Blown Dimmer Switch / Bulb - Lights now always on?

[Mac Sami]

Thursday night one of 4 50W GU10 bulbs blew. Very loud bang, glass fell out of the bulb(!) and tripped lighting circuit fuse at the fuse box.

Swapped bulb out for a new one (again 50W GU10), switched fuse back on at the fuse box, all lights are on.

Went to press the dimmer switch to turn them off, nothing happened.

Pressing and holding the dimmer switch again didn't dim the bulbs.

Therefore seems that there is a fault with the dimmer switch (assume it's blown?), and is therefore keeping the lights on (does this make sense - I would have thought the lights wouldn't switch on if the dimmer was broken)

Here's a photo of the lighting switches - the rightmost one is the dimmer switch (it's a recessed push button type - press it to turn lights on / off or press and hold to dim them up, then down, then up and so on)

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• Mac Sami
• 30 Sep 2011
• 1

As I didn't really fancy having lights on all night, I turned the lights off at the fuse box again and then swapped the blue + brown wires from a working light switch that ran to a single ceiling bulb fitting with the blue + brown wires from the dimmer. I removed the bulb from the single ceiling fitting, and then turned on the lights at the fuse box. All yellow / green wires go to a single point in the back of the faceplate (for earthing I assume).

Now I can at least switch my GU10 lights on and off (and have a ceiling light that I don't use as otherwise it'd always be on).

I think my next step is to therefore get a replacement dimmer switch - but the bit I'm unsure about is why the lights would remain on and the dimmer not work. Is it the dimmer switch that's the problem or do I need to get something else checked first?

Any advice much appreciated - apologies for the late night rambling, having just done the above work myself.

 

[ericmark]

A MCB will not work fast enough to protect semi-conductors to do that one needs a semi-conductor fuse. In theory lamps should have a built in fuse so if there is ionisation when they fail the internal fuse in the bulb will blow but it seems this is not included in many of the cheap imports.

The other thing is ceiling roses and many lighting parts are only rated at 5/6A so although the upper limit is 16A (559.6.1.6) often the limit is reduced due to manufactures recommendation (134.1.1) to 6A. In most houses this is not a problem but where GU10's for example have been added to original design sometimes a lighting circuit can use over 6A and it has been known for the MCB to be swapped from a B6 to a B10 or B16 and this can cause problems with bulbs welding themselves onto contacts and switches burning out when bulbs blow.

Semi-conductor devices are not permitted as isolators because they may not fail safe and you have prove that this is the case with your switch (which are permitted as they are not isolators).

 

[Mac Sami]

The GU10 bulbs I've purchased have an internal fuse, but this bulb was in the property when I purchased it (new build) around 3 / 4 years ago, and has never needed replacing - so perhaps a chance it didn't have an internal fuse in the bulb.

The bulb itself wasn't difficult to get out and replace - and looking at the dimmer switch it's rated for between 0.1A and 10A from memory (was looking at it very late last night!).

Excuse my stupidity but not sure what you mean about semi-conductors and isolators! Does that mean it makes sense that when the dimmer blew that the lights remained on once I'd flicked the switch at the fuse box (because it had switched off)?

 

[ericmark]

Because a dimming switch uses semi-conductors it can fail on as well as failing off. In other words it does not always fail safe.

 

[Mac Sami]

Thanks for explaining it - understand it now.

Have a new dimmer switch ordered so that should do the job.

Thanks again.

 

[ban-all-sheds]

And re fuses in bulbs - even if they have them they don't always work to stop an MCB tripping.

 

[johnnykc]

And re fuses in bulbs - even if they have them they don't always work to stop an MCB tripping.

No.....but they should at least protect your expensive dimmer.