Replacing a VOELCD ?

[Nemo2000]

The electrical installation in my house was re-wired about 20-odd years ago and appears still to be in good order. The energy supply company replaced the old black meter by a digital 'white box' some years ago, but that has been the only change.

All circuits are supplied through an 8-way Wylex consumer unit with re-wirable fuses. There is no RCD in the CU. However there is a separate, 60A black trip switch (known as a VOELCD I believe?) in the tails between the meter and the CU. From what I have been able to gather about the history of the installation, this had been inserted into the previous installation at least 30 years ago, then retained when that installation was replaced some year later.

There is an earth wire from the VOELCD to an earth rod under the floor, another earth wire from the VOELCD to the earth rail in the CU, and a third earth wire from the CU to the earth rod. There is NO earth connection to the incoming undergound supply cable.

I know that the VOELCD does work - it tripped last year when there was a short in an electric kettle - but I wonder if it is necessary/prudent to replace it now by a modern RCD? If so, to avoid more extensive disruption to the CU etc, can this be done simply by replacing it with a 'stand-alone' RCD (plus any necessary adjustment to the earth wiring). I was thinking of something on the lines of the 63A RCD-only Wylex Shower Unit, which is about the same physical size and layout as the VOELCD and would fit the space easily.

Your advice please?

 

[bernardgreen]

It would be highly advisable to replace the voltage operated circuit breaker with a modern RCD

The VOELCB detects when voltage on the "earth" wire ( the CPC circuit protective conductor ) inside the house raises to a voltage of 50 volts above true ground potential.

If there is NO connection between the CPC and true ground then all earth fault current flows from CPC to the VOELCB and the voltage rises to 50 volts with a small fault current, That is good and how they were designed to work before the fault current became dangerously high

BUT in most modern houses there are many routes for connections from CPC direct to ground and these route take some of the fault current away from the VOELCB so the fault current has to be higher, in some cases very much higher, before there is enough current to raise the CPC voltage high enough to operate the VOELCB

 

[ban-all-sheds]

The electrical installation in my house was re-wired about 20-odd years ago and appears still to be in good order.

Appearances can be deceptive, as I think you realise.

There is NO earth connection to the incoming undergound supply cable.

Contact your DNO and find out if you can get PME.

If so, to avoid more extensive disruption to the CU etc, can this be done simply by replacing it with a 'stand-alone' RCD (plus any necessary adjustment to the earth wiring).

Not really, as it's unlikely that a single device for the entire house would comply with the Wiring Regulations. I know that's what you have now, and I know that there is never an obligation to bring things up do date per se, but any new work done needs to comply with the current regulations. So at the moment, for example, you cannot add any new sockets, or install any new concealed cables.

It's new CU time, really.

 

[Nemo2000]

Contact your DNO and find out if you can get PME.

In what way would that affect changing the VOELCD please?

There is a problem with that suggestion.....

Most of the houses in my road seem to have individual feed cables from the main supply cable in the road. However my house is one of a group of 3 set back from the road some distance, so when they were built there was a feed cable from the road to the first house, then daisy-changed onwards to the second house and finally to mine.

Some years ago the owner of the middle house had an extension built and his electrician wanted to get the supply changed from TT (?) to PME. This involved some work in my house and more in the first house. The owner of the first house refused to have the work done and the owner of the middle house refused to pay for the work needed on the other two houses, so nothing was changed.

I suspect therefore that I would get the same problem, or else have to have an entirely new supply cable fitted from the street?[/quote]

 

[westie101]

Contact your DNO and find out if you can get PME.

No No No, contact the DNO and ask if they can provide an earth terminal! There is no magic about PME and a non PME earth from a DNO is just as good. In this case it may not need the other properties changing to PME either! Just confirmation the earth sheath on the service cable is continuous.

Looking at the OP case it would be as well to confirm the present TT earth is good enough, yes changing the VOELCD to a modern RCD is a good idea.
In many areas these devices were installed by the then electricity boards so it might be worth asking the DNO anyway as it may be their responsibility

 

[Nemo2000]

Thanks for the advice and I will try to follow up the DNO in due course.

What I am NOT clear about tho', so please bear with my ignorance, is WHY - apparently - replacing just the VOELCD with a RCD is not workeable and needs the CU replaced instead? I'm trying to minimise the potential disruption.......

In the meantime, to take this matter on a bit further……..

One of the fuseways in the CU feeds a 20A dedicated linear circuit to the garage and garden shed. [ There are separate CUs in the two out-buildings with re-wireable 15A and 5A fuseways for their power and lighting circuits. ] Whenever I use one of the power sockets for a lawn-mower or power-tool etc, then I always use a plug-in RCD adaptor rather than just relying on the VOELCD. One reason for thinking of replacing the VOELCD by a RCD was to eliminate the need for the adaptors.

Where the circuit cable leaves the house there is a separate 1-gang CU (fused 20A) to act as a ‘master isolator’ switch. Rather than disturb the main VOELCD arrangements at the moment, I was wondering about using the Wylex RCD mentioned above as a replacement for this ‘isolator’ CU. [ I realise this will only safeguard the shed/garage circuits, but these are my main concern at the moment as the garage can get a bit damp! ] Would this be OK, or would the existing CCC/earthing need attention/modification?

 

[JohnW2]

What I am NOT clear about tho', so please bear with my ignorance, is WHY - apparently - replacing just the VOELCD with a RCD is not workeable and needs the CU replaced instead? I'm trying to minimise the potential disruption.......

In electrical terms that would be 'workable'. You would have changed the current situation in which your entire installation was protected by a single VOELCB (which, as Bernard has explained, is far from satisfactory) to one in which your entire installation was protected by a single RCD (much better and 'safer').

