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[sunny]

I have come across an installation with an old style 'voltage operated elcb' between the meter and the CU. I am intending to change the old CU for a new one with modern RCDs. I was going to remove the old elcb. Then though if I removed the earth wires to it, it could remain in the role of 'isolating switch'. Can't find anything in the regs to say if this is acceptable or not, any ideas? I am only recently 'part p' so older stuff is a bit of a mystery sometimes. :?

[ericmark]

Its a long time ago but I seem to remember the main problem was if the Earth in and Earth out became connected together the earth leakage part would become disabled and they only worked as a switch. Earth bonding often stopped them working. Which would do as you say but should another electrician come along he will tell house holder you should not be using them any more and it may cause all sorts of confusion

[333rocky333]

An isolator would I think need to be of a bs standard, your old unit may not conform and the contact gap in the off position may not be suitable for present standards.
The short circuit rating of the device may also be too low for modern standards

[333rocky333]

That elcb was before the cu for a reason.
And seems like a TT supply.

it is essential that what you replace it with, gives the equiv or better cover to ALL the circuits of the install.
Unless you have designed the board to do so.
A split load set up will not give sufficient cover

It was most likely a 100 ma time delayed AND would have covered ALL circuits INCLUDING lighting circuits

An isolator is less than £20 and SUITABLE for that purpose,if that was what you want

In your case, I think you may need to replace it with the CURRENT operated version of what you have now.

[ericmark]

Good one rocky I missed that but if sunny is Part P then as he used his meters he would see that hopefully. I was also caught out with a TT supply on coaxial cable I had assumed TN-C-S and 100ma RCD's are not cheap. I got away with it, persuaded the supply authority to supply an earth. It will be interesting to see if sunny confirms it to being TT supply.

[sunny]

Sorry chaps, should have given more info from the start.
Yes it is TT and it has always been my intention to have all circuits in the new board on appropriate RCDs
I'm a bit unsure why Rocky says a split load board is unsuitable, I thought that was the norm for a TT system.
I only thought of using the old ELCB as an isolating switch (and nothing else) to save getting the supply disconnected when I change the board. Of course if I change the ELCB for a switch then I am back into that hassle.
I assumed, possibly wrongly, that if the ELCB was of a sufficient rating to switch the whole installation when it was first installed then it would still be good enough now. After all I am not adding any extra circuits or load. Rocky makes the point that it may not comply with current regs, if it doesn't then must it be replaced? This is another generally confusing area. After all you can note things on a periodic as conforming to an earlier edition but that doesn't necessarily mean those things need changing. You don't have to bring everything in a property up to the current regs just because you've changed a light bulb! So when do you need to upgrade and when don't you?
Interesting stuff, thanks for your responses, I look forward to your thoughts.

[ericmark]

I have not seen anything to say any old system should be removed. Although I have seen something about making it no worse. But as far as the voltage sensing unit is concerned although it would work as an isolator it would have to have the earth wires disabled at very least. This leaves a problem in that you have deliberately disabled a safety device.
There is some debate as to the using two earth leakage trips with no time difference between to two trips. The method which has been used for years is the 30ma trip is feed from the 100ma trip but both were at 40ms the change seems to be the 100ma trip would need at 70ms delay.
Of course if in parallel rather than series then the differential is no longer a factor but the existing consumer units are not built that way. I am not sure if they ever will be built that way as if an isolator is fitted instead of a RCD using two switches to isolate instead of one also presents problems.
Using two RCD’s in series is a problem as most the retail outlets I have looked at don’t give the time, only the current setting. You can get for example Crabtree 310/100 or 310/100TD but as far as what the time is, on time delay types, this does not seem to be published it only says it has a delay.
But there is one get out of jail card. 536.3 manufacturer’s instructions shall be taken into account. So if a consumer unit is designed for two RCD’s in series that is how they should be fitted.
At the end you need some common sense. Will leaving the old RCD in place cause problems latter? Will it confuse some electrician in the future by what he sees? Could some other person look at the shorted or missing earth wires and try to re-instate them so reducing the system earth to an unacceptable level.
In the end you’re the man on site you have to decide and make the executive decision. No real rights and wrongs just a load of gray area’s