RCBO Blew Up

[LewisM]

Hello I you are all well

This is hard to explain but basically doing a three phase install and there is no power yet so I wanted to test two lighting circuits before the lifter went, I used an extension lead and connected live to phase L2 (that's what the lights were on) Neutral to the neutral bar and Earth to the earth bar (see attached picture) however the extension lead I was given had been wired with reverse polarity L-N switched so when powering on with all circuit breakers OFF one of the RCBOs blew up as it had a backfed neutral, power was quickly removed however has the integrity of all the RCBOs been compromised?

Thanks Lewis

 

[pc1966]

Revered polarity is a very bad thing but I'm struggling to work out why it would over-current the RCBO as surely the load side of the RCBO would be isolated from earth on both L & N?

An extension lead would (I presume?) have no more than a 13A fuse which is going to limit the fault current somewhat so the RCBO ought to be OK, but obviously you should test it anyway for trip-times and mechanical action just as you would do with any new install.

 

[pc1966]

Those Hager RCBOs have the functional earth, though should force them to trip on reverse polarity so maybe the "bang" was not over-current but just the breaker going.

Was the extension lead's fuse blown?

 

[LewisM]

All circuits were in the off position and yes the RCBO Blew up I mean smoke and sparks as my head was about 30cm from it, somebody else had plugged the lead it as I was to turn the two lighting MCBs on to test the lights

 

[pc1966]

Are you sure it was the extension lead that was reverse-polarity and not the supply to it?

No fuse in the N side might cause a whole lot more fault current that way than someone swapping brown & blue in the back of the 13A plug.

It still is not explaining where the current went to blow up the RCBO. Had all those circuits been IR tested before this?

 

[LewisM]

No it was the lead as it was fitted with one of those Dura plugs, it had been cut off and old work light. The safe end was wired into DB to test lights, the other end was plugged into an extension lead after the fire works I took it apart and the live end had been wired incorrectly probably from factory

 

[Vortigern]

So you put L onto the N busbar and E onto E busbar with a FE from the RCBO and the N fly lead from the RCBO if I understand you correctly, and there was a bang. Not really surprised at that thinking it through. The PEFC would need to be calculated and match it against Icn of the RCBO in question to see if it needs replacing I imagine.

 

[pc1966]

Having 240V on N is not the only reason.

I have 500V tested N-E on such a board and found the functional earths act like a few hundred kOhm loads. Yes, probably not a good idea in retrospect, but they were fine afterwards and tested out perfectly on trip-times & ramp once all was done and energised.

So if high-current fireworks there has to be some path for the current that is not limited by the normal load, which is why I asked if the loads had all been IR tested before hand to rule out a N-E short in one of them.

 

[LewisM]

All passed on insulation resistance testing prior to connection to RCBO this was to simply check all fittings were working

 

[LewisM]

So you put L onto the N busbar and E onto E busbar with a FE from the RCBO and the N fly lead from the RCBO if I understand you correctly, and there was a bang. Not really surprised at that thinking it through. The PEFC would need to be calculated and match it against Icn of the RCBO in question to see if it needs replacing I imagine.

Inadvertently yes earth to earth bar, N to L2 bus and L to N bar RCBO has been removed already

 

[LewisM]

Fireworks may have been an exaggeration however it was more a sustained arcing and smoke until the Power was removed, I have spoken to Hager who said to simply test trip times of other RCBOs and via the test button

 

[pc1966]

Hmm, odd. It sounds like the FE causing trouble but as far as I understood them they simply trip the RCBO if the neutral goes open.

 

[telectrix]

ramp test it as well.

 

[loz2754]

This would not have happened if the full sequence of initial inspection and testing had been followed, even (perhaps especially) with a temporary supply.

 

[LewisM]

This would not have happened if the full sequence of initial inspection and testing had been followed, even (perhaps especially) with a temporary supply.

Yes you are correct it's the usual case of rushing to get things done, full testing is to be undertaken this was to see if there was enough light or if more were needed

 

[Lucien Nunes]

This would not have happened if the full sequence of initial inspection and testing had been followed

But at this point the question is what caused the RCBO failure, as even though there was a procedural deviation leading to a reverse polarity connection, I agree that this alone shouldn't make it go bang. It might have been defective and gone bang anyway with a normal supply, or there might be a subtle shortcoming in the design that is not generally known.

Can you break open the RCBO so we can study the failure mode?

 

[suffolkspark]

Would also be interested if you can drill it open and get some pics

 

[LewisM]

Will do on Monday

 

[LewisM]

Got nice and hot then

 

[Lucien Nunes]

Hmm. Are you sure that was running from a 13A extension lead?
The initial failure led to a more extensive power arc that will have covered much of the evidence in soot. For forensic purposes I am sure it would yield information given enough time and care to extract.

It would be interesting to know what Hager technical say. If it were mine I would contact them, state that an RCBO failed on a reverse-polarity supply and see if they can can add anything. They might even be interested to have the failed device back for inspection.

 

[AJshep]

Seems very odd. Out of interest where was the extension lead plugged in... was it plugged into a single phase circuit on a 3 phase system?
Did you verify the voltage of the extension lead between all conductors?

Poor neutral connections can present themselves as reverse polarity and cause higher then normal voltage to run through the coils. Maybe the problem is with the existing circuits.

 

[LewisM]

Yeah was a 13A plug that was wired to a Dura plug, the Dura plug live had been wired incorrectly as I tested it after and there was reverse polarity, I took it apart after and the lead was wired wrong

 

[LewisM]

Yeah was a 13A extension lead that had been wired to a Dura plug, the Dura plug live had been wired incorrectly as I tested it after and took it apart

The Dura plug lead was plugged in to a an extension lead had been used to charge batteries etc for two weeks

 

[pc1966]

Did the fuse in the extension lead blow?

I'm struggling to see how it was a high-current fault and beginning to think it is the FE that can't stand long-term reverse power. Which is odd as the last measurement I did (by accident) suggested no real issue in terms of dissipation on a open neutral sort of fault.

 

[LewisM]

Did the fuse in the extension lead blow?

I'm struggling to see how it was a high-current fault and beginning to think it is the FE that can't stand long-term reverse power. Which is odd as the last measurement I did (by accident) suggested no real issue in terms of dissipation on a open neutral sort of fault.

No fuse didn't blow, breaker replaced and all working and tested fine now