Switched fused connection unit with RCD?

[murraysnudge]

My house has an 80mA RCD between incoming mains and consumer unit. I want to wire some external lights and a pond pump - all mains voltage. I can fit a Fused Connection Unit as a spur off a ring-main socket on the external wall, and take the cable through the wall to a weatherproof junction box. Am I correct in thinking that the FCU should have a 30mA RCD as well? I have done a search on the site and that appears to be the recommendation when the property does not have an RCD, but is it still the recommendation here because my unit is an 80mA one. I can only find an un-switched unit made by PowerBreaker but would prefer a switched one. I could have a 20A DP switch between the ring socket and the FCU but does anyone know if a switched FCU with RCD does exist? I’ve looked on many supplier’s sites but can’t find one.
Thanks for any advice.

 

[plugwash]

afaict there is no such thing as a 80ma rcd

im guessing its an 80A currecnt rating 30ma trip device

but you need to look more closely...

 

[breezer]

afaict there is no such thing as a 80ma rcd

plugwash, nil points, advantage origonal poster

see here

 

[BR]

yes, they exist, but are very uncommon at least here. I have seen one once, maybe twice but that's it. (they are appropriate for certain installations but a 100MA is in compliance and a lot more common. RCD's can actually be got in many other tripping ratings such as 40MA 50MA 70Ma, 72MA (don't ask!) & 80MA.

 

[murraysnudge]

It's a 1960's house and the RCD was there when I bought it so I don't know how old it is. It definitely says 80mA on it though. It doesn't really matter if it's 80 or 100 because my question is because the whole house is not covered by a 30mA RCD do you all advise that I fit a Fused Connection Unit with a 30mA RCD before I take the cable through the wall?
Also does a switched one exist or do I fit the PowerBreaker one with an additional 20DP switch as well. I would like the facility to isolate the external wiring without switching the whole house off.
Thanks.

 

[ban-all-sheds]

I would like the facility to isolate the external wiring without switching the whole house off.
Thanks.

Just press the Test button on the RCD...

 

[murraysnudge]

That will trip the RCD before the consumer unit as well though so all the house is then off.

 

[securespark]

Why?

 

[plugwash]

no it will not

rcd test buttons will only trip that rcd

though there is no point in having 3 rcds of teh same trip current in sequence as it is pot luck which one goes anyway (and likely both will)

rcds work by detecting an imbalance between live and neutral current
in a fult condition this is caused by current to earth

the test button creates this condition by conencting live-neutral (with a resistor) accross the rcd

so from the pint of view of a rcd before it there is no imblanace

 

[murraysnudge]

I understood how the RCD worked as regards the current imbalance transformer, but assumed that the test button linked live to earth. You are saying that the link is between live and neutral, presumably linked so that current flows through the winding on the live side but not through the winding on the neutral side to give an imbalance.
Are they all made like that? If so, why doesn't the test button link live to earth (as I had thought) which does represent what would happen in a fault condition.
Thanks.

 

[ban-all-sheds]

Dunno how the test button works, but I can't see how connecting a resistor between live and neutral would do it - that, after all, is just a normal load....

 

[plugwash]

they link LN accross the rcd

basically the test button trips the rcd in the same way a borrowed neutral in house wiring does

one end of the resistor is on the supply side of the rcd
the other end is on the load side of the rcd

 

[murraysnudge]

OK, thanks plugwash for explaining the workings of RCD. Thanks also to ban-all-sheds for suggesting to just use the test button to isolate outside installation, so no additional DP switch is required as I had first thought.
Nobody has answered my question about whether the 30mA RCD is essential for the external wiring, as I have the original 80mA RCD on the incoming mains.
Thanks for help so far.

 

[ban-all-sheds]

Yes - the outside stuff should be protected with an RCD with a sensitivity of no more than 30mA.

 

[murraysnudge]

Thanks ban, that's me sorted on this one then. I've only just discovered this forum; there are some good guys on it.