Discuss Intermittent RCD tripping. in the Electrical Forum area at ElectriciansForums.net

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HappyHippyDad

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I have been asked to investigate a tripping RCD for a family member. I haven't been out to view yet and have asked very few questions but the scenario sounds like an RCD as a main switch. The homeowner opens all MCB's and then closes one by one until the RCD trips, finding that the 'lights in extension' is the culprit. He leaves this one open and the RCD does not trip. The trip has occured twice both times within the last week at times when it has been raining heavily.

No recent DIY/professional work has taken place
Trips spontaneously, i.e nothing is switched on at exact time of trip.

I will take a look on friday and I am happy about the steps to take if the fault is still there when I arrive as I can IR test and break down the circuit, but if the fault isn't there when I arrive I am not sure what to do apart from visually inspect as much as I can (especially any outside lights)?

I wasn't going to add this next bit as it shows my ignorance but am I right in thinking I should be looking for IR less than 0.008Mohm (very approx) which would be 30mA at 240V (so probably 0.01Mohm as will trip at less than 30mA). If the resistance is over that then it might be something plugged in (and disconnected at time of test).

I'm actually looking forward to it!! :smiley2:
 
I had Rcbo tripping on lighting circuit in extension other day, water seaping down caverty some how soaking wall then into wall lamp. Was small amount of water on junction enought to trip. There was been a lot of water this last few weeks
 
Having spent eight hours today ripping my own house apart to find water causing my kitchen RCD to trip....we have, as said above, had a heck lot of rain recently!!
 
you're correct in saying that 0.008Meg. is approx. the leakage required for a 30mA RCD. however, in practice it may be quite a bit higher, due to some leakage elsewhere. if you go by recommendation, anything under 1Meg. needs addressing. and agree with applemac that water is the most likely culprit. i always feed outside lights etc. via a 2 pole FCU so that both poles can be isolated just by throwing the switch. start with a IR test ( LandN shorted together and test to E) on the circuit, then break it down, dis. anything outside first.
 
Thanks chaps,

So if the fault is not there on friday then basically it's a visual inspection, starting with anything outside? Thats kind of what I thought but still very green when it comes to fault finding! Think I'll still do all my IR tests (even if RCD isn't tripping) just incase the culprit is still wet and perhaps showing decreased IR.

Thanks for the IR bit Tel, I didn't think about leakage from elsewhere increasing the IR figure!

Forecast for friday is rain! Which is good... the fault cant escape me!! :smiley2:
 
well...i`d say it was pretty obvious that if theres tripping problems just after rainfall then the first place to be looking is outdoor enclosures

what do you say Hippy?
 
if i tripped after rainfall, i'd be looking for compo from the council.:fuk2:
 
well...i`d say it was pretty obvious that if theres tripping problems just after rainfall then the first place to be looking is outdoor enclosures

what do you say Hippy?

Yes Glenn, I guess it is obvious but I like to hear it in black and white from the experts (that just about includes you :smilewinkgrin:) when it comes to fault finding in case I've missed absolutely anything!
 
Had one Christmas Eve which was eventually tracked down to a jb under the floor which had been flooded. Was a pain because all IR tests were clear, well, 7M ohm but something was tripping it. Good luck mate:
 
Could be underfloor JB....seen more than a few where builders have jointed cables underfloors with them, low enough for water to get into them.
If its not the outside lights, could be rain penetrating into the roof space / switches for the lights (old man is currently at war with the builder who 3 years on still can't figure out why rain is getting into the soffits on his extension...)
 
get the offender zapped Hippy....soon find out which cable`s gone to earth...

then narrow it down till you find which leg it is....

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dont IR through the RCD will you...

and check for dimmers as well beforehand...
 
get the offender zapped Hippy....soon find out which cable`s gone to earth...

then narrow it down till you find which leg it is....

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dont IR through the RCD will you...

and check for dimmers as well beforehand...

dimmers, neons, fluorescents... they just don't like it up em!!
 

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