RCBO Tripping

[martinxxxxxx]

Right then, I had a problem yesterday that had me beat and it is annoying me a lot.
Fully tested an installation in preparation for a new fuse board. This included ir testing between the neutral, live and cpc conductors of all circuits and further testing between the above conductors to the same conductors in the other circuits to ensure all circuits were completely independent from each other no shared neutrals etc.
Plus all of the other tests in GN3. No abnormal results recorded apart from an o/c phase on the up circuit which was traced and rectified, loose connection in socket.

There are 3 ring circuits
Down
Up
Kitchen

Kitchen is fine

If any socket on the Up circuit is used - the RCBO for down trips, but the up circuit stays on and continues to work
If any socket on the down circuit is used the down RCBO trips.

Have checked that the conductors for each RCBO are the correct ones and that they have a near infinite IR from all of the other conductors in the neighbouring RCBO - and they do.
Have checked tightness of all connections to busbars and sequence of fly leads from RCBOs.
If I remove the conductors from the down rcbo and run UP circuits - all operates OK.
And (and here is a big clue that I have not been able to digest yet) if I remove the N and L from the outgoing of the UP the down now works fine.

Baffling that the there is interaction between the circuits when they megger out ok from each other.

Also the RCBOs have been tested with the outgoing circuits removed and go through the test sequence fine no trip at 1/2 both ways and 29ms and 30ms at 30 and 150ma respecttivly

 

[davelx]

Long shot trying to think outside the box - is there anything that when plugged in could somehow be linking the up and down circuits? I'm thinking something like a TV aerial amp powered from the upstairs ring feeding a TV on the downstairs ring - which might possible link the CPCs, or even theCPC to neutral via the screen of the coax (or even the neutral!).

 

[martinxxxxxx]

Yes I am thinking the link must be capacitive or inductive because of my IR testing.
I was thinking the the order of the neutrals on the busbar might have an effect, but that would require a high resistance in the neutral to have the effect. I will check again the tightness of the neutral connections

 

[RF Lighting]

Have you made absoluotely certain you've not got one or both of your lives / neutrals mixed up at the CU?

 

[martinxxxxxx]

Oh yes there is no muddle up, that was my first thought and it has been very well eliminated. with long leads to the sockets on low ohms and separations on IRs.each circuit is definatly separate from the other on all conductors. I am thinking N or E borrowed between circuits some where now or reversed

 

[JohnW2]

If any socket on the Up circuit is used - the RCBO for down trips, but the up circuit stays on and continues to work
If any socket on the down circuit is used the down RCBO trips.

Fascinating one! It's the asymettry of the behaviour which is perhaps the most intriguing - have you tried swapping the two RCBOs to see if the behaviour then 'reverses'?

Kind Regards, John

 

[EFLImpudence]

Has the householder had this problem or has it only started since you rejoined the phase conductor?

Are there other RCBOs (or RCCBs) in the board?

 

[securespark]

No matter how unlikely, I think you have to eliminate the possibility of an appliance issue.

Disconnect ALL loads and recheck.

 

[EFLImpudence]

Do a ramp test on both RCBOs with and without the load conductors connected.

 

[bernardgreen]

Yes I am thinking the link must be capacitive or inductive because of my IR testing.

IR tests with 500 volt DC give no indication of the capacitive coupling between isolated circuits.

Test with an AC voltage and the amount of capacitive coupling and thus the resultant current can be found. A floating 230 volt source ( from a 1 to 1 transformer ) with an AC milliamp meter can be used to measure the current flowing through stray capacitance between conductors when 230 volts AC is applied. I have a small 230 volt lamp in series to limit current to be within the safe range of the meter in case there is a short across the conductors being tested.

The fact that a load has to be connected does suggest there is path for some of the load current on either live or neutral to be by passing the RCBO.

The fact that two RCBOs are involved makes me think there are two faults. maybe neutral to something other than CPC.

Any sections of pipe work or other conductive material that are not connected to the CPC but could be fault connected to the neutral or live.

