RCBO keeps tripping without reason

[EFLImpudence]

Quite possible. Copper superconducts at 0 Kelvin and the OP does say that the boiler is on the faulty circuit. I believe Copper Oxides exhibit superconductivity at higher temperatures, so older cables would help.

Ah. that must be it - my mistake.

 

[JohnW2]

I didn't think that (m)any 'standalone' IR testers had such a facility?

Oh. I thought they all did. I may be wrong.

I really don't know for sure, but I had assumed that a standalone IR tester (I've never owned one) was just a standalone IR tester. To be able to measure very low resistances as well as IR, it presumably would have to contain some separate, different, technology - since using 500V to measure very low resistances would result in some very interesting currents - IR testers themselves 'cutoff' below a certain resistance, for that reason.

No. I meant, whilst looking for a fault, how can he be certain.

I'm a bit confused. If, as I have 'assumed' (I know!), he disconnected the circuit in question from the CU and then found no continuity between that (disconnected) circuit and any other circuits, could he not then be certain that there were no 'cross-connections' (which is what I'm assuming was the point of the exercise)?

Kind Regards, John

 

[EFLImpudence]

Edit - misunderstood readings.

I don't know your level of experience but -

Are you sure it is Mega ohms?
Most continuity meters go up to 2000 ohms.

240V / 2001Ω = 120mA

 

[JohnW2]

Thanks John for making mostly correct assumptions

Thanks for confirming.

Fuse--------Circuit--------------- LN------------NE------------LE
6 -----------Gnd Ring ------------ 390 ---------- 34 ----------- 420
The results above indicate the Gnd floor 'NE' is suspect - but cannot see anything. By the way all wiring is exposed in basement so didn't have to lift floor boards to visually inspect.

It is certainly the circuit which has produced the lowest IR readings, but they really are not low enough to explain an RCBO trip. Even with a full 230V across it, the current though a 34 MΩ 'leak' would only be about 0.007 mA - hardly enough to trip a 30mA RCBO.

Kind Regards, John

 

[JohnW2]

Are you sure it is Mega ohms? Most continuity meters go up to 2000 ohms.

He said that the IR measurements were undertaken with a 500V IR tester, the multimeter only being used for the ring 'continuity' measurement.

You surely aren't suggesting that the sub '2000' IR readings for all three of his ring finals are all <2000&#937;, are you - were that the case there would surely be an awful lot of 'bangs' happening

Kind Regards, John

 

[EFLImpudence]

No, but there are inconsistencies in the thread.

Are we then to assume that all the circuits are fine and the fault is to another earth source?

or -

"It always happens with vacuum (1320W). But remember not on upstairs circuit.
Kettle (2000W) plus microwave (1000w) no effect.
But sometimes the microwave on its own causes problem.
Sometimes computer (600w) switched on at socket but not powered up causes problem.
Then sometimes it just flips without any noticeable appliance change - say noticeable as the boiler may have fired up."

just accumulative leakage?

 

[JohnW2]

No, but there are inconsistencies in the thread.

I agree - but, as I said, I don't think it makes any sense to suggest that all those recently-presented IR readings were in ohms, rather than Mohms - that just can't be the case!

Are we then to assume that all the circuits are fine and the fault is to another earth source? or - "It always happens with vacuum (1320W). But remember not on upstairs circuit.
Kettle (2000W) plus microwave (1000w) no effect.
But sometimes the microwave on its own causes problem.
Sometimes computer (600w) switched on at socket but not powered up causes problem.
Then sometimes it just flips without any noticeable appliance change - say noticeable as the boiler may have fired up."
just accumulative leakage?

Who knows - it's mysterious. The vacuum cleaner itself clearly cannot be the source of 'leakage', since (IIRC) it's only '2-core' - all it really could be doing (particularly at start-up) would be to 'reveal' a N-E fault elsewhere by putting a transiently large load on the circuit. The fact that switching on the computer socket, but not the computer, (thereby probably exposing the installation to capacitors at the power input to the computer PSU) can cause trips does, indeed, suggest that there is a pre-existing total leakage quite close to 30mA - but goodness knows (on the basis of what we've been told) where!

Kind Regards, John

 

[pads69]

I have fingers crossed - think I have found it.
There is a fused spur outlet which has not ben used for about 15 years (and I had almost forgotten about).
Had some panelling put in recently and guess what must have nailed through the cable.
Luckily the outlet is actually on a spur (not directly on the ring). So I've disconnected t.

On testing the cable to the fused outlet (now hidden behind panelling) I get those very same low insulation results.

So hopefully case solved.

Thanks for hints folks

 

[JohnW2]

I have fingers crossed - think I have found it.
There is a fused spur outlet which has not ben used for about 15 years (and I had almost forgotten about). Had some panelling put in recently and guess what must have nailed through the cable. Luckily the outlet is actually on a spur (not directly on the ring). So I've disconnected t. On testing the cable to the fused outlet (now hidden behind panelling) I get those very same low insulation results.

I hope you're right - but if 'those very same low IR results' your talking about are those ones in tens of M&#937;, that really shouldn't be enough to trip an RCBO.

I suppose one interesting question is whether disconnecting this spur has resulted in the IR readings for the rest of the circuit going up to much higher figures.

Kind Regards, John

 

[EFLImpudence]

As John has said, those results would not cause the tripping.

As you have tested all the conductors in isolation I would now test from each Neutral first and then Line to the MET with all of the CPCs, bonding and earth conductor connected.

However, this still may not detect the fault if it is due to movement.

If all the circuits are satisfactory as tested and it is not accumulated earth currents, as appliances are all unplugged, then the problem is more likely to be a worn or damaged cable resulting in leakage caused by something else intermittently.

This could be, for example, movement caused by treading on the cable below a floorboard or the cable contacting a pipe.