weired rcd tripping.

[A1Electrical]

Hi all, would like to pick your brains with this problem.
got called out to a tripping rcd. Separate rcd connected to 15yr old cu by mk.
the cu services the main house electrics in a quite frankly over-extended single phase system, consisting of 1 main incomer, split into 3 separate legs each with it's own cu and separate rcd's for stables and other outbuildings, all tt'd although it's not agricultural, just big...
Any way, the rcd for the house cu - 80A 30ma apparantly started tripping intermitently on 1 socket ring main. No recent work or changes done for at least 3 years at all. This intermittent tripping getting worse and now spread to second ring main. I get there to find a 10 way mk board with 3 light circuits, 5 32a circuits and a 16a garage door circuit. 3 of the five 32a circs serve ring mains but 2 have an extra radial live connection shoved into the mcb's. 1 32a is for the cooker, the other is shower-which is rarely used. When the 32a circs are energised the rcd trips on 2 of them immediatly(sockets). 1 has a straight ring, the other with the extra live. the other circs seem ok until i turn the cooker on which then trips the rcd making 3 faulty circs. removing the extra cable and leaving one leg out on the rings makes no difference.
Noticed on some of the cabing the green residue from that 60/70's cable that breaks down the insulation is part of the system but from what i've read that usually isnt an issue.

I've run the usual tests inc cont/IR/R1+R2/loop etc and found nothing really untoward except the main earth loop is a little high at 220 ohms.
swapped out the rcd with no change. this has got me frustrated as it doesnt make sense to have 3 circs out of 9 causing this that should be separate and dont throw odd readings. any ideas greatly appreciated

 

[tonyelectric]

Its of course difficult to say without seeing the install, but this sounds a similar scenario to a large country house (20 bed!!!) that i visited last year, this time of year. On the face of it the install looked good, 16th ed boards c1995 with RCD protection, but tripping for no decernable reason on random circuits, scratch the surface though and a large portion of the wiring was original 1928 rubber in conduit, it had held up well in the summer and drier winters of late, but this dank humid cold weather of the past few winters allowed sufficient current to pass to trip the RCDs, nothing for it but to re-wire.

First thing i'd want to do is check each circuit one by one, look inside JBs in lofts, they often have tight spiders nest cobwebs in them which when damp conduct.

How old is the property, if it was built pre 60s there is a real possibilty of cable recycling.

Or it could be something completely different!, good luck

 

[ericmark]

My first thoughts are neutral - earth fault. I have had it before where the RCD would not trip on the fault until the current drawn raised the earth - neutral voltage difference to a critical level at which point it tripped.

The same of course if there is a neutral to neutral fault.

I would switch off and test the neutral to earth.

Do let us know what you find.

 

[mikeyd]

Sounds like a classic N-E fault. It is probably only one circuit faulty and might not be one of the 3 you suspect.

It took me a while to get my head round why RCD's can trip when turning on circuits where the fault eventually turns out not to be.

Here's the way I look at it:

Imagine a light circuit with a low resistance neutral to earth fault which lets say 1% of the current flowing through the the neutral go to earth. You turn the cooker (different curcuit) on and it pulls 10A. 10A returns to the CU but then 1% of it goes down the lighting circuit neutral and through the fault to earth instead of through the RCD. So RCD sees 100mA difference in live & neutral current so trips.

 

[maguire]

Very well explained mickyd I have tried often to explain to apprentices what happens but that is pretty good answer
DM

 

[JB666]

My Metrel multi function tester can show live leakage to earth to give you a real idea of what is causing leakage and where which allows you to check all circuits live. Have you got similar or can you borrow? Would save you a nightmare of searching...
I've had similar problems in the past which have turned out to be VERY varying faults from screws through cables to deteriorating insulation. Good luck in your hunting!

 

[mikeyd]

Very well explained mickyd I have tried often to explain to apprentices what happens but that is pretty good answer
DM

Thanks maguire, I never was any good at explaining things, just hope the OP understood.

 

[pjcomp]

Just another thought to stir in the pot ... the RCD gets triggered from mutiple rings, so are they separate rings? I've met boards where the legs from different rings have been crossed in the MCBs so you need to turn off two breakers to isolate one RFC, and if the mismatch has gone further it can get even more complicated.

PJ

 

[A1Electrical]

Gents,
thanks very much for the info. gave me some good insight to the problem.
Unfortunately i still haven't got much further on this.
The lady is very reticent about the disruption and cost of more extensive investigations and as she comes across as a very anxious sort i dont want to scare the bejeesus out of her.
I have managed to check for N-E faults, but came up with nothing so far. I will be borrowing a live earth leakage testor this week, but as the temps down here are still in our boots, i think i'll leave it until the temps rise later this week.
The rings all test as separate as far as i have been able to ascertain.
I have swapped out the RCD for a normal switch for now so at least she has central heating.
Thanks again, and i'll keep you posted.

 

[Buzx1313]

i agree probably e n fault

i had one on a motorised valve on the heating, kept triping out when enrgised...

kept going out to site to test but couldnt find it, by chance went once and the heating came on..

 

[1john]

.... could be the heating pump too, or the boiler fan.

If the circuits all read well then it is an appliance fault, or she has alot of appliances which 'leak' to earth.

You have checked the sensitivity of the RCD have you, perhaps it is tripping at 10mA??