However, that would mean that you still had a situation in which a fault anywhere in your installation would cut off the entire electricity supply. Current regulations do not like that, and therefore require circuits in the house to be split between two (or more) RCDs - so that a fault on one circuit does not result in all the electricity in the house being lost.

Kind Regards, John

 

[EFLImpudence]

I thought VOELCBs had to be replaced.

Replacing it with an RCD may not be ideal, for reasons stated, but, as that is all that is being done, the installation will be 'no less safe', indeed safer, than before.

Compulsory replacement of the consumer unit would be unreasonable.

 

[Nemo2000]

However, that would mean that you still had a situation in which a fault anywhere in your installation would cut off the entire electricity supply.

I could live with that

To add a bit more to this (something which I did not mention originally, in order to keep the question ‘simple’) in actual fact the tails from the VOELCD go to a sealed terminal block (which originally had a DNO’s lead seal on it, suggesting that they may have fitted the VOELCD as well), and then there are 2 pairs of tails to 2 separate CUs. The 8-way CU supplies all the main circuits, the smaller 3-way CU supplies dedicated circuits such as the central-heating, freezer socket and alarm system. (I understand the idea was the main 8-way could be turned off when the owners went on holiday, just leaving the ‘essential’ devices still powered.) So, in theory maybe the VOELCD could be replaced by two RCDs , one for each of the CUs? Would that meet the spirit of the regulations, even if by perhaps a rather unorthodox method?

 

[JohnW2]

I thought VOELCBs had to be replaced.

As we know, they certainly ought to be replaced, but I'm not sure where the "had to be" would have come from. Indeed, I think I would probably struggle to think of anything for which there was a mandatory "had to" in relation to an existing installation.

Replacing it with an RCD may not be ideal, for reasons stated, but, as that is all that is being done, the installation will be 'no less safe', indeed safer, than before.

Indeed so.

Compulsory replacement of the consumer unit would be unreasonable.

Indeed - and, as above, I'm not sure that 'compulsory' would actually mean. It (or a complicated equivalent) would obviously be required in order to achieve full compliance with current regs, but there is, of course, no such general requirement in relation to an existing installation.

Kind Regards, John

 

[JohnW2]

However, that would mean that you still had a situation in which a fault anywhere in your installation would cut off the entire electricity supply.

I could live with that

I'm sure you could - after all, it's no worse (merely safer) than your present situation. However, as I said, the current regulations are not necessarily quite so easily pleased! There is, however, no general obligation to bring an existing installation up to the standard of current regs - so, if it were a choice between replacing the VOELCB with a single RCD and 'doing nothing', then it's clear which would be the way to go.

To add a bit more to this (something which I did not mention originally, in order to keep the question ‘simple’) in actual fact the tails from the VOELCD go to a sealed terminal block ... and then there are 2 pairs of tails to 2 separate CUs. The 8-way CU supplies all the main circuits, the smaller 3-way CU supplies dedicated circuits such as the central-heating, freezer socket and alarm system. .... So, in theory maybe the VOELCD could be replaced by two RCDs , one for each of the CUs? Would that meet the spirit of the regulations, even if by perhaps a rather unorthodox method?

That would certainly be moving in the direction of the spirit of the current regs - but to be fully compliant with that spirit, you might have to consider redistributing some of the circuits between those two CUs. I think it fair to say that the spirit of the regs would like to see at least some lights (and, ideally, at least some sockets) still usable if one RCD trips - so maybe one of your lighting circuits could be 'swapped' onto the smaller CU? You really need to discuss all this with your electrician.

Kind Regards, John

 

[ban-all-sheds]

No No No, contact the DNO and ask if they can provide an earth terminal! There is no magic about PME and a non PME earth from a DNO is just as good. In this case it may not need the other properties changing to PME either! Just confirmation the earth sheath on the service cable is continuous.

Sorry - I did not think that new TN-S was provided any more.

 

[ban-all-sheds]

I'm sure you could - after all, it's no worse (merely safer) than your present situation. However, as I said, the current regulations are not necessarily quite so easily pleased! There is, however, no general obligation to bring an existing installation up to the standard of current regs - so, if it were a choice between replacing the VOELCB with a single RCD and 'doing nothing', then it's clear which would be the way to go.

So what kind of electrician is going to install a single front-end RCD and sign a document to say that to the best of his knowledge and belief it complies with BS 7671?

 

[westie101]

So what kind of electrician is going to install a single front-end RCD

From what the Op says this device could well be the DNOs, so what kind of electrician would change it anyway, possibly breaking seals to do so?

Op your first port of call must be the DNO to change it, it should not cost you anything!

Sorry - I did not think that new TN-S was provided any more

Not for new supplies but from existing cabling if appropriate. There are still a lot of old properties where no earth was provided and a simple earth connection to a lead sheath solves.

 

[JohnW2]

... There is, however, no general obligation to bring an existing installation up to the standard of current regs - so, if it were a choice between replacing the VOELCB with a single RCD and 'doing nothing', then it's clear which would be the way to go.

So what kind of electrician is going to install a single front-end RCD and sign a document to say that to the best of his knowledge and belief it complies with BS 7671?

Interesting question. (S)he obviously could not sign a document saying that the installation was compliant with current BS7671, but I'm not sure that they would have to - would they not be declaring that, to the best of their knowledge and belief, the work for which they had been responsible was compliant with BS7671?

The implication of what you're suggesting is that it is never possible for an electrician to, say, replace a defective item in an installation (say the RCD in a single-RCD CU) if the initial installation of that item would not be compliant with current regs. Is that what you believe?

Whatever, I think that, if the householder were not prepared to have any other works done, an electrician who refused to replace a VOELCB with an RCD for procedural/'bureaucratic' reasons would be plain irresponsible, and could end up being morally responsible for a death.

Kind Regards, John