What caused this pcb to blow out?

[MonkeyFinker3]

Hi. A few days ago my cooker extractor fan made a loud bang and sparked when I tried to turn it on.

The trip had tripped on the fuse box.

I turned off the electric and it dissembled it enough to remove the control box which is basically just a pcb with 5 switches mounted to it.

On inspecting the pcb it was clear that there had been a major spark/arcing which had left some scorch marks on the pcb. The track looked bad in a couple of places.



I tested for continuity and that confirmed it had burnt the track so I soldered some wires to bridge the damaged tracks then added arrive hot glue to insulate and secure everything. Put it back together and it all works fine.

So that's all good but I'm wondering what caused this. I read that tracks can degrade with age, humidity which would be appropriate to the situation.

Bit perplexed that the tracks have out in 2 places at once and also AFAIK it's on the circuit which is the light switch hence low current and the bulb hadn't even blown.

Thoughts on the cause and safety of continuing to use?

[MonkeyFinker3]

Sorry was struggling to add picutres on my mobile.

I should mention that at the time the spark/bank happened I was just turining on the appliance, had slightly wet fingers altough very much doubt any water got past the buttons, switches and through into the pcb. Was also possibly trying to turn the light on and the fan at the same time ie pressing 2 buttons at once...

the damaged board

1670023757788.jpg (132.37 KiB) Viewed 1127 times

my repair

1670023757765.jpg (145.35 KiB) Viewed 1127 times

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the circuit diagram

1670023757797.jpg (183.03 KiB) Viewed 1127 times

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the flip side showing the switches

1670023757775.jpg (230.77 KiB) Viewed 1127 times

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[big-all]

no expert at all but perhaps moisture and enough contamination to cause bridging
a path burnt out on a printed circuit board can only happen when the load exceeds the ability to conduct the required load or a stray path causing a short to another path are there any discolouration off components on the board especially on the circuit route near that point
as the board looks otherwise clean i assume the path was via a connecting metal surface close to the point off damage when secured in position
but purely a guess

[aeromech3]

As with big-all, I would be thinking about a low rated fuse (3amp) in circuit; the bridge you have made might have been the weak link to a current draw to earth; have you got RCD protection as you only mention fuse box?
Very often a light bulb goes in an extractor hood causing a trip, but your lights all working in the hood.
The other possibility is that one of a switch contacts are high resistance and causing hi current local overheat.

[Someone-Else]

The trip had tripped on the fuse box.

What trip? (MCB or RCD)

Either way what you have done is dangerous and has made matters worse. (I shall explain)

When something goes wrong, the weakest link fails, in most cases this will be a fuse. But in your case it was a track on the PCB. As you have now replaced the track with something thicker (The piece of wire) if the problem occurs again, it will be something else that fails which will result in a bigger bang and more damage/danger.

What you should have done (and can still do) is to replace the piece of wire you soldered in, with something smaller, e.g. a single strand of the wire you used.

As for the cause, I do not have enough information to make a suggestion.

[MonkeyFinker3]

no expert at all but perhaps moisture and enough contamination to cause bridging
a path burnt out on a printed circuit board can only happen when the load exceeds the ability to conduct the required load or a stray path causing a short to another path are there any discolouration off components on the board especially on the circuit route near that point
as the board looks otherwise clean i assume the path was via a connecting metal surface close to the point off damage when secured in position
but purely a guess

No other components on the board - its just the switches. Theres clearly scorch marks/discolouration you can see in the photos, the shape does seem to be centered around one of the traces but the tide mark does extend to the other failed trace. The board sits within a fairly tightly fitting plastic box (which also has some scorch marks) so no path via an external metal surface.

[MonkeyFinker3]

As with big-all, I would be thinking about a low rated fuse (3amp) in circuit; the bridge you have made might have been the weak link to a current draw to earth; have you got RCD protection as you only mention fuse box?
Very often a light bulb goes in an extractor hood causing a trip, but your lights all working in the hood.
The other possibility is that one of a switch contacts are high resistance and causing hi current local overheat.

No, nothing modern like an RCD, literally just an old fuse box. The circuit has a 3A breaker in it. Actually there are 2 lights, one failed probably about 18 months ago and I hadnt got around to replacing it but the other is absolutley fine.

Yes I was wondering if it might be some consequence of operating the switches. Not sure how foolproof they are particularly if you operate 2 at once as i believe i did

[MonkeyFinker3]

The trip had tripped on the fuse box.

What trip? (MCB or RCD)

Either way what you have done is dangerous and has made matters worse. (I shall explain)

When something goes wrong, the weakest link fails, in most cases this will be a fuse. But in your case it was a track on the PCB. As you have now replaced the track with something thicker (The piece of wire) if the problem occurs again, it will be something else that fails which will result in a bigger bang and more damage/danger.

What you should have done (and can still do) is to replace the piece of wire you soldered in, with something smaller, e.g. a single strand of the wire you used.

As for the cause, I do not have enough information to make a suggestion.

Neither an MCB or RCD just an old fuse box with the plug in fuses.

Yes I take your point re increasing the load capacity of that part of the circuit and how it would lead to escalation to the next weakest part.

THat said putting in a thinner piece of wire might encourage future failures esp as I don't know the load carrying capacity of the traces so might underrate the wire and it might then routinely blow out. Apart from the risks of that - sparks/fire etc its a pain in the backside to remove this switch (took about 2 hours due to its fiddly inaccesibility) so dont want to create a constant maintaince issue.

The thing is though would the bulb not act like a fuse in a very high current scenerio?

Indeed the more I think about this the fact that the bulb didnt blow makes me think that a direct short- possibly due to water ingress might have been the cause. Still can't fathom why the board failed in 2 places but maybe that is the effect of shorting?

[ericmark]

I would expect some where ionisation took place. It could have been anywhere, any one of the switches could have caused the spark, which caused the ionisation, but there should have been a fuse some where that should have ruptured rather than take out the track.

Normally air is a good insulator, but some times it ionised, electric storms are a good example.

It also happens with old tungsten light bulbs, some times you would get a bright flash when the filament ruptured, often causing the MCB to trip, as it would act faster than the fuse which should have been built into the bulb.

As to if the 3 amp fuse would blow before the track is hard to say, or even if fitted with 3 amp fuse, I have never checked my house fuses, could not say if any are too big.

[OnlyMe]

Nice soldering job

[Someone-Else]

THat said putting in a thinner piece of wire might encourage future failures

It, wont.

The thing is though would the bulb not act like a fuse in a very high current scenerio?

No. It is a lamp, not a fuse. Also a fuse is in series with what it is designed to protect, when it fails it cuts the supply. A lamp is in parallel with the supply, when it fails everything else still continues to work.

Don't complain you were not told.

I will leave you with this thought. The best time to stop a failure is before it happens.

[OnlyMe]

THat said putting in a thinner piece of wire might encourage future failures

It, wont.

The thing is though would the bulb not act like a fuse in a very high current scenerio?

No. It is a lamp, not a fuse. Also a fuse is in series with what it is designed to protect, when it fails it cuts the supply. A lamp is in parallel with the supply, when it fails everything else still continues to work.

Don't complain you were not told.

I will leave you with this thought. The best time to stop a failure is before it happens.

Apart from saying the OP is wrong do you have anything positive to say?

[Someone-Else]

Apart from saying the OP is wrong do you have anything positive to say?

Yes, but it would be pointless.

[OnlyMe]

Apart from saying the OP is wrong do you have anything positive to say?

Yes, but it would be pointless.

That translates as you have no idea

[Someone-Else]

You are using the wrong translator.