V O E L C B and C O E L C B

[JohnW2]

I see what you mean, it could be an issue when you come to exit the eq. zone or use a class 1 appliance outdoors or touch the outside tap.

Indeed, but those are situations which also become very dangerous with a TN-C-S system which develops a neutral fault.

I still think that VOELCBs were next-to-useless, particularly in the presence of parallel paths - since, as I said, the one thing they can protect against is equally protected against by adequate bonding (with the 'outdoors' exceptions you mention).

Kind Regards, John.

 

[Spark123]

Yep, however they will be a lot more apparent on a TN-CS when the installation ceases to function properly. If it occurs on the network then hopefully there will be enough PME points to tie it down reducing the fault scenarios that could cause mains to become apparent on ECPs.

It is basically a game of maths, there is a greater frequency of things going wrong in an installation than CNE conductors going open circuit I hope!!

 

[JohnW2]

Yep, however they will be a lot more apparent on a TN-CS when the installation ceases to function properly. If it occurs on the network then hopefully there will be enough PME points to tie it down reducing the fault scenarios that could cause mains to become apparent on ECPs. It is basically a game of maths, there is a greater frequency of things going wrong in an installation than CNE conductors going open circuit I hope!!

I'd be interested to hear what the DNO folks have to say, but the impression I get is that the PME points would often not hold the voltage downstream of a CNE break down to a very low level - and,of course, in the worst case scenario, the CNE fault occurs downstream of (m)any PME points.

Ironically (in the context of this thread), if people are concerned about this, it is one situation in which resurrection of VOELCBs could serve a purpose. Such a device with its coil connected between a TN-C-S 'earth' and an earth electrode could presumably be used to disconnect the installation from the supply in the (I agree probably very unlikely) event of a CNE fault resulting in a substantial rise in potential of a DNO-supploed 'earth'.

Kind Regards, John.

 

[westie101]

Someone call.

From a practical viewpoint, the only cases of significant voltages being present on CPCs is following the loss of neutral/earth on a single phase service.
(one of the reasons for all of the bonding requirements)
I can't recall a case of it occurring following the loss of the neutral/earth on a main.

 

[Spark123]

Ironically (in the context of this thread), if people are concerned about this, it is one situation in which resurrection of VOELCBs could serve a purpose. Such a device with its coil connected between a TN-C-S 'earth' and an earth electrode could presumably be used to disconnect the installation from the supply in the (I agree probably very unlikely) event of a CNE fault resulting in a substantial rise in potential of a DNO-supploed 'earth'.

To me it would have a limited effect, given that the ECPs would remain connected to the CNE conductor.
If you wanted to go to the trouble of installing an electrode you may as well opt for a TT system.

 

[Paul_C]

Sure, but did they not have ('main') bonding in those days?

Certainly there was.

Provided one had an equipotential zone, the one thing these devices were designed to protect against was something which shouldn't happen, or be a risk to human beings - there's no risk (to human beings) in the internal earthing system of an installation (i.e. CPCs and exposed-conductive parts) rising to a 'dangerously high potential' relative to true earth if true earth potential does not exist within the building.

That's not peculiar to a voltage-operated ELCB-protected installation. Ask yourself why, after connecting all the earth terminals from sockets, metal light fittings and so on together, and connecting all of those to the pipework, we then still connect everything in the house to a supplier's provided earth terminal, be it the cable sheath with TN-S, or the neutral with TN-C-S.

 

[JohnW2]

Someone call. .... From a practical viewpoint, the only cases of significant voltages being present on CPCs is following the loss of neutral/earth on a single phase service.

That's the very scenario that some people 'fear', which we are talking about. How rare (or common) is it?

Kind Regards, John.

 

[westie101]

The last I can remember locally was over 2 years ago.

In practice as the N/E conductor in a service cable is copper and the L aluminium usually for most faults the aluminium fails first

 

[JohnW2]

That's not peculiar to a voltage-operated ELCB-protected installation.

Exactly my point. The risk of having dangerous pds between different pieces of touchable metal within a premises is essentially eliminated by having adequate bonding. A VOELCB therfore is not necessary to minimise that risk, and I'm not clear as to what else useful it would/could achieve.

Ask yourself why, after connecting all the earth terminals from sockets, metal light fittings and so on together, and connecting all of those to the pipework, we then still connect everything in the house to a supplier's provided earth terminal, be it the cable sheath with TN-S, or the neutral with TN-C-S.

I thought we were talking about TT (the only context in which I've heard/thought of VOELCBs being used). I suppose the main answer to your question (with TN system) is to achieve acceptable disconnection times in response to L-E faults, something one obvioulsy could not hope to achieve with a TT installation. Unless one is contemplating the vanishingly unlikely scenario of someone simultaneously touching and unused TN earth terminal and something else, connection to the TN 'earth' is not required to eliminate dangerous pds between simultaneously touchable bits of metal within a premises.

Kind Regards, John.

Kind Regards, John.

 

[westie101]

To continue from above one thing I cannot remember is there ever being a case of a fatal or serious (requiring hospital in-patient treatment) electric shock in these circumstances.
So yes it could happen but the likelihood if it is very, very low.

 

[Paul_C]

A VOELCB therfore is not necessary to minimise that risk, and I'm not clear as to what else useful it would/could achieve.

{.....}

I thought we were talking about TT (the only context in which I've heard/thought of VOELCBs being used). I suppose the main answer to your question (with TN system) is to achieve acceptable disconnection times in response to L-E faults, something one obvioulsy could not hope to achieve with a TT installation.

I think you've partly answered your own question there John. If you didn't have the ELCB, how else would the supply be disconnected even with a bolted L-E fault on the installation?

 

[westie101]

The biggest issue seems to be the number of VOELCBs in service in, now, inappropriate situations. (we remove them where we find them and can do so)
I have friends down south that have two (one on the on peak and the other on the off peak) along with a TNC-S supply! I think that my next visit will see me investigating them a bit closer!

Though it is fair to say that the wiring is interesting in that property!

 

[JohnW2]

I think you've partly answered your own question there John. If you didn't have the ELCB, how else would the supply be disconnected even with a bolted L-E fault on the installation?

That's obviously true, in the same way that we now use RCD to provide disconnection in TT installations in that situation.

However, I think we've probably been talking at cross-purpoes. A bolted L-E fault does not represent any hazard (of electric shock) to someone within an equipotential zone - although it may be undesirable to leave the fault uncleared for other reasons, such as fire risk. I think you led me down this track when you wrote or VO ELCBs, a few messages back:

It's sole purpose was to prevent the internal earthing system of the installation from rising to a dangerously high potential

..since when I've been thinking/talking about direct risks to people (electric shock); let's face it, as I've been saying, there isn't really any such thing as a 'dangerously high potential' within an equipotential zone.

Kind Regards, John.

 

[ban-all-sheds]

To continue from above one thing I cannot remember is there ever being a case of a fatal or serious (requiring hospital in-patient treatment) electric shock in these circumstances.

What about fires?

 

[ban-all-sheds]

It was also important that the reference electrode be outside the area of possible influence from any earth electrode of a neighboring installation, otherwise earth faults on that neighboring installation could cause it to trip (by way of the bonded pipework on the "F" side of the coil then acting as the true earth reference).

Were they not also prone, or at least known, to trip due to nearby lightning strikes, particularly with overhead TT supplies?