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.

--
Cheers, Rob

RJH wrote:

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

Given the ablation on the earth and live terminals, I'd suggest
something bridged between them, that screw wasn't rattling around loose
in there, I presume?

On 30/12/2020 18:37, Andy Burns wrote:
RJH wrote:

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

Given the ablation on the earth and live terminals, I'd suggest
something bridged between them, that screw wasn't rattling around loose
in there, I presume?


I would be tempted to test the fuse.


A LE short at that point may not have taken out the fuse.

--
Adam

ARW wrote in news:Nd4HH.155906$GUR9.144313
@fx16.ams4:

https://www.icloud.com/sharedalbum/#B0zGf693Zhiyh7

/Or had the screw been missing and it had been arcing for ages?

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.

Looks to me like it was tracking between L and E *before* the fuse.

My money is on the fuse in the plug itself still being OK - apart from
the layer of carbon on its surface. It is scrap now.

I reckon the plug has been soaking wet at some time in the past and was
just damp enough to allow a carbonising arc path to form. The damage is
consistent with a fault inside the plug and before the fuse inside.
Brass doesn't melt all that easily so there was a high current flowing.

Last time I saw one similar a CH leak was dripping water onto it. It
went with a hell of a bang when switched on (took two goes to find it).

--
Regards,
Martin Brown

On 30/12/2020 19:10, JohnP wrote:
ARW wrote in news:Nd4HH.155906$GUR9.144313
@fx16.ams4:

https://www.icloud.com/sharedalbum/#B0zGf693Zhiyh7

/Or had the screw been missing and it had been arcing for ages?


It looks more like a short than a lot of arcing.

ie too much melted metal and soot as opposed to a overheat that would
normally be caused by arcing.



--
Adam

On 30 Dec 2020 at 19:07:25 GMT, "ARW" wrote:

On 30/12/2020 18:37, Andy Burns wrote:
RJH wrote:

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

Given the ablation on the earth and live terminals, I'd suggest
something bridged between them, that screw wasn't rattling around loose
in there, I presume?


The live screw was rattling about inside, half the thread stripped and charred
(you can just see it in the pic). To my eye it looks forced out. But maybe it
was cross-threaded and worked loose? Cable was securely clamped.



I would be tempted to test the fuse.

Fuse (13A) had blown.



A LE short at that point may not have taken out the fuse.

FWIW, the cassette deck wasn't harmed. I've replaced the plug/3A fuse.


--
Cheers, Rob

On 30/12/2020 21:13, RJH wrote:
On 30 Dec 2020 at 19:07:25 GMT, "ARW" wrote:

On 30/12/2020 18:37, Andy Burns wrote:
RJH wrote:

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

Given the ablation on the earth and live terminals, I'd suggest
something bridged between them, that screw wasn't rattling around loose
in there, I presume?


The live screw was rattling about inside, half the thread stripped and charred
(you can just see it in the pic). To my eye it looks forced out. But maybe it
was cross-threaded and worked loose? Cable was securely clamped.



I would be tempted to test the fuse.

Fuse (13A) had blown.



A LE short at that point may not have taken out the fuse.

FWIW, the cassette deck wasn't harmed. I've replaced the plug/3A fuse.



Thanks for the update and thanks for posting the photo. I found it
interesting.



--
Adam

On Wed, 30 Dec 2020 19:38:55 +0000, ARW wrote:

https://www.icloud.com/sharedalbum/#B0zGf693Zhiyh7

/Or had the screw been missing and it had been arcing for ages?

It looks more like a short than a lot of arcing.

Agreed and how would it short to the earth terminal with the plug
cover properly fitted? The earh terminal is enclosed by the mouldings
in the cover.

ie too much melted metal and soot as opposed to a overheat that would
normally be caused by arcing.

And the metal is ablated on the earth terminal, metal that looks like
it might be deposited on the plug base between the earth and live
terminals. With burnt plastic (soot) being blasted against the top
left side of the fuse. There is also a "shadow" in the soot mark
below the lower right corner of live wire terminal.

Looks like a short between the live wire terminal and earth wire
terminal. The live wire terminal is post fuse so that may be a goner
as well. I'm a bit stuck for why or how that short occured. I don't
think that water would be conductive enough or hang around long
enough with the sort of power levels required to blast molten brass
about the place...

--
Cheers
Dave.

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.

Agree with the rest of posts, but would also note that the transverse
"crack" in the insulation of the brown suggests that this may have been
overheating for a while, and hence become embrittled. A slack screw (and
long-term mild arcing within the live terminal) could have caused this.

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.


--
Cheers,

John.

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

On 30 Dec 2020 at 21:44:01 GMT, ""Dave Liquorice""
wrote:

On Wed, 30 Dec 2020 19:38:55 +0000, ARW wrote:

https://www.icloud.com/sharedalbum/#B0zGf693Zhiyh7

/Or had the screw been missing and it had been arcing for ages?

It looks more like a short than a lot of arcing.

Agreed and how would it short to the earth terminal with the plug
cover properly fitted? The earh terminal is enclosed by the mouldings
in the cover.

ie too much melted metal and soot as opposed to a overheat that would
normally be caused by arcing.

And the metal is ablated on the earth terminal, metal that looks like
it might be deposited on the plug base between the earth and live
terminals. With burnt plastic (soot) being blasted against the top
left side of the fuse. There is also a "shadow" in the soot mark
below the lower right corner of live wire terminal.

Looks like a short between the live wire terminal and earth wire
terminal. The live wire terminal is post fuse

It's not, actually.


so that may be a goner
as well. I'm a bit stuck for why or how that short occured. I don't
think that water would be conductive enough or hang around long
enough with the sort of power levels required to blast molten brass
about the place...
The missing screw might be a candidate but unless the plug cover is broken
leaving a gap a loose strand of the brown wire is likelier.

