30 mA RCD

sclement · 18 Oct 2004

Hi

I am upgrading our current shower to a high power shower. We are limited on the MCB that we can use as our crabtree consumer unit is fairly old, and I could only manage to get a 40 amp MCB. The unit itself has
a 100 mA RCD.

Should I install an additional 30 mA RCD
inline with the cable from the consumer unit to the DP shower switch, or is the
100 mA one sufficient. If so has anyone got any recommmendations on what to use ?

ryanj · 18 Oct 2004

You could split the metre tails and use a small shower consumer unit. With a 30mA RCD and whatever rating MCB required.

sclement · 18 Oct 2004

Yes I thought of that, but was not too happy fiddling with the meter tails.
Is that allowed anyhow ?

plugwash · 18 Oct 2004

the tails after the meter are yours so yes you can tee them

the trouble is that unless you fancy doing it live (which i don't reccomend) then the only means of isolation is often the service fuse

technically you are not supposed to pull this but getting the REC/DNO out to do it can be a real pain with the result that many sparkys just get on and pull it

the issue of pulling the service fuse has been argued over many times on this forum but the general consensus seems to be it is not reccomended for a diyer to do it

sclement · 18 Oct 2004

Thanks for the info

As I'm a DIYer I will not attempt teeing them. I saw somebody get a
massive shock off three phase once, so I am very cautious doing
this sort of stuff.

To go back to my main point, is it necessary to install a seperate 30 mA RCD in the cable going to the shower if there is already a 100 mA RCD in the consumer unit.

If the answer is yes what exactly should I use ?
Should I use one of the shower consumer units,
like the ones advertised in screwfix, with a 30 mA rcd, between the main
consumer unit and the DP switch for the shower ?

ban-all-sheds · 18 Oct 2004

What is the rating of the new shower? 40A gives you 9.2kW at most.

sclement · 18 Oct 2004

Shower is 9 kW mira sport. Manual suggests using a 40 amp MCB
with 30 mA RCD

sclement · 20 Oct 2004

Ok

so this is what I'm planning to do:

meter tails------100 mA RCD ------ Consumer Unit (40A MCB) ------ 30 mA RCD ------- DP Switch -------- shower.

I'll only be fitting the 30 mA RCD, DP swicth and shower.

Perhaps the 30 mA RCD is overkill, but it recommended in the
Shower installation notes. I was planning to put this RCD in an enclosure
next to the Consumer unit. I can't get hold of an RCBO for the
particular consumer unit I have, so fitting a 30 mA rcd inline with the
cable going to the DP isolation switch seems the only option.

Any comments on this ?

ban-all-sheds · 20 Oct 2004

Why has your board got a 100mA RCD? That should be 30mA as well, unless you've got a TT supply, in which case you should also have a 30mA one for the sockets...

If you're fitting a separate enclosure, why not go the more common route of a "shower CU" - RCD + MCB, connected into the tails. You could then install 10mm² cable on a 50A MCB, and be in a position to fit a larger shower in the future.

sclement · 20 Oct 2004

Hi

I think it is TT supply. The CU was installed 14 years ago, before we moved into the house. The 100 mA RCD was what was installed
at that time. Are you saying it is non-compliant ?

I did consider fitting a shower CU, and having a splitter and extra tails
installed, but thought this may be more expensive, as I would have to get
a spark out to do this.

Fitting an RCD as described in my previous post, was something I would be happy doing

ban-all-sheds · 22 Oct 2004

sclement said:
Hi

I think it is TT supply. The CU was installed 14 years ago, before we moved into the house. The 100 mA RCD was what was installed
at that time. Are you saying it is non-compliant ?

Yup.

sclement · 22 Oct 2004

Do you think it would have been compliant 14 years ago ?

ban-all-sheds · 22 Oct 2004

I'm only guessing, but probably.

t4msy · 9 Jan 2005

You must use a 30mA RCD. Also be careful on your MCB size as it depends on the size of the cable u can use. Using a 50 Amp MCB with 10mm cable would unlikely be compliant with the regulations as correction factors for installation method and length of run would need to be considered. I'd advise 32Amp mcb with 6mm cable, or 40Amp mcb with 10mm cable.
Also if you are on a TT system you would need to check that the rod resistance is less than 1666 ohms or u could be creating a dangerous situation with regards to touch voltage.
With the 100mA rcd u currently have u are only allowed 500 ohms.

breezer · 9 Jan 2005

i bet his shower has been working for ages, that was originaly posted in October 04

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30 mA RCD

ryanj

breezer

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