RCD Question!

[r10ley]

Hi new to the forum so Hi again. Quick background of myself, I am a time served electrican but still new only 22 I have the 17th and 2391 however my background is industrial hence havent done much with houses in the past anyway....

Im looking to put a 2 way garage RCD consumer unit into my garage (obviously). The issue I have is that I currently have a new style split load dual RCD cosumer unit in my home. I am led to believe that having two RCD's in series in a single circuit is a no no.

I intend to run the supply from my garage from one of the two RCDs in my split load board however these MCBs are obviously RCD protected and the problem then comes by the fact the garage cosumer unit also is RCD protected. They are both 30mA rated.

I see no way round this as an unprotected MCB cannot be put into my new consumer unit in my home unless I go to stupid lengths regarding earthed conduit. The only other solution I can see is by buying a garage consumer unit without a RCD in however I dont seem to beable to find one on the net.

Can anybody help

Thanks

r10ley

 

[nickblake]

go to your local electrical supplier and get a main switch and replace the RCD one in the garage distboard with that , although when i do garage supplies i do like garages to be on their own RCBO so as not to cause nesance tripping

 

[DjSparkos]

Garage C/U units typically have a higher ip rating. In my opinion you'll be fine putting in a 2 way C/U with main switch for the garage, with the RCD protection coming from your Dual RCD 17th edition board.

Just consider where you situate the new Sub board due to the lower IP rating of the enclosure.

 

[r10ley]

Thanks for your input ill probably pop in a RCBO for the formentioned reason.

Thanks

r10ley

 

[r10ley]

Hi just another quick question. Regarding two RCDs in series being a no no what about having sockets supplied off a RCD via a split load board and then putting an external socket outside with a RCD incorporated?

Thanks

r10ley

 

[nickblake]

Put in an IP rated socket without RCD, IE MK masterseal easiest option not cheap but good

 

[Addlight]

This is good timing. Honestly, I didn't know about two RCDs in same circuit being a 'no-no'. Is there a reg somewhere about that - I must have missed that class. ...

Coincidentally I found this thread when trawling through looking for an answer to my question on testing a submain off a dual RCD DB.

The submain supplies the bathroom - shower and lights. It's a Starbreaker and was fitted for protection before the consumer unit was upgraded to 17th Edition.

I was running my RCD tests on the two circuits in the bathroom - at shower and light point - and noticed that both the starbreaker and house RCDs tripped on x1 and x5 - but on 180 degrees at x1 only the submain RCD tripped - I repeated (up and downstairs a lot for the exercise) and got same result.

The main DB has an 80A 30mA RCD, the submain has a 63A 30mA RCD.

So two questions (now) ....

(1) why not two RCDs in same circuit (isn;'t it double the protection) ?

and

(2) why would both RCDs trip on testing - shouldn't only the submain (63A) RCD trip?

 

[Sintra]

Reg 531.2.9 discrimination between RCD's in series.

 

[kung]

Is the sub swa through out its length ? if so no rcd protection from house main is needed but if its not the sub will need rcd protection !

 

[kung]

If its detached from house i mean and under ground !

 

[kung]

Let me word that better ! is the garage detached from the house and is the sub going to run under ground ? if so my reply is ok and if so have you considered removing meter tails from house cu and henley them then connecting house cu and a non rcd cu to terminate the swa into then fit your rcd garage cu ? ! if not you didnt see me here !

 

[Addlight]

Thanks Sintra! just read and re-read 531.2.9 - not exactly lucid, is it?

I assumed discrimiation meant the RCD rating - this one has 80A then 63A.

I can't see how it discrimination here can mean residual current - otherwise I'd need to put a 100mA in house RCD!

I'd do some more reading ....

 

[Sintra]

Thanks Sintra! just read and re-read 531.2.9 - not exactly lucid, is it?

I assumed discrimiation meant the RCD rating - this one has 80A then 63A.

I can't see how it discrimination here can mean residual current - otherwise I'd need to put a 100mA in house RCD!

I'd do some more reading ....

The 80A & 63A are overcurrent ratings nothing to do with the residual current rating of 30mA. The discrimination is to do with the residual current ie if you have a fault on RCD 1 it should not cause tripping in another RCD. As for using a 100mA RCD these do not fulfil the requirements of additional protection and are generally only used in a TT sub main etc and are also time delayed to comply with 531.2.9.

 

[WayneL]

The 80A & 63A are overcurrent ratings nothing to do with the residual current rating of 30mA. The discrimination is to do with the residual current ie if you have a fault on RCD 1 it should not cause tripping in another RCD. As for using a 100mA RCD these do not fulfil the requirements of additional protection and are generally only used in a TT sub main etc and are also time delayed to comply with 531.2.9.

Didn't think RCDs offered overcurrent protection.

 

[Sintra]

Didn't think RCDs offered overcurrent protection.

Thanks Wayne didn't word that one correctly so here goes again.

The 80A & the 63A refer to the maximum current that can loaded onto the RCD.