[Steve T]

I've just remembered something that happened quite a few years ago, I was at a mates house when all the lights (halogen and incandescent) went dim suddenly, no flickering or anything though. We checked the voltage at a socket (was a plastic socket so not an immediate shock risk) and it read a steady 120v according to his multimeter. We turned the main switch off and rang the DNO after checking with some neighbours to see if the same thing had happened, just to rule out a fault in his wiring. The strange thing was it only affected every 3rd property in the row of houses, so made me think it could have been a problem with only 1 of the 3 phases. So I thought at the time one of the transformers at the substation had failed, but looking back could it actually have been a broken neutral??

 

[pc1966]

If it only affected 1 in 3 it sounds more like a phase fault than an open neutral, as the PME fault case you still see the 400V phase-phase so if your 230 has gone down to 120, then the others have gone up quite a bit to match (depending on the balance between the phases).

 

[Steve T]

If it only affected 1 in 3 it sounds more like a phase fault than an open neutral, as the PME fault case you still see the 400V phase-phase so if your 230 has gone down to 120, then the others have gone up quite a bit to match (depending on the balance between the phases).

Makes sense, certainly his (apparently unaffected) neighbours didn't report any broken appliances or much brighter lights or anything like that.

 

[marcuswareham]

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.

And the main switch / isolator is now probably in a bonded metal enclosure so you get a shock when opening the flap, doh

I am keeping my plastic consumer unit as long as i can

 

[marcuswareham]

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.

I did think capacitance, but I dont know enough about how that kind of shock occures, so didn't want to sound silly ?

 

[LastManOnline]

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 ary concrete floor.

While we all understandably conclude that standing "outside" is very dangerous and standing "inside" is very safe, it is, nt always, as clearcut as that. "outside" earth will be very dry and very unconductive if you have a few weeks of good dry summer days. Add on to that someone wearing good shoes and sockets (not after a hard day's work and perspiration) and you will certainly have a high resistance path. On the "inside" as mentioned by other posters, if you have an older home with a concrete floor with no damp coarse you will certainly feel a tingle and perhaps something more. I recently "felt" 27 volts due to a faulty shower while wearing shoes and socks and my feeling afterwards was "Well I can understand the homeowners panicky phonecall now" as he received the same shock with water flowing down around his feet.
I must also say that regardless of any situation I encountered there was never any mystery to the shock felt. It could all be explained by Ohms law.

 

[LastManOnline]

4) What, if anything, is in place to protect against these sorts of dangerous situations occurring?

I, ve just read the detail regarding that very, very sad case of the 7 year old electrocuted in a pub garden due to a seriously underspec electrical intallation.
Its sobering to consider that the exact same problem could occur with the same fatal consequences in a PME installation considered up to spec.

 

[marcuswareham]

I, ve just read the detail regarding that very, very sad case of the 7 year old electrocuted in a pub garden due to a seriously underspec electrical intallation.
Its sobering to consider that the exact same problem could occur with the same fatal consequences in a PME installation considered up to spec.

Quite! although rare, it seems strange too me so many people accept that risk !

 

[LastManOnline]

Quite! although rare, it seems strange too me so many people accept that risk !

Yes.I think though the tide of opinion is gradually changing towards "accepting that risk" within our industry.

 

[timhoward]

I, ve just read the detail regarding that very, very sad case of the 7 year old electrocuted in a pub garden due to a seriously underspec electrical intallation.
Its sobering to consider that the exact same problem could occur with the same fatal consequences in a PME installation considered up to spec.

Very tragic case. Unless I'm missing something I guess you mean with broken upstream Neutral as per this thread or exposure to live parts (as was case in the water filled light you referred to).

Also sobering, this isn't exactly a new debate - I spotted the following discussion elsewhere several years ago which is rather poignant:

 

[marcuswareham]

Very tragic case. Unless I'm missing something I guess you mean with broken upstream Neutral as per this thread or exposure to live parts (as was case in the water filled light you referred to).

Also sobering, this isn't exactly a new debate - I spotted the following discussion elsewhere several years ago which is rather poignant:
View attachment 65376

Exsactly whey my outside lights are SELV or double insulated, they are in easy reach and one of them is likely to be touched

 

[LastManOnline]

Also sobering, this isn't exactly a new debate -

Correct. It's an old debate. However, far from being yesterday's news it has the potential to become very current with one single high profile incident. The example below (outside light) would probably have constituted the vast bulk of "at risk" metalwork under open PEN , 25 years ago.
Today the level of "at risk" exposed metal parts outside the home has changed enormously (electric gates, heat pumps to mention but a few). Add to that the DSO, s approach of employing largely TNC-S for new installs plus converting existing TNS cables to TNC-S when repairs are required.The electrical landscape is starting to look very different and with that change the potential for open PEN issues can only increase.

View attachment 65376

 

[marcuswareham]

Correct. It's an old debate. However, far from being yesterday's news it has the potential to become very current with one single high profile incident. The example below (outside light) would probably have constituted the vast bulk of "at risk" metalwork under open PEN , 25 years ago.
Today the level of "at risk" exposed metal parts outside the home has changed enormously (electric gates, heat pumps to mention but a few). Add to that the DSO, s approach of employing largely TNC-S for new installs plus converting existing TNS cables to TNC-S when repairs are required.The electrical landscape is starting to look very different and with that change the potential for open PEN issues can only increase.

The conversion of old TNS to TNCS in the street and the head staying the same inside the property I feel is a concern as a spark could install something (hot tub, heat pump, etc etc) outside believing the loss of PEN conductor risk is not an issue due to the fact it is TNS, him being unaware it is really now TNCS

 

[LastManOnline]

The conversion of old TNS to TNCS in the street and the head staying the same inside the property I feel is a concern as a spark could install something (hot tub, heat pump, etc etc) outside believing the loss of PEN conductor risk is not an issue due to the fact it is TNS, him being unaware it is really now TNCS

I suspect that an experienced spark will likely test the supply and be pretty sure what he is dealing (or perhaps ring the DSO) but, yea to make a major change in the supply system by converting a TNS to a TNC-S and not leaving a very clearly marked label or"record " of it behind for the homeowner (and future electrician) is I feel very, very poor practice.

 

[newfutile]

how can you test the supply to find out earthing type or maybe we just assume a ZE less than 0.35 is most likely some variation of TNC-S.