100mA Main RCD Cut Out before 17th Edition Consumer Unit?

[1shortcircuit]

Just a quickie please:thumbsup

If a seventeenth edition consumer unit (with 30ma RCD's) is fitted after an already installed 100mA RCD will there be any discrimination issues?

The reason I ask is because I'm sure I remember a while back when I was doing a job with somebody else that the MAIN RCD unit kept tripping when the RCD's were being tested on RCD x5 test (TT System). This was quite a while back so I'm a little unsure???

I ask because I will be replacing a Consumer Unit that has one of these 100mA RCD main cut outs and it's not a TT system. I'm looking at the safe isolation benefits of this being in place at the moment but just want to confirm that I am not going to be shooting myself in the foot at the same time by leaving it in place.

Obviously I would like to replace this with an Isolation switch but then we go back to the "call the DNO" scenario

I have a picture that I can post up in a short while if required

Regards

1SC

:thumbsup

 

[Deleted member 26818]

A 100mA RCD will trip when testing 30mA RCDs at 5x, because the current used to test is 150mA.
Discrimination will only be provided if the 100mA RCD is time delayed.
If all the circuits, are to be protected by 30mA RCDs in the CU, then there will be no point in having the 100mA RCD.
If you remove the 100mA RCD, and install an isolator, the tails from the isolator to the CU will have to be the sheathed type, and the CU will have to be plastic.

 

[malcolmsanford]

First off is it an actual RCD or is an ELCB, so post that picture and we will tell you.

If it is a 100mA RCD then I'm afraid you will have discrimination problems unless it is a time delayed or S type kind.

If it is a ELCB, there are 2 kinds the Voltage operated and the new current, you normally tell a voltage one as it will have a yellow button on the front and the main earth conductor goes through it. These types are now no longer recognized in the regs and need to be changed. What often happens is that you can remove the earth conductor and leave them in situ to act as a double pole isolator.

If they are Current activated, they can be tested with your MFT as you would with any other RCD, if they activate within time then you can leave them in situ, but the rule of thumb is that you should still change them for a modern RCD.

 

[Knobhead]

Personally I’d leave the 100mA in place for the simple reason you have a ready installed isolator. As has been pointed out if it’s the old type voltage operated ELCB remove the earth through the unit and divert the earth around the unit. They are down right dangerous! Make sure you clearly label it as an isolator, test button inoperative.


From a piece I wrote for another site

I suppose this is where my age and the age of the ELCB (earth leakage circuit breaker) comes in.
In the 60's ELCB's came in two guises, current operated and voltage operated.
The current operated working on core balance became the RCCB (RCD) that we know today.
Voltage operated relied on there being only one earth path from an installation for it to work effectively.
So for example your house would need to have plastic gas and water mains so that there was no parallel earth paths. The only earth path would be via the main earth bonding wire through the ELCB's voltage coil out to an earth spike.
Now think of the problems;
You run a copper pipe to a tap in your garden. You build a metal-framed garage and install power. There were 100's of ways of creating parallel earth paths all of which made the ELCB inoperative.
Another failing was under heavy fault conditions the voltage coils internal resistance caused the installations potential in relation to earth to rise, also under heavy fault the coil could burn out (I'll leave you to figure out what the result is).

 

[1shortcircuit]

Thanks for the informative posts so far, I'm very pleased that I asked this question now :thumbsup

If these photo's are not clear enough then I have now collected the keys to the property so I am able to get a better one if required

Thanks again

 

[1shortcircuit]

Ooops! Sorry, didn't mean to show off my NEW Regs book that I received this morning lol

 

[malcolmsanford]

It looks like an RCD rather than an ELCB, so the only hassles you have is the discrimination part as that will trip as well as the new 30mA if it's not a S type or time delayed type.

 

[1shortcircuit]

It looks like an RCD rather than an ELCB, so the only hassles you have is the discrimination part as that will trip as well as the new 30mA if it's not a S type or time delayed type.

Oh Bollards lol

Oh well, this is what I was concerned about. Kinda suspected it would be too good to be true lol

I guess it's a call to the DNO then to see if they'll fit an isolator so that I can work safely when changing the board over

 

[1shortcircuit]

I took a closer picture today and it would seem for sure that this item will need to be replaced:****:


What a day I've had today lol

 

[ash_s3]

Im sure those main fuse tags have already been cut and poked back through the holes... :jester:

 

[1shortcircuit]

Im sure those main fuse tags have already been cut and poked back through the holes... :jester:

Ha ha, you can't imply such naughty things on an open forum lol

I did have a quick glance and they appear to be the self sealing type lol

 

[gooner85]

What are "seals"