Discuss Occasional mystery trip in the UK Electrical Forum area at ElectriciansForums.net

The ramp test probably tests between N and Cpc ?
I know this is a tangent and a rabbit hole, but you know me - I had to explore this idea further....!
A non-rcd protected socket, two different MFTs, a calibrated multimeter in series with CPC (set to Max), and a clamp meter (set to Max):
View attachment 111884View attachment 111886View attachment 111885View attachment 111887

Conclusion: in this environment the the clamp meter is pretty close to the multimeter. The cause of the effect we both saw is likely not due to the clamp meters accuracy.

I set the clamp meter aside, and introduced an RCD that ramp tests at 25ma, then used one MFT in calibration mode to perpetually leak 15ma to earth, then the other MFT in ramp test mode. Interestingly the latter still ramp tests at 25ma even though it's counterpart is leaking 15ma. So I can reproduce the issue at home very easily.

I'm wondering if there is some sort of phase offset shift inside the MFT when ramp testing as what seemed like a perfectly logical idea simply doesn't work! So it seems we use one or the other but don't try and be clever with both, for reasons as of yet unknown.
Did you try using the mft to leak up to the tripping point of the rcd ? And if so was the tripping point of the rcd the same as the ramp test ?
 
Did you try using the mft to leak up to the tripping point of the rcd ? And if so was the tripping point of the rcd the same as the ramp test ?
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.
 
I have had something very similar, actually it was a call out last Christmas. Turned out to be a faulty cable supplying the boiler...
The boiler was wired in T&E a service engineer had been out the month before and changed a pump, he had also trapped the cable between the casing, ot started it off as a high resistance short between L,N & E. My earth leakage clamp meter didn't pick this up. I presume it was because the imbalance between both L&N was shared down to earth...

in your case it doesn't explain your clear IR values but I'd definitely check the boiler if you get called back.
 
Nothing relevant from me Tim.
Looked at your title this morning with my cup of tea, and thought.. oo, he's suggesting forum members meet up for an occasional mystery trip :)
 
Nothing relevant from me Tim.
Looked at your title this morning with my cup of tea, and thought.. oo, he's suggesting forum members meet up for an occasional mystery trip :)
There are many on here, yourself included that I’d love to have a beer with.
I fear until retirement it’s unlikely to happen in most cases.
Pint in Ludlow sometime?!
 
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.
 
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.
 
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 :)
 

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