Four RCBOs trip at the same time - IR is good on all circuit

martinxxxxxx · 25 Jul 2014

Hi ya all

Bit of an unusual one - I have a board that has been tripping on the socket circuit and the two light circuits and the shower circuit for some time. The house is a student let and is now empty for the summer so it is time to get to the bottom of what is happening.
All the IR's are greater than 100MOhms.

The shower circuit was switched off at the isolator and still the RCBO was in the off position when I arrived today. The whole run of the cable is largely visible as it runs across the roof space and is not damaged. So today I remove the outgoing wires from the RCB for the shower and put a pendant light with a few inches of cable as a load instead.

The customer has told me that it has tripped when he poppped in to look.

So I am wondering anyone has had experience of an external factor such as surges causing RCBO's to nuisance trip?

I have replaced the rcbo 's and used the ones I took out on other jobs with no problems at all so I don't believe there is a fault with them

Any Idea?

flameport · 25 Jul 2014

Do these RCBOs have the white functional earth leads?

JohnW2 · 25 Jul 2014

Odd. Are these a reputable brand of RCBOs, and do you know if they came from a reputable source? I take it that you are reasonably confident that drunken students have not be pressing the test buttons?

Kind Regards, John

RF Lighting · 25 Jul 2014

Duff neutral connection somewhere upstream of the RCBOs. Check neutral bar, link to main switch, main switch, tails, meter connections, cutout connections...

I've an old video of this happening somewhere.

JohnW2 · 25 Jul 2014

RF Lighting said:
Duff neutral connection somewhere upstream of the RCBOs. Check neutral bar, link to main switch, main switch, tails, meter connections, cutout connections....

ISTR you mentioning that before, so we've probably also been through this before as well, but I can't remember the answer - how can that make an RCBO trip?

Kind Regards, John

RF Lighting · 25 Jul 2014

Not sure how, but I've come across this twice now.

A good test for the op is to turn all your breakers on, and then really go at the cu, meter and cutout or their backing board with a rubber mallet or a nut driver in an sds drill on roto stop and see if the vibration has any effect!

JohnW2 · 25 Jul 2014

RF Lighting said:
Not sure how, but I've come across this twice now.

Interesting, but very odd - I'll have to think more deeply!

Kind Regards, John

RF Lighting · 25 Jul 2014

This is from back in the day when phone cameras had just been invented but it shows the principle in action.

http://i69.photobucket.com/albums/i62/RFLighting/IMGP2057.mp4

JohnW2 · 25 Jul 2014

RF Lighting said:
This is from back in the day when phone cameras had just been invented but it shows the principle in action.

Thanks - and I wasn't really doubting you

In terms of a momentary disconnection of the N when it was waggled, one could understand if it were an 'active' RCD, but I presume it wasn't. One could also understand if it contained electronics which needed to remain powered to prevent the device tripping - but I don't think they work that way around. ... I could even imagine it happening if there were a highly reactive load on the circuit - but, again, I doubt that would have been the case ... so I'm still thinking.

Kind Regards, John

Adam_151 · 25 Jul 2014

I imagine the effect is that the arcing of the connection generates frequencies much higher than 50hz, perhaps a few hundred hz to a few khz which get superimposed on the mains waveform. These higher frequencies mean that anything that is reactive has a much lower impedance than it does at 50hz and sufficient current flows to trip the RCBO. Remember also that the cabling itself forms a capacitor

ericmark · 25 Jul 2014

I have long realised there are up stream faults which can trip RCD's mine have been in around 25 years and every so often we get a batch of tripping then it just stops clearly something on the road causes it but never found out what. Also common if reset one without opening MCB's first the other will trip.

I know it shouldn't but it does.

Welder lives next door and I have wondered if it's his welder but unlikely as not the same phase.

JohnW2 · 25 Jul 2014

Adam_151 said:
I imagine the effect is that the arcing of the connection generates frequencies much higher than 50hz, perhaps a few hundred hz to a few khz which get superimposed on the mains waveform. These higher frequencies mean that anything that is reactive has a much lower impedance than it does at 50hz and sufficient current flows to trip the RCBO. Remember also that the cabling itself forms a capacitor

True. As I said, I could probably understanding it happening with a 'highly reactive load', even at 50Hz - so, as you say, if there were significant higher frequency components present, then the loads would not have to be quite so reactive for that to happen.

However, I still would have thought that it requires a pretty reactive 'true load'. I haven't bothered to do the sums, but I would have guessed that, even at 'a few kHz', the reactance of the capacitance of the cable would probably be far too high for 30mA to flow through it.

Kind Regards, John

bernardgreen · 26 Jul 2014

Question to the OP .... Is this a TT installation ?

If it is TT then a significant potential difference between the functional earth to the RCBO and the Neutral through the RCBO could either [1] be seen as a good reason to trip the RCBO or [2] be confusing the electronics into a false trip.

I really must find out exactly what function the functional earth provides. The obvious one is as a source of "neutral" to power the trip mechanism if the real Neutral goes open circuit leaving only Live at the RCBO. But it may also be intended to enable a trip when Neutral is floating to far from ground potential.

bernardgreen · 26 Jul 2014

Adam_151 said:
These higher frequencies mean that anything that is reactive has a much lower impedance

As you say capacitive reactance will be lower but inductive reactance will be higher so the current transformer that forms the sensor element will be higher impedance.

martinxxxxxx · 26 Jul 2014

Thanks for your replies, especially Rob.
The house is now unoccupied this month, the owner wants me to rewire it to get rid of the fault. Which I am not going to do unless I can satisfy myself that this would not be an expensive waste of time.
My next step will be to change the board for an MK. But I am loath to do this without a reason.
The board is a Pro-elec (see Amazon or Farnel) for details. I have used hundreds of them without a problem. And As I say, the "faulty rcbo" removed from this site have been recycled into another student house and have performed well for over 6 months.
I will go back and check the neutral connection, this might be a suspect especially since the supply is split (by the DNO from the downstairs flat when the property was split into 2 flats - same guy owns both and both are empty - so I can go and wiggle things around down there with no issues)
I am hoping this is the case because overnight the only RCBO that has flipped is the one with my lamp on it (ie the only one passing current), where as normally all of them are going. Bit of a clue that. So I am going to put a light on each of the other circuits to excluse all of the wiring and see what happens - obviously I will check the neutral connections out first.

Thanks again

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Four RCBOs trip at the same time - IR is good on all circuit

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