Ceiling Light Fitting Blowing Bulb when Switch was off? Ask questions and find answers to many subjects relating to electrics and electrical work

[Emjones23]

Hi, I had a light bulb blow yesterday in our bathroom which tripped the fuse box. We removed the bulb and reset it. Tonight we tried to fit a new bulb. With the light switch off we tried to screw the bulb in but it had an almighty flash and popping sound and tripped the fuses again. Why would this happen when the switch was off??

[ericmark]

I would say clearly not off even if switch in off position. When a tungsten lamp filament ruptures you can get ionisation of the gas in the bulb often a very bright flash. This is what blows the fuse or trips a trip. The current for these few seconds is very high. A B6 MCB is a double unit the thermal bit is set to 6 amp but the magnetic bit is set to between 3 and 5 times that value so 30A can flow but only for 0.01 of a second.

Some people have put C6 or D6 MCB's in the lighting circuits which means 60 or 120 amp can flow or swapped to a 10 or 16A version again current well above the normal 30A. This can cause bulbs to weld into holders and switches to weld in the on position.

Why the second bulb failed I don't know, but any crack in glass will cause them to blow.