Can I use VOELCB as Main isolation switch

[JohnW2]

Thanks for the reply. As you say fitting circuit breakers to the old box adds nothing and they appear to be unavailable anyway.

As you've been told, they wouldn't improve safety, so are really not worth installing (only advantage is that they can easily be 'reset', avoiding the need to 're-wire a fuse') but they are pretty easily available - as per this from TLC ...

So thinking of fitting an upfront 100 amp 100 mA RCB at this time and then maybe swap over the CU later with MCB's and 30 mA RCB in the box and maybe a surge protector as well if I can run to that.

It would make absolutely no sense to fit a 100 mA RCD. If you feel that you must, at least for the time being, put off replacing the CU with a modern one (with all the modern devices in it), in order to avoid the need for notification, then for goodness sake at least put a 30 mA RCD up-front

Kind Regards, John

 

[securespark]

However, 537.2.1 allows that a device used for isolation "... shall be of a type for which the isolation function is explicitly recognized by the
relevant product standard or as identified in Table 537.4". We know that it is not "identified in Table 537.4", but since I haven't got a clue as to which was/is the 'relevant product standard' (do you know?)

I believe it was BS 842.

 

[securespark]

The standard was BS842, the most recent version being from 1965, and the majority of what's in it is based on the original 1939 version.
Nothing in it specifies anything about contact gap or mechanical design of the contacts, and it's not stated that it's even suitable for isolation.

Obviously late to the party again....

I know we all hate the assume word, but I would have assumed the VOELCB would be suitable for isolation given its job.

 

[JohnW2]

Obviously late to the party again.... I know we all hate the assume word, but I would have assumed the VOELCB would be suitable for isolation given its job.

Indeed - and same here - I have always 'assumed' (same caveat!) that they were suitable for isolation.

I think that some people are probably just quibbling about the 'bureacratic' (not really 'legal', since Standards, including BS7671, are not law) question of whether the relevant Standard actually states explicitly that they were suitable for isolation.

Let's face it, these things were introduced as 'protective devices', and if their contacts could not be relied upon to stay open after they had operated, they would not really have been fit for that purpose, would they? !!

Kind Regards, John

 

[Stuart981]

As you've been told, they wouldn't improve safety, so are really not worth installing (only advantage is that they can easily be 'reset', avoiding the need to 're-wire a fuse') but they are pretty easily available - as per this from TLC ...
View attachment 293541

It would make absolutely no sense to fit a 100 mA RCD. If you feel that you must, at least for the time being, put off replacing the CU with a modern one (with all the modern devices in it), in order to avoid the need for notification, then for goodness sake at least put a 30 mA RCD up-front

Kind Regards, John

Thanks will do regards Stuart

 

[Stuart981]

Hello chaps - me again with a follow up question if I may.

So I fitted the 30mA RCD no problem. Switched it on and then switched on the fuse box and the RCD immediately tripped. I pulled out all the fuses and RCB still trips so I am assuming I have some sort of earth/neutral leakage on one of the house circuits. I have removed the RCD and reverted to the VOELCB until I have time investigate the fault.

My question is as follows - I assume that to trip the RCD there has to be a circuit that runs from the transformer on the pole in the road, down the overhead cable to my CU, then along the neutral on one of the house circuits where it finds a fault path across to the earth. It then runs down the earth cable to the spike in the ground and then back through the ground to the earth cable on the pole in the road and then up to the transformer. If that assumption is correct where does the power to trip the RCD come from as I have pulled all the fuses?

Hope that makes some sort of sense - thanks Stuart.

 

[bernardgreen]

The incoming Neutral will not be at the same potential as the Ground surrounding your Ground Rod. This difference is due the voltage drop along the Network Neutral due to current in other houses.

Current flows from the incoming Neutral, ~~ through the RCD sensor,~~ through the Neutral to CPC fault ~~ through the CPC to the Ground Rod

 

[JohnW2]

.... My question is as follows - I assume that to trip the RCD there has to be a circuit that runs from the transformer on the pole in the road, down the overhead cable to my CU, then along the neutral on one of the house circuits where it finds a fault path across to the earth. It then runs down the earth cable to the spike in the ground and then back through the ground to the earth cable on the pole in the road and then up to the transformer.

Yes, that's roughly right - but you are only considering your own installation.

If that assumption is correct where does the power to trip the RCD come from as I have pulled all the fuses?

Bernard has partially explained. Assuming that the transformer on the pole serves other installations in addition to yours, then the main reason why the neutral will be a little above earth potential is that current being drawn by other installations will create s small voltage drop in the neutral supply cable (which is connected to earth at the transformer).

As bernard has explained, in that situation, some current (effectively a small proportion of the neutral current from your neighbours' loads) will flow 'backwards' through the neutral side of the RCD, thence through the N-E fault to earth, and thence back to the neutral side of the transformer.

If yours were the only installation fed by the transformer, then I can't see how you could get a trip.

Does that make sense?

Kind Regards, John

 

[Stuart981]

Yes, that's roughly right - but you are only considering your own installation.

Bernard has partially explained. Assuming that the transformer on the pole serves other installations in addition to yours, then the main reason why the neutral will be a little above earth potential is that current being drawn by other installations will create s small voltage drop in the neutral supply cable (which is connected to earth at the transformer).

As bernard has explained, in that situation, some current (effectively a small proportion of the neutral current from your neighbours' loads) will flow 'backwards' through the neutral side of the RCD, thence through the N-E fault to earth, and thence back to the neutral side of the transformer.

