Exploding Light Bulb

[Lorenzo]

The light bulb in one of the ceiling lights (at the top of the stairs) just went "bang" and the glass section blew off leaving the metal part still in the fitting. The bulb was burnt looking along one side.

Can anyone suggest what might have caused this? I took the light fitting apart and all the connections are intact and there is no sign of any short circuiting. The fitting is old, but I can't see any problem. I put in a new bult and everything seems fine.

Earlier today I re-wired another light fitting (at the bottom of the stairs) because the wiring in that one had come apart and was no longer working. This is on a different fuse circuit from the one that blew, although they do share a double light switch. Is it likely to be a coincidence that this one blew so soon after?

I am not an electrician (even an amateur one) so any advice would be appreciated. Thanks!

[Martin]

I have known this happen - very rarely - I don't think it's related to the wiring or fitings.

However, I don't like the idea of two separate fused circuits feeding the same double switch.

Typically, someone working on the switch in the future might satisfy themselves that, say, the downstairs light has safely gone off when they pull the fuse - and fail to check more thoroughly before tampering with the still-live switch.

[Andrew Gabriel]

Yes -- it's the same issue which sometimes results in MCB's and fuses blowing when a bulb blows. The filament breaks and causes a small spark. The spark keeps the current flowing. The spark is lower resistance than the filament (gross over- simplification), so it grows in length, walking along the filament back to the lead-in wires. At this point, it is shorting out the supply. This all happens extremely quickly, and you will see the bright flash from the spark (arc). Normally this blows the built-in fuse in the lead-in wires. Sometimes it also trips an MCB or fuse for the circuit. If the fuses/MCB don't operate fast enough, the heat generated by the arc will cause the gas to expand in the bulb and blow the glass off. This is quite rare; I've only personally experienced it once in the UK. It's more common in countries where filament lamps don't routinely include fuses.

Ignore the incident. It might have been a faulty bulb, in not having fused lead-in wires.

[Lorenzo]

Thanks for the re-assurance Martin.

I see what you mean. I am not sure the wiring can be trusted here, which is why I put the main switch off in the house (in fact two switches - don't know why either!) before going near the fitting. I hope that everyone else is as paranoid in the future! I don't suppose I can do anything about this.

[Lorenzo]

Thanks. You have put my mind at ease that I haven't caused a problem with my tampering!

[the_constructor]

I have never seen in any house that I have been in where they have a fused lighting circuit for upstairs and a fused lighting circuit for downstairs, a separate switch just for the stairs lights. Infact, allot have downstairs hall light and upstairs hall lights on same switch.

[ARWadsworth]

I have seen this. It was quite remarkable. The lamps were new but the building was old and damp. I do believe condensation may have caused the glass to shatter in this case. The best bit was that the light stayed on for a good 30 seconds before the filament died. There was a bang, the room went dim then bright then slowly faded into darkness

Pure coincidence. I cannot see how you could make a lamp explode on a single phase circuit no matter how it is wired up.

Adam

[Martin]

Sorry if I wasn't making it clear. Yes - separate fused circuits for upstairs and down are common IME. But where a double switch is operating upstairs on one gang and downstairs on the other (common enough when stairs / halls etc can be controlled from up- and down- 2-way switching) then AIUI both gangs (and hence both up and down lights) should be supplied from the same fused circuit.