Bulbs blows out of bayonet fitting when they fuse?


Postby Qwe » Sat Apr 18, 2015 10:50 am

I had a partial rewire and new consumer unit fitted
Since then when a bulb fuses it blows out of it's Bayonet fitting

Any ideas on what may be causing it ?
Thanks
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Postby ericmark » Mon Apr 20, 2015 1:35 pm

Yes likely nothing to do with re-wire.

When a bulbs element ruptures it can either just cause a little spark and stop working or it can ionise the gases in the bulb when this happens you get a very bright flash and the built in fuse in the bulb should also rupture.

However the MCB is two devices built into one. And the magnetic part will normally work at 5 times rated value (B type) which is often lower that the current even if for a very short time that a bulb which ionises will draw. So it is common when moving from fuse to MCB to find the MCB trips when every bulb blows.

Today we buy most electrical stuff from China so the fuse may be missing.

The regulations say lighting must be 16 amp or less for fuse or MCB but often we use ceiling roses and junction boxes rated at just 5 or 6 amp so in real terms the MCB/Fuse should be either 5 amp for fuse or 6 amp for MCB. However many people put in larger units and the standard lamp holder is only rated at 2A so as a result if the fuse is missing from the lamp it can melt the contacts and mean you have to change holder as well.

Quartz bulbs are very bad and as a result you have to have a glass shield to stop the very hot quartz escaping when they blow.
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Postby Qwe » Mon Apr 20, 2015 7:27 pm

Thank you
That is very reassuring. I just have to hope that the shooting bulb doesn't do any damage ☺️
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Postby ericmark » Wed Apr 22, 2015 7:24 pm

I have seen many times where MCB's or RCBO's greater than 6A are used for lighting. I personally feel it is wrong but unless one can identify a junction box which is rated at 6A it's hard to say it does not comply.

With a 6A MCB/fuse/RCBO then any bulb should not fail in a manor which releases any matter which can cause combustion. But where a larger device is used then it may allow the circuit to be energised when the bulb is no longer able to retain it's contents.

What you have to decide is the bulb or protective device the problem?

If over 6A then likely the protective device, if under 6A then likely the bulb.
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Postby Qwe » Thu Apr 23, 2015 8:19 am

This incident happened with a spot lamp as well as an ordinary Bayonet fitting bulb .
Do I understand correctly that I need to ensure that the MCB in the new consumer unit is rated no more than six amp. which incidentally trips when bulb blows

Thank you for your help
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Postby ericmark » Thu Apr 23, 2015 11:50 pm

Normally yes. There are exceptions but in the main either 5A fuse or 6A MCB.

Bulbs should not brake when they blow. But if the fuse or MCB is wrong size then it would be hard to blame bulb manufacturer.
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