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Change 30Ma Upfront To 100Ma Td




ok, so I have a job with a SWA feeding an outbuilding,

30mA RCD feeding a sole feed to this,

nuisance tripping is a massive issue,

Ive measured the leakage current at 22mA [ive started another thread on this]

does anyone see why this really needs to be a 30mA RCD? and not a 100mA TD?

in the outbuilding we have 5 x double sockets, all 30mA self contained RCD, and 1 x maintained emergency light,

all wired in SWA or armoured flex,

although the terminations of the flex or existing SWA is TBC, I'm doubtful if its been done correctly TBH,

2 of the SWA connections [i havent looked at any more] , on a visual, have been banjo'd to a plastic enclosure,,,,,,,,,,

the armoured flex simply appears to have been stuffing glanded,,,,,,,,,


now, the point of asking is this,

this in an educational establishment, does that throw up extra regulations?

kids/the general public having access?

I wouldnt have an issue with a normal commercial/industrial environment,

just wondering if the supply cable requires 30mA protection due to the type of installation,

Im not 100% on the source protective device,

but the 16mm sub to the outbuilding is terminated to an enclosure with a main switch and a B20,


many thanks.




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6 answers to this question

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Off the top of my head, I don't see an issue going to a 100mA S type for the sub main as long as the rest of the installation of the sub main cable is up to scratch.

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what's causing the earth leakage?


schools have no more requirements than any other building, the fact it's full of kids does tend to encourage RCDing everything - risk assessment is that no one wants to kill a child.


As the sockets are are RCDed anyway, does the supply cable need RCD at all?

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I would say the same, does the SWA actually need RCD protection anyway, but no harm if 100mA T/D is used.

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TNCS to outbuilding?

As it stands it is not complying with discrimination of RCDs.

Is the nuisance tripping just occurring at the upstream RCD?

Is the 22mA measured with (computer?) equipment plugged in? Or is leakage occurring without anything connected (maintained light?).

Apparently, reading rocknroll's post here http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=20736 , although bs7671 RCD testing requires that an RCD mustn't trip at half rated current for 2 seconds, the design of RCD means that it should trip at half rated current after a time specified on the time-curve (eg 30mA rcd should trip at 4mA in >5000ms), but I can't find any rcd curves at all, so can't corroborate this.

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I agree. Cannot see why the submain would need an rcd.. [unless it is fed from a TT supply perhaps]


What is the earthing type of the installation feeding the submain?? Could you not do away with that, install your 100mA TD and then TT the outbuilding. Just thought that this might just give the advantage of a "standard" installation nobody can pick holes in.... But then again it is trusting protection to unreliable rcd's...



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Evans Electric

That was my first thought TBH   why fit an RCD to the sub-main .


Just quoting ,as I read this , for a 4mm  sub main to a stable ...without RCD , feeding a 4 way RCD board in the stable itself .

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