On 31 Dec 2020 at 11:03:29 GMT, "No Name" wrote:
On 30/12/2020 22:36, John Rumm wrote:
On 30/12/2020 17:34, RJH wrote:
Not known this before. Friend's cassette deck is chewing tapes, so I
said I'd
take a look. Plugged it in and flash/bang from the plug, blew the 13A
fuse in
the extension, and tripped the socket circuit on the main board.
Anyone know
the cause?:
https://www.icloud.com/sharedalbum/#B0zGf693Zhiyh7
Looks like the fuse exploded.
One guess would be that the live wire was fitted to the terminal when
removed from the plug, and then the terminal replaced and the fuse
inserted.
If they had left some long strands on the wire end poking out the back
of the life terminal, they would be folded up and sandwiched between the
end of the live terminal and the plastic moulding.
Putting the top on the plug could have folded a strand towards the earth
terminal. Then it would only take a small shift in the wire, with a
knock or something, to create a L&E short via the strand. Once power was
applied, it would blow the strand and start an arc, which then
established between the edge of the earth terminal, and the screw in the
live terminal. The vaporised metal and soot being the end result.
There was actually two stages to this event. (I ams speaking as a
doctoral level materials scientist here who has done failure analysis
using scanning electron microscopes with EDS/WDS onboard and XRF)
An arc/plasma has formed between the earth terminal and the live wire's
terminal. As suggested by othe, this may have been formed via a stray
strand.
It requires less energy to sustain an arc than to create the arc in the
first place.
This produces a vapourised & ionised metal arc plasma.
The clue is in that the earth pin has molten metal on the bottom right
hand corner and bottom edge only and you can see the molten metal also
on the top left hand corner of the live wire's terminal. Both regions
are pointing to each other and the closest physically.
A layer of brass (copper & tin) metal has been deposited on the
insulative area between the earth terminal and the live terminal.
This deposited brass will be electrically conductive......
So that live wire terminal actually becomes part of earth once that 13A
fuse blows.
(In PVD, PLasma vapour deposition, a plasma is used to deposit metal
onto a non-conducting surface.)
you then get a second flashover event from the now earthed live terminal
to the top of the plug's live pin which is also the bottom cap on the
now open circuit fuse
Then there was an explosion which then deposited carbon all the way down
to the top of the plugs live pin and the bottom cap on the fuse and it
is this event that popped the 32A breaker. I seem to recall that the
real current achieved will be several times higher than that 32A rating
(John Rumm and ARW can probably work this out from the Adiabatic
equation! :-) along with the time it took for the 32A MCB to open and
break the arc..... )
You can see the carbon deposited all around the plug cover's fuse "shell"
So what initially started as a molten copper-tin arc plasma between then
became a carbon arc plasma. (don't forget, if you already have hot "air"
with metal particles in it, its easier to sustain an arc than to
initiate it !)
EDS/WDS in a SEM will confirm the surface region between the earth pin
and the live wire pin to be coated in copper and tin, and that the
region between the two fuse ends is carbon.
Thanks, interesting!
To the OP:
1. I would consider testing the "blown" 32A MCB and replace if necessary
as its had a few hundred amps pass through it transiently
It tripped and seems to have reset successfully. Not sure how to test it
though - the doesn't have a test button like the RCD.
2. If you have an RCD before the MCB, I would get this tested as this
*should* have tripped with the first brass arc as this is a L-E Short.
This would have protected both the 13A fuse and the 32A MCB and
prevented the carbon arc from forming.
The MCB tripped too. I'll test it tomorrow.
3. If you don't have an RCD, then consider getting one installed!
Perhaps you have that Wylex board where the rewirable carts were
replaced with those plug in MCBs? In this case you have no RCD.....
I think that side of it is OK - sockets and shower (but not lights) are on an
RCD protected circuit.
--
Cheers, Rob