blown bulb and poor connection

[IanJ]

I've sorted this problem already and all is well, but I'm just curious ...
(apologies in advance if I'm being thick )

Bulb blew in hall light today. Straightforward one-way switch on downstairs lighting circuit. Replaced bulb - didn't work. Tried bulb known to work from another light - still didn't work.

Traced problem to loop-in connection inside ceiling cavity, where the switched live into the light had come slightly adrift from its connection.

Question is - was the blown bulb the symptom or the cause of this?
Will a dodgy connection in a lighting circuit always blow a bulb? (although I can't think what would have caused the connection to come adrift - no recent work or movement in that area). Or are there circumstances in which a blown bulb might cause a physical failure in a connection (maybe in a connection that's already prone to failure)?

As I say, just curious, and hopefully not a stupid question ...

Cheers

 

[andy]

a dodgy contact can damage the filament, which over time will fail (blow)

 

[ban-all-sheds]

Had you noticed any flickering before it failed?

 

[IanJ]

Nope, no flickering beforehand - so just seemed like a straightforward blown bulb ...

 

[Damocles]

Did changing the bulb disturb the wiring so that the already loose connection came completely apart?

 

[Panjandrum]

Here's a theory.
Sometimes bulbs blowing cause fuses to blow too - I guess because of something odd happening inside the bulb as it flashes to oblivion.
Now, suppose there was a dodgy connection in the rose - a wire under tension under a screw that was almost holding it. Suppose this was kind of making OK contact for normal purposes. Suppose the bulb failed in one of those "blow the fuse" moments. Suppose that the excess current in fact blasted that almost-but-not-quite-good connection to bits - instead of blowing the fuse? You'd look and find a wire that appeared to have come adrift from its connection. Wouldn't you?

 

[felix]

A working bulb filament is a long length of thin tungsten wire coiled into a small space. When it blows, there's a possibility that many turns will melt into each other to form a shorter, thicker length of molten metal. If this happens, the current shoots up and the fuse can blow before the filament. Halogen bulbs are notorious for this. Have you ever looked at a blown halogen bulb and wondered where the filament went?

In the case of the loose connection, I think it's much more likely that the wire came loose when the bulb was changed but, if you want to be Sherlock Holmes, study the wire and terminal carefully for signs of burning

 

[securespark]

Ooooh, look!

I've just opened my cloakroom door, and, on a hook at the back, I can see my Osram hat.......!!!

If you buy good, branded lamps, they should have fuses built in to prevent the circuit device failing.

Osram are very good, and are double fused. This is NOT the opinion of everyone here, but where my customers have had problems with surging lamps killing the circuit, I have suggested or installed Osram, and so far, touch wood, I have had a 100% success rate. Well, 99.999999999.

One customer insisted she has put Osram in, but the failed lamp proved to be Nisa.... Boooooooooo!!

 

[Damocles]

Um, no. A broken filament bulb turns into an arc lamp. The current bridges the gap between the ends of the broken filament and the arc gets bigger and bigger until it fills the bulb. The gas/metal ionises. It is extinguished when the AC voltage drops to zero. While this is going on it has a very low resistance. The current is enough to activate the fast trip element of a 6A type B mcb, but it does not go on for long enough to melt a 5A wire fuse.

 

[felix]

Um, OK. Coiled filament -> molten filament -> vapourized filament -> ionized filament -> blown fuse.

 

[ban-all-sheds]

No.

Coiled filament -> molten filament -> vapourized filament -> ionized filament -> tripped MCB.

 

[Panjandrum]

Or are there circumstances in which a blown bulb might cause a physical failure in a connection (maybe in a connection that's already prone to failure)?

I raised the point about short-term high-current on bulbs failing as a possible sensible answer to IanJ's question. Hence I said:
"Now, suppose there was a dodgy connection in the rose - a wire under tension under a screw that was almost holding it. Suppose this was kind of making OK contact for normal purposes. Suppose the bulb failed in one of those "blow the fuse" moments. Suppose that the excess current in fact blasted that almost-but-not-quite-good connection to bits - instead of blowing the fuse? You'd look and find a wire that appeared to have come adrift from its connection. Wouldn't you?"
Take for granted that I know that bulbs blowing often cause plug fuses to blow. Now, apart from dissertations on bulbs blowing, does this make sense?

 

[felix]

I have seen bad connections that have obviously burnt away over time. The insulation is discoloured and the wire ends are blackened. If such a joint was hanging on by a whisker of copper, I think it's quite possible that a dying bulb could be the last straw. You won't need to look too hard at the wire and terminal to see the evidence/