On 31/12/2020 10:49, Caecilius wrote:
On Wed, 30 Dec 2020 17:34:01 +0000 (UTC), 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.

The damage to the head and threads of the loose screw make me wonder
if that screw was involved in the L/E short.

Is there an RCD on that circuit? The damage suggests a high fault
current, but I'd have expected something in the 30mA RCD + MCB + 13A
Fuse to have limited the fault current & duration.

Well the fact that the plug is still in one piece and the house is not
on fire, one might argue it all worked as it should.

The RCD is possibly a diversion since they have no ability to limit
fault current - only fault duration.

In high current fault scenarios the fuse and MCB would operate before
the RCD will get a look in generally, since RCDs need to see a fault for
a good proportion of a mains cycle before tripping.

As others have suggested the carbon deposits on the fuse would suggest
that ultimately there was a flashover between the earth and neutral
*pins* (i.e. not just the screw terminals[1]) of the plug - that
basically puts the plug fuse out of the picture as well, once the arc is
established. So now its all down to the circuit MCB.

[1] even though the flash over between terminals started the ball rolling.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/