RCD blowing cutout fuse

[JohnW2]

I assume it is a spring driven opening but I was looking at all options as I find it hard to believe that the design would allow copper spluttering ( from the damaged Live ) to get that close to the contacts.

The damaged Live is well away from the arc chamber.

BTW, I've looked a little more carefully at that damaged L, and the damage is actually quite substantial. Nearly the whole depth of the conductor (I'd guess at least 80%) has 'blow out', seemingly 'from within', since the strands around the edges of that hole in the conductor are all splayed outwards, and there are globules of (previously) molten copper at the bottom of that hole. I'm therefore not totally convinced that it was not a major part of the 'big bang', although I admit that the soot/whatever track theory also seems a strong contender (although, as I've asked, what then initiated the arcing?).

Kind Regards, John

 

[JohnW2]

I suppose if the spring was weak and the contacts not forced apart far enough then the arc may have not been passed to the arc chamber - from the original pics the arc chamber looks relatively clean.

Yes. much cleaner than most other parts within the device - even parts which were much more distant from the contacts and damaged cable - virtually everything other than the chamber was heavily covered in soot.

Another option is that the cable (connected to one side of the contact) passed closer to the other side of the contact than the contact opening, the insulation being damaged caused a shorter air gap hence the arc not being passed to the arc chamber. When the contacts opened an arc formed between the two closest points and didn't quench. That would be my favoured guess now.

Credible, although I can't quite see how that damaged cable could have got into the position you postulate.

Kind Regards, John

 

[JohnW2]

Another question which occurs to me is 'where did all the soot come from?'. Something must have degraded/burned/whatever to produce it, yet there really is no evidence I can see of anything have suffered in that way - everything was 'coated' in soot, but underneath that soot there was no obvious thermal damage. Are there perhaps components of the device which had simply 'vanished' by the time the incident was over and I was able to inspect the aftermath? [for example, as already mentioned, I could not convince myself that I could find any remains of the 'test switch' mechanism]

Kind Regards, John

 

[SimonH2]

Well there's a lot of copper missing from that damaged cable. Very hot copper + oxygen gives a messy black oxide which I believe looks just like any other (carbon) soot - dunno how conductive it is.
If you look at some of the videos where they've initiated arcing across copper busbars, you get a lot of black "smoke" as well as globules of molten metal.

 

[Lectrician]

I was called to a site once that had no power. They said they had had the lecky board out and they did something, and said they needed a sparky.

I took one look, the RCD was tripped, so reset it. Thundering great bang and flash, pooped by self, and the main fuse blew.

Called lecky board, the same chap came out, and said to me "yea, it did that a couple times to me, that's why I got them to call you". Nice to have some warning!

 

[JohnW2]

Well there's a lot of copper missing from that damaged cable. Very hot copper + oxygen gives a messy black oxide which I believe looks just like any other (carbon) soot - dunno how conductive it is. If you look at some of the videos where they've initiated arcing across copper busbars, you get a lot of black "smoke" as well as globules of molten metal.

Yes that's all true. I suppose it strengthens the argument that the development of that 'hole' in the cable may, indeed, have been a major (maybe the only) cause of the problem - even though there are obviously some people here who feel that is unlikely. The hole in that conductor (and missing copper) is certainly the only truly 'damaged' component I can find - unless, of course, as I've suggested, some components are completely missing because they have totally 'vapourised'!

I guess we'll never know the answer with any certainty.

Kind Regards, John