What caused this?

[RF Lighting]

Got called out to a job where they had lost some power.

Aparrently an isolator went bang when they switched it on.

It was a 50A isolator supplying a radial socket circuit wired in 4.0mm² LSF singles in steel trunking.

The fault had taken out the C32A RCBO supplying the circuit, and also the submain breaker which was either 50A or 63A type 3.

There was no fault on the circuit which I can find.

The isolator was at most 6 months old, and not often used.

It looks to me as an arc has jumped between the L & N terminals on the outgoing terminals within the isolator, but I have no idea how or why.

 

[Robin0577]

I must admit that I'm clutching at straws here but is/was there anything about the environmental conditions that could have allowed some kind of contamination into the isolator?

Can't think what though, it looks far too clean for most contaminants I can think of....

 

[RF Lighting]

Nothing that I noticed. It was in a theatre control room in a heated building. Very odd.

 

[Robin0577]

Hmmm.... I have once known a stray flake from a glitter bomb to find it's way into some theatre switchgear but that made a great deal more mess and I wouldn't have thought the design of that switch would allow such a thing anyway.

 

[disinfo]

Had something weirdly similar to this happen at work on Thursday, but this was on a 230v extension reel, the lad using it was unreeling it (plugged in, switched on ) when it went with a loud bang and he threw it on the floor.

On inspection there looked like there had been an Arc between the L/N bars.

I put it down to a bit of swarf.

 

[westie101]

I've seen similar on our gear where an arc was set up for no apparent reason!

Only possibility I can think of is that that contamination built up on the contacts from load switching. When it was switched on this led to a poor contact and an arc, this would have ionised the air in the region of the contacts which probably caused the L-N flashover.

 

[canduit]

Don't know why it went bang but nice photography.

 

[ricicle]

What make is that switch Rob ?

 

[RF Lighting]

MK metalclad

 

[JohnW2]

MK metalclad

Are you going to send those excellent piccies and/or the physical evidence to MK for comment?

Kind Regards, John

 

[bernardgreen]

Any motors, wound transformers switched by the isolator ? They could create a large arc when turned off. What was the gap between the open contacts. 3mm is adequate for breaking current to resistive loads can be jumped by an arc from an inductive load.

 

[plugwash]

I'm wondering if a stray strand of fine stranded wire somehow got inside during installation then some time later happened to short it out (and then vaporise).

 

[JohnW2]

Any motors, wound transformers switched by the isolator ? They could create a large arc when turned off. What was the gap between the open contacts. 3mm is adequate for breaking current to resistive loads can be jumped by an arc from an inductive load.

Does not the pattern of damage suggest arcing between L and N, rather than across opening contacts in N and L paths (simultaneously)?

Kind Regards, John

 

[Spark123]

My guess is the arc started at the contacts by not closing properly, ionising the air making it conductive and eventually allowing an arc across L-N.

 

[JohnW2]

My guess is the arc started at the contacts by not closing properly, ionising the air making it conductive and eventually allowing an arc across L-N.

Yes, possibly, particularly if the contacts 'bounced'. Otherwise, I would have thought the arc would be more likely to have occured due to too slow (or inadequate) opening, rather than closing, of the contacts - but again, as you say, with the resultant ionisation perhaps extensive enough to facilitate an L-N arc.

Kind Regards, John