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New GU10 50W Spot lights Stop working Just After Putting in?

Postby Ressull » Tue Aug 04, 2015 6:22 pm

Hi, I'm a novice when it comes to electrics and looking for some help with a problem.

I have 9x spot lights (gu10 50w bulbs) in my living room that were installed around 2 years ago. I replaced 2 bulbs the other night and an hour later they all stopped working. It has not tripped the fuse and all other rooms on the same fuse are working ok. There is a 2 gang switch and a dimmer at the other end of the room.

I've tried removing all the bulbs and putting 1 new one in and also disconnecting the dimmer switch in case it was causing the issue but no luck.

Any suggestions what it could be?

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Postby ericmark » Wed Aug 05, 2015 10:59 am

Likely the dimmer switch, but not enough info to really say. However some pointers.

The Quartz bulb is designed so the quartz is so hot that the tungsten which migrates from the filament can't adhere to the quartz so it instead returns to the filament. Over time the filament will vary in thickness so will rupture. So quartz lamps should not be used with a dimming switch.

When a lamp blows normally the filament just parts but some times it can ionise the gas inside the bulb which then becomes conductive this causes a bright flash until the fuse or trip disconnects the circuit. The current flowing when this happens is only limited by the loop impedance of the circuit and can take out dimming switches specially if the wrong size fuse or MCB has been used.

Although the official limit for lighting is 16A in most domestic wiring ceiling roses and other items are only rated at 6A so normally a 6A MCB or 5 amp fuse is used with domestic. With a type B (will have B6 on unit if 6amp) it needs up to 30A to trip the magnetic part of the trip with a type C then it's 60A to trip magnetic part.

There has been a move to fit type C RCBO's in some houses which unless the loop impedance is below 3.8 ohms means the magnetic part will not operate only the thermal part will work which in turn means lamps popping often takes out the dimmer switch. In fact even had it weld the contacts of a lamp so have to change whole fitting.

The LED lamps do not work well with dimmers even if you get a dimmer that will reduce the output then colour stays the same and so you lose the ambulance from the warm reddish lamp. However LED lamps last a lot longer than tungsten and I now have very few tungsten lamps left.

I would replace the dimmer as really 1960's today we use the 1/3 and 2/3 twin switches so what more light then switch on more lights simple. And also move to LED or cold cathode bulbs.

The problem is the spread. With MR16 bulbs (stands for multi-factiated 16/8 of an inch why not called 2" I don't know) the reflector give a set angle of light. With LED bulbs the LED tends to give more light at the centre but not the very defined cut off point of the reflector type. So there tends to be a very different light patten.

With lights aimed at pictures this can be a problem, but where the light is aimed at a white wall so the reflected light gives general lighting the angle is not a problem.

It seems many people when installing spot lights did not realise the whole idea of having the light reflect and the ceiling looked like a planetarium however there are some GU10 to E14 adaptors and using candle LED bulbs instead of the spot lights gives a far better spread of light. Before LED the heat from bulb would have been a problem but that's not the case with LED.

The ones I did worked well it looked as if the fitting was designed to take candle bulbs. It also reduced size of bulbs I used 10 x 3W candle bulbs for a large living room. It was really too bright. Originally used 35W bulbs.
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Postby Ressull » Wed Aug 05, 2015 5:51 pm

Bio ericmark, thanks for the detailed reply. I bought a new switch today to replace dimmer and it now works so your hunch was right

Thanks for your help
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