If yours were the only installation fed by the transformer, then I can't see how you could get a trip.

Does that make sense?

Kind Regards, John

Thanks for that John and bernard. That explains things for me perfectly. John there are three other houses connected to the pole by overhead cables and they are all roughly the same distance from the pole at about 100 feet.

There is one odd thing though. The cable from the pole to my house is two strands of cable twisted together for its whole length. It connects to a connector high on the gable wall of the bungalow then the two cables run around the outside of the house for some 50 feet or so clipped to the soffit board and then pass through the attached garage wall and into the main fuse.

The other three houses connected to the pole each have a single thicker cable.

The network operator restrung these four cables about 5 years ago because we were getting regular blackouts which are not so frequent now but still happen from time to time.

Many thanks Stuart

 

[JohnW2]

Thanks for that John and bernard. That explains things for me perfectly.

You're welcome - I'm glad that our explanations were clear enough.

John there are three other houses connected to the pole by overhead cables and they are all roughly the same distance from the pole at about 100 feet.

Thanks for confirming. As I said,had yours been the only installation supplied by the transformer, I don't think it would be possible for you to get an RCD trip when all your fuses were removed. In that situation, I don't think that there could have been any current at all flowing from the transformer (to anywhere, even if there were an N-E fault present within your installation..

There is one odd thing though. The cable from the pole to my house is two strands of cable twisted together for its whole length. It connects to a connector high on the gable wall of the bungalow then the two cables run around the outside of the house for some 50 feet or so clipped to the soffit board and then pass through the attached garage wall and into the main fuse. ... The other three houses connected to the pole each have a single thicker cable.

There obviously must be two conductors (L and N) from pole to each installation. For some reason (don't ask me!) you appear have separate cables (each with one core), whereas the other consumers must have two cores within a single cable. There are a good few different types of cable in use but, if I understand correctly you to be saying that all four (including yours) were replaced at the same time, it would seem odd that they did not use the same type of cable for all of them. Is your cable perhaps appreciably longer or shorter than the other three?

Kind Regards, John

 

[Stuart981]

You're welcome - I'm glad that our explanations were clear enough.

Thanks for confirming. As I said,had yours been the only installation supplied by the transformer, I don't think it would be possible for you to get an RCD trip when all your fuses were removed. In that situation, I don't think that there could have been any current at all flowing from the transformer (to anywhere, even if there were an N-E fault present within your installation..

There obviously must be two conductors (L and N) from pole to each installation. For some reason (don't ask me!) you appear have separate cables (each with one core), whereas the other consumers must have two cores within a single cable. There are a good few different types of cable in use but, if I understand correctly you to be saying that all four (including yours) were replaced at the same time, it would seem odd that they did not use the same type of cable for all of them. Is your cable perhaps appreciably longer or shorter than the other three?

Kind Regards, John

I had a look around the village and it's strange how you do not normally notice the mess that all those overhead cables impose on the environment. Anyway I've noticed that the houses that are furthest from the poles do have dual twisted cables so I guess they need to be tensioned more to prevent the occasional double decker bus becoming an inadvertent route to earth. Maybe double stranded cables allow extra tensioning?

Anyway when it gets a bit warmer and her indoors can agree to having the power off for an hour or so (or several hours in my case) I will try to track down the fault.

Regards Stuart

 

[Stuart981]

Yes me again chaps with yet another question.

As I understand these things the problem with those old VOELCB’s is that most homes have more than one earth and because of that an earth fault can bypass the device and prevent it from tripping.

My question is would that actually happen in the real world. I mean if there was a stonking great short circuit wouldn’t it eventually trip the device even if it did take a bit more oomph and a few more milliseconds?

Thanks for your forbearance.

 

[flameport]

They rely entirely on a voltage appearing between the installation earth and the electrode outside.
That can happen with a building that only has a single earth electrode and is far away from any other items such as metallic pipes and other properties.
In reality, there are many parallel paths to Earth via water pipes, gas pipes and other metallic items such as a metallic backbox in a damp brick wall.
The electrodes for other properties may or may not be far enough away to avoid problems, and even if they are today, they can be moved, replaced or altered in ways which you have no knowledge of.

The second problem is that these are mechanical devices and are all well over 40 years old. Operation cannot be guaranteed under any circumstances.

The third problem is that they only disconnect for faults to the earthed metalwork of an installation.
They do not operate when a person contacts a live part, or where a live part is connected to Earth via some other means such as a damaged cable in damp soil, or a fault to a conductive item which is not directly connected to the installation earth.

They don't work for faults between N&E either, such as that which exists in your installation. Even worse, such faults can prevent the VOELCB from working as the voltage between the Earth outside and the installation earth will be significantly reduced under fault conditions - most of the fault current will flow through that N-E connection and very little via the earth electrode.

VOELCBs were got rid of 40 years ago because they were not fit for purpose.

 

[bernardgreen]

A rough sketch where a single bonded water pipe creates a short across the coil. As Flameport has said there are other items whose bonding will create a short across the coil

 

[Stuart981]

most of the fault current will flow through that N-E connection and very little via the earth electrode.

VOELCBs were got rid of 40 years ago because they were not fit for purpose.

Thank you for that detailed reply. You say most of the fault current will flow through the N-E connection and very little via the earth electrode. My question is would that small amount flowing through the earth electrode build to a level that would eventually trigger the device?