Broken PEN conductor

[bernardgreen]

Looks like there is some ambiguity, surprised no one has come out with a device to mitigate such situations,

Such a "device" is achieved by a TT configuration where the CPC ( Earth) is not connected to the Neutral but is a true Earth connected to Ground.

In some locations such as, among others, petrol stations and wireless communication stations the use of the Neutral as an "Earth" is expressly forbidden and these omstallations must be TT installations. At some sites the potential difference beteen Neutral and Ground is continuously monitored and alarms raised if the difference exceeds safe iimits

 

[John D v2.0]

Looks like there is some ambiguity, surprised no one has come out with a device to mitigate such situations, however rare they are. Maybe a device that monitors L-N-E and trips if E > 50v?

Even if you did trip your own supply, there's still a risk, perhaps even higher because you have removed your loads. If the main earth terminal and your ecps are all live from elsewhere, your supply won't make much difference by switching it. As Bernard says the way round is not to connect the neutral to all your metallic incoming services and class 1 equipment ie TT, but even then you'd still have to hope that any ecps were not importing a potential. If someone else connects the gas supply to 240v, TT won't help you much.

 

[JohnW2]

When the neutral supply is broken it all flows through the bonding and extraneous-c-ps - any extraneous-c-ps within the premises and exposed-c-ps become live waiting to be touched.

"Waiting to be touched", yes, but in your diagram you label them as "shock risk", but that surely should not be the case within the property if all earthing and bonding has been done properly? - there should still be an equipotential zone, the only change being that it has become 'equipotential' at a higher potential than the usual ('fairly near true earth') potential.

In other words, although it would be possible to touch 'live parts' (how 'live' depending on the resistance to ground of the extraneous-c-ps and the number/size of the loads switched on), it should not be possible to find any true earth within the property to touch simultaneously - hence, at least theoretically, no risk of shock. Bernard will probably mention damp walls etc. as a potential path to true earth, and it's obvioulsy true that things like outside taps which were connected to the MET would be particularly dangerous (since they are outside of the equipotential zone).

Of course, the real-world situation can be 'worse' (in terms of MET potential) than your diagram illustrates, if the break in the neutral is in such a place that one's installation's neutral feed is still connected to a number of other consumers, but not to the DNO's neutral. In that situation, the number of 'switched on loads' can be much greater, hence a higher MET potential in all those properties.

Kind Regards, John

 

[ban-all-sheds]

Maybe the forthcoming amendment should be to convert installations to TT when the new rod is installed, not to make that rod another E in PME?

 

[JohnW2]

Maybe the forthcoming amendment should be to convert installations to TT when the new rod is installed, not to make that rod another E in PME?

Despite your toungue being in your cheek, given the very widespread, and increasing, deployment of RCDs, I suppose that is not a totally silly suggestion.

The main think that would be lost would be the redundancy of fault protection which exists when it is (often) provided by both OPDs and RCDs.

Kind Regards, John

 

[JohnW2]

Even if you did trip your own supply, there's still a risk, perhaps even higher because you have removed your loads. If the main earth terminal and your ecps are all live from elsewhere, your supply won't make much difference by switching it.

True. However, as BAS has just reminded us, it looks as if there will soon be a requirement for all TN-C-S installations to have a local earth rod, which will open up the possibility of N-E pd sensing. If there were then a requirement for all installations to have automatic (DNO-supplied?!) isolation in the event of a high N-E potential, then that would largely solve the problem.

However, I don't see it happening - at least, not any time soon!

Kind Regards, John

 

[aesmith]

True. However, as BAS has just reminded us, it looks as if there will soon be a requirement for all TN-C-S installations to have a local earth rod ....

Was that clarified as such? Comments that I saw suggested that it would apply to TNS as well.

 

[ban-all-sheds]

So has it gone open-circuit there?

 

[dobby1]

In normal circumstances a small current is flowing in the bonding conductors and extraneous-conductive-parts (if any) in the inverse proportion of the resistance of the neutral and ground - so not much.
The CPCs are, of course, all dead ends that don't go anywhere.

When the neutral supply is broken it all flows through the bonding and extraneous-c-ps - any extraneous-c-ps within the premises and exposed-c-ps become live waiting to be touched.

The CPCs are only dead though until someone touches them? Sure the resistance may be high but ultimately it is one jumbled parallel circuit. if under non-fault conditions I stood outside, bare foot, in a puddle, with wet hands, and touched a CPC I should at least have some current running through me.

 

[bernardgreen]

The CPCs are only dead though until someone touches them?

NO, if the PEN has failed then the Neutral and hence the CPC will be pulled up to the potential of the Live by any load still connected. That could be a lamp still switched ON ( but not lit ).

I stood outside, bare foot, in a puddle, with wet hands, and touched a CPC I should at least have some current running through me.

Yes you would, quite a lot, if you connected the CPC to a low impedence Ground then possibly enough current would flow from CPC to Ground to light the lamp. If ypu make that connection CPC to Ground through your body it wpuldn't be enough to light the lamp but would be a severe, possibly non recovereable ( lethal ) shock

 

[dobby1]

NO, if the PEN has failed then the Neutral and hence the CPC will be pulled up to the potential of the Live by any load still connected. That could be a lamp still switched ON ( but not lit ).

I am talking under non-fault conditions. Obviously, under PEN fault the CPC will be at live potential.

 

[JohnW2]

Was that clarified as such? Comments that I saw suggested that it would apply to TNS as well.

Assuming that it remains per draft, it will apply to all installations - i.e. TN-S as well as TN-C-S (and TT is obvioulsy already TT!). However, I mentioned only TN-C-S, since that is the context of this thread.

Kind Regards, John

 

[EFLImpudence]

"Waiting to be touched", yes, but in your diagram you label them as "shock risk", but that surely should not be the case within the property if all earthing and bonding has been done properly? - there should still be an equipotential zone, the only change being that it has become 'equipotential' at a higher potential than the usual ('fairly near true earth') potential.

Ok. Lost supply neutrals are not a hazard, then. Why the proposed requirement for earth rod, I wonder?

Of course, the real-world situation can be 'worse' (in terms of MET potential) than your diagram illustrates, if the break in the neutral is in such a place that one's installation's neutral feed is still connected to a number of other consumers, but not to the DNO's neutral. In that situation, the number of 'switched on loads' can be much greater, hence a higher MET potential in all those properties.

For goodness' sake. It's just a simple diagram to explain to Dobby.

Even Appendix 15 doesn't show everything

 

[mwatsonxx]

So with a broken conductor the circuit is still closed?

That's correct.

 

[EFLImpudence]

Oh dear. Trump is on the forum.