RCCB burn out

[incognito1]

hello, can anyone tell me most probable reason for this kind of RCCB(4P, 400V, 63A, 100mA, 3kA) burn out. there was isolator in the system before this RCCB.

can this happen due to a lightening strike? then why isolator didnt burn out first?

 

[ScottyUK]

Too much load and over-sized overcurrent protection device perhaps? Remember that the RCCB does not provide any overcurrent protection at all.

 

[incognito1]

but why only line terminals are burnt? not load terminals or any other cable burnt.

can heat from a overload current melt the tightening screws also?

 

[itsmoked]

No1 I'd say number one cause would typically be loose screws. Are you sure they were tight?

No2 would be water getting into the box and allowing a soggy short getting tracking and corrosion on the fast track leading to arcing.

No3 lightning. It could be just the leader that sets up main current arcing. This would not necessarily cause problems with nearby equipment.

No4 the overload protection has failed or is incorrect and an overload like a stalled motor runs thru that RCCB for an extended time. I'd expect the load to show some overheating signs.

No5 something that causes the supply voltage to more than double briefly. This would cause other things in the immediate vicinity to be toasted also. This could be a tree branch connecting the HV to LV.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Perhaps not, but an arcing fault certainly can. The real question is "Why did this develop into an arcing fault?"

 

[Mbrooke]

How likely are arcing faults at 230 volts?

 

[Paulusgnome]

I note that while the RCCB is well broken, there doesn't seem to be a lot of heat distortion. The plastic case is still square and largely intact, but covered on black arc soot.
This leads me to the thought that whatever did this, it was most likely a short-duration high-intensity event. Possibly a lightning strike?
A few years back, I saw something similar where a 3-phase MCB-type isolator was similarly destroyed.
The cause was determined to be the set of tin-plated busbars which were fed by direct connection to the isolator, these separated by a ~8mm air gap.
Tin-plated copper grows metal whiskers out of the plated surface. When you get a forest of these whiskers growing between busbars, eventually something will allow them to touch, and when they do, you get something like photoflash going off. The enclosure got its insides liberally coated with black soot, the isolator got blown apart, but the upstream MCCB protecting the circuit was not fast enough to react and stayed untripped.