Have you tested for IR to ground ( not to the CPC or "earth" ) In theory the ground and the "earth" should be the same but with some installations and supply systems they can be considerably different.

 

[martinxxxxxx]

No matter how unlikely, I think you have to eliminate the possibility of an appliance issue.

Disconnect ALL loads and recheck.

They were all disconnected (also confirmed by a high IR on L-N of every circuit) apart from the one appliance I carried around to test the circuits (hammer drill)

But I do take your point, and will check that there is not a hidden low current device plugged in some where hidden, like a mice deterrant wall wart in the loft or some thing.

I will also take my circuit tracer with me to to trace the current back to earth when I put a signal on the live/neutral and earth wires each in turn from the furthers point of the ring on each circuit, which I will break at the that point. There should not be any signal on the other circuit.

 

[JohnW2]

They were all disconnected (also confirmed by a high IR on L-N of every circuit) apart from the one appliance I carried around to test the circuits (hammer drill) ... But I do take your point, and will check that there is not a hidden low current device plugged in some where hidden, like a mice deterrant wall wart in the loft or some thing.

I guess that you have to consdier 'anything & everything' but, unless there were loads which are only connected to the supply via a capacitor (very unlikely), your very high IR would have excluded the possibility of their being present.

Although it's very unlikely (and wouldn't explain everything, anyway), I assume that you have confirmed that the problem doesn't relate to a fault with your 'test appliance' (hammer drill) by trying with some other load?

Although it's been discussed, I seriously doubt that capacitive or inductive coupling between domestic wiring would result in currents high enough to trip an RCBO.

Have you tried with a very small, simple, load (like a 15W or 25W incandescent light bulb)? Does the behaviour you've described then still persist? If it did, then I think that all thoughts of capacitive/inductive coupling (and a number of other hypotheses) could probably be dismissed.

- and, as I asked before, have you tried swapping the two RCBOs?

Kind Regards, John

 

[martinxxxxxx]

The hammer drill was employed as it was a heavy portable load, the kettle initially caused the problem.
I won't have the opportunity to do further test quite yet. Which is bugging.

I wonder if a loose neutral to the busbar or a faulty mainswitch could cause this effect. and a the fact that I have only seen it on the ring circuits so far is load dependent as you have pointed out.

I have a drummond tester I'll give that a go with break out plug on each socket

All RCBOs thats the way I roll.

Regards

Martin

 

[JohnW2]

The hammer drill was employed as it was a heavy portable load, the kettle initially caused the problem.

That's fair enough but, as I said, you'll learn something (and probably be able to eliminate some possibilities/hypotheses) but determining whether the same happenes with a very small resistive load.

The I wonder if a loose neutral to the busbar or a faulty mainswitch could cause this effect ....

The main thing to be explored/explained is obviously the apparent 'linkage' between two circuits which are meant to be totally separate. One subtle way in which a 'linkage' might occur would be if the neutrals of both circuits were satisfactorily connected (via the RCBOs) to the neutral bar, but there were a significant impedance (e.g. due to loose connections, damaged main switch or even damaged cable) in the connection between that neutral bar and the Neutral of the supply. I haven't yet thought through whether that could explain any of what you are seeing, but the 'common impedance' (common to both circuits) of that iffy connection obviously would create a degree of 'linkage' between the two circuits.

If you can totally isolate the board, then you could check for that by measuring resistance between neutral bar and the supply side of the main switch (or, even better, the load side of a pre-board isolator), which obviously should be 'near zero'. Failing that, you could see if you can measure any significant voltage between the neutral bar and the earliest point in the N supply you can get at (incomer to main swith, pre-board isolator/Henley/whatever) with a high load applied to the circuit(s). The voltage obviously should be near zero.

Kind Regards, John

 

[securespark]

I suppose a loose connection could cause heat build-up, tripping the device.

I had an RCD that was tripping on a shower, the connections were loose. Tightened up the connections, the tripping stopped.