Occasional mystery trip

[timhoward]

I'm writing this mainly hoping something occurs to me while writing it!

I got called to an occasionally tripping RCD. It's a Hager double height board, house is about 4 years old, 7 circuits on a type A RCD at top.
Global IR on top row of 0.07M. So fault not present, therefore I could only hope that the low IR was related to the actual fault.
Clamp meter around the RCD tails showed 4ma. Ramp test showed 23ma.

I IR tested all the circuits, L+N to E. Only circuit of interest was kitchen and utility sockets which was 0.07 (obviously still not RCD-trippingly-low).
I found the reason for that (old tumble drier). Then global IR rose to 85M.

Next thing I noticed was when the oven was on, the clamp meter around L and N RCD input shot up to 37ma. And the RCD still didn't trip....
So I thought I'd "help it" with a ramp test. But the ramp test STILL took 23ma to cause a trip. Brain turning to jelly now.
Faulty RCD?
It tripped this morning when they weren't using the oven, and the circuit has 200M IR when the oven is off.

Only other clue is that oil fired boiler was serviced last Thursday, first trip was Thursday night. The boiler circuit had >200M IR (it's wet underfloor heating).
It's tripped again tonight after I sorted out the sockets circuit, and drove away with a healthy global IR.

So I'm thinking
-this is a classic needle in haystack unless I manage to be there when fault is happening
-maybe faulty RCD, still confused by ramp test not tripping quickly when cooker is apparently leaking a lot.
-I should probably have a look inside the boiler next time
-how long do I fight this before converting top row to 7 x RCBO's at £21 a pop...

So far the only remotely suspicious thing has been the Beko oven range, and they have agreed to see if it still occurs with it's isolator off but that is rather clutching at straws.
Any other thoughts or diagnostic ideas most welcome!

 

[mainline]

No but I like the idea and I will when possible.
I have a naughty 16A trailing socket that gets attached to the top of my SPD for calibration and testing purposes and I was using that.
The cooker plate socket/circuit is the only one that the family won't shout at me for tripping and there isn't room to setup too much test equipment near there without SWMBO conveying constructive criticism.

Does the ramp test vary the current clamp down the earth.

I wonder on TT earthing if you will get the same results.

 

[nicebutdim]

Pint in Ludlow sometime?!

Had a few too many in Ludlow on more than one occasion. Haven't been in 20 years, but there used to be some incredible restaurants close by - think I remember 3 with Michelin stars, which was hard to believe in such a small area.

 

[Rockingit]

Talk of pints aside (it is only coffee time on a Tuesday, after all..) the only way to know what is leaking down a cpc is to clamp that cpc (or have absolute confidence that you've got a correct L-N combo). And the only way to know the total cumulative leakage is to clamp the main earth and at the MET and compare the two to take account of bonding. I'd suspect that the reason your ramp test wasn't working as you expected is because of how your meter applies the current v relation to phase cycles and is probably adding some blinding or bias into the system to account for what it's already detecting.

However... back to first principles of what makes an RCD trip:
1) A 'new' fault on an appliance
2) Mechanical damage to a conductor
3) Water ingress

In reverse order - 3) We have had a lot of rain lately; 2) You've tested sufficiently to have spotted that; 3) You've also tested sufficiently to find anything obvious.

Additionally - intermittence is normally caused by either something moving or something on a timer (eg heating).

Trust your gear, trust your skills - the rest is patience and methodical thinking.

 

[brianmoooore]

Fridge or freezer with a light that comes on when opened? Ice build up around the lamp, partly melts when lamp is on, water runs into live bits, trips RCD, then freezes again before you get the meter connected.
The above is a summary of one I solved years ago, after others had tried and failed.

 

[HappyHippyDad]

Im not as clever as you guys. I would be doing what somw others have said, I would be leaving certain items switched off for a period of time. Depending on how often the trip occurs of course. The boiler cant stay off for long in the middle of winter for instance, especially if an elderly couple.

It does increase the chance of it being the boiler, with the fact someone has just been working on it. Far from definite, but certainly more likely than if no-one had been near it.

Forget all the clever tests that are eating up your time and just switch it off