--
Roger Hayter

On 31 Dec 2020 00:51:48 GMT, Roger Hayter wrote:

Looks like a short between the live wire terminal and earth wire
terminal. The live wire terminal is post fuse

It's not, actually.

Er, wall socket live plug pin live fuse live wire terminal

Live wire terminal post (aka after) fuse...

The missing screw might be a candidate but unless the plug cover is
broken

Screw isn't missing it's in the photo(*) and was apparently rattling
around inside, the screw shows signs of melting. The mouldings in the
plug cover look undamaged, well apart from the results of the
failure.

leaving a gap a loose strand of the brown wire is likelier.

Don't think a single strand would survive long enough to do that much
damage to the terminals. I suppose it could have formed an ionised
path to sustain an arc between them.

(*) You don't get to see the full image by default, scroll around
abit.

--
Cheers
Dave.

On 31 Dec 2020 at 01:20:20 GMT, ""Dave Liquorice""
wrote:

On 31 Dec 2020 00:51:48 GMT, Roger Hayter wrote:

Looks like a short between the live wire terminal and earth wire
terminal. The live wire terminal is post fuse

It's not, actually.

Er, wall socket live plug pin live fuse live wire terminal

Live wire terminal post (aka after) fuse...

Yes, sorry you're right. Not thinking straight.





The missing screw might be a candidate but unless the plug cover is
broken

Screw isn't missing it's in the photo(*) and was apparently rattling
around inside, the screw shows signs of melting. The mouldings in the
plug cover look undamaged, well apart from the results of the
failure.

leaving a gap a loose strand of the brown wire is likelier.

Don't think a single strand would survive long enough to do that much
damage to the terminals. I suppose it could have formed an ionised
path to sustain an arc between them.

(*) You don't get to see the full image by default, scroll around
abit.


--
Roger Hayter

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.

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.

I agree 100%.

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.

Yes. again 100%. That should NOT have happened on an RCD protected
circuit. Worrying



--
"First, find out who are the people you can not criticise. They are your
oppressors."
- George Orwell

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.

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

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.

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.....

P.S.

The distance between the two fuse caps is approx 4 x the distance
between the bottom fuse cap and the "now earthed" live wire terminal.

So once the fuse wire had melted, it would have formed an arc within
the fuse casing. There is ceramic powder within the fuse casing which
act as an arc quencher.

It would have quickly transferred itself to the outside of the fuse as
the ditance is 4x shorter..... (the energy required to initiate an arc
reduces as the seperation distance decreases)

On 31/12/2020 11:03, 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.

Yup, hence my guess at a stand of flex, and as you say the erosion of
the metal on the edges of the terminals does suggest those as the
initial flashover point.

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

Yup in fault current scenarios the magnetic part of the MCB trip
mechanism will activate. That is at a nominal 5x In for a type B MCB. So
a current of 160A or more will do it.

(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..... )

Difficult to know without also knowing the loop impedance at the socket.
However since it was at the socket and not the appliance end of the
flex, it will presumably be within the normal design parameters for a
32A ring circuit - so no more than 1.37 Ohms. Chances are (looking at
the damage done) that it was a good deal lower than that. So you could
have ~500A of fault current to play with. If the MCB opens in the
nominal 0.1 sec "instant" part of the trip, that would suggest a let
through energy I^2t or 250K x 0.1 = 25,000 J, which (to put it mildly)
ought to get things toasty pretty sharpish! :-)

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.

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

In reality it would be cheaper to replace than test - since there is not
much testing you can easily do other than see if it will operate again.
Chances are that it will be fine - a few hundred amps is not going to
stress a device with a 6000A breaking capacity.

However if its a "high energy" installation - i.e. close to a
substation, or fed from a more industrial sized feed, and so with very
low external Ze, then you could get close to or even exceed the breaking
capacity of the MCB and damage it. (particularly so if its one of the
"plug in" wylex MCBs designed for the old BS3036 rewireable fuse boards,
since many of these have lower maximum breaking capacities.

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.

Chances are a normal RCD may not be fast enough to trip in these kinds
of circumstance. It will normally take at least one mains cycle (20ms)
to detect the imbalance, and the MCB will have it all done and dusted
long before that.

3. If you don't have an RCD, then consider getting one installed!

+1

Perhaps you have that Wylex board where the rewirable carts were
replaced with those plug in MCBs? In this case you have no RCD.....

and MCBs with less breaking capacity...

http://wiki.diyfaq.org.uk/index.php/....2C_6kA_etc.29

(although you still get 3kA on the 30A versions - and that is probably
adequate unless its a high energy install)

--
Cheers,

John.

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

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 |
\================================================= ================/

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

On 31 Dec 2020 at 13:12:35 GMT, "John Rumm"
wrote:

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.

Thanks - and to everyone else.

As to the 'cause of the cause', does it seem likely that there was a stray
strand at the live wire? So whoever wired the plug? I'd add it was a 13A fuse
fitted - which might suggest some lack of attention to detail.


--
Cheers, Rob

On 01/01/2021 18:21, RJH wrote:
On 31 Dec 2020 at 13:12:35 GMT, "John Rumm"
wrote:

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.

Thanks - and to everyone else.

As to the 'cause of the cause', does it seem likely that there was a stray
strand at the live wire?

Its quite possible - something caused a flashover between the L & E
terminals - you can see that from the erosion of the metal in the
location. The most likely cause I can think of is a strand of wire - but
it could possibly have been moisture in the plug... or perhaps it have
had a previous fault and that left a conductive film on the plastic in
the area.

So whoever wired the plug? I'd add it was a 13A fuse
fitted - which might suggest some lack of attention to detail.

Perhaps...

--
Cheers,

John.

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