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eskimo39

Hi guys, got a problem in the hotel yesterday which I could do with a little help on. Late last year a local company came and installed 2 dimmer switches (Cheapo rotary dials) to two of our confrence rooms. However pretty much since they have been fit, there has been numerous faults with the MCB tripping (10A with both circuits on it)

I have tested the current and am getting 3.8a and 3.6a on the 2 circuits which at 7.4a is well below the rated 10a MCB. I have also replaced the MCB to rule out a faulty MCB.

The lighting in the 2 rooms controlled by the dimmers is low voltage downlighting approx 20 x 35w 12v lamps each with its own transformer (I have no idea of the quality of these transformers either).

The dimmers are rated at 1000w each. At 230v the 3.8a would be around 870w however I have tested the voltage as high as 250v which gives a load of 950w whichmakes me wonder how well these dimmers are rated.

Has anyone come accross dimmers that cause MCB's to trip???? Is it possible that under load a cheap dimmer can start to cause it's own problems on a circuit?? Sometimes they trip straight away, other times it can be 20-60 minutes.

To test, I bridged both switches out last night and they have stayed in all night with no problems, unfortunatly due to the nature of the hotel I cannot split the 2 rooms onto 2 MCB's so have to find a way to keep them both on one. Also the manager wants dimmers so replacing with a normal switch is not an option.

Does anyone know of any decent dimmers switches rated at 1000w or above?? Or would my best option to replace all low voltage lighting for GU10 lamp holders and use 7w LED lamps???

Cheers Guys
 
could you split the lights into 2 separate circuits and fit an additional MCB? or, if the EFLI is low enough, fit a C type.?
 
When halogen down lighters are used, i'm afraid I'm still on the side of derating a switch by at least 50% to stop things like this happening.

I'm sure we will get the normal thing about how that is a myth and halogens are this and halogens are that, but it is amazing to me how often fitting a 50% larger wattage device, stops this.

I would try Lutron or Varilight Dimmers, a tad expensive but a good makes, I would also try and get a unit that is 50% above the 900W, so a 2000w unit.
 
think of a number and double it comes to mind.
 
When halogen down lighters are used, i'm afraid I'm still on the side of derating a switch by at least 50% to stop things like this happening.

I'm sure we will get the normal thing about how that is a myth and halogens are this and halogens are that, but it is amazing to me how often fitting a 50% larger wattage device, stops this.

I would try Lutron or Varilight Dimmers, a tad expensive but a good makes, I would also try and get a unit that is 50% above the 900W, so a 2000w unit.

Quite agree, this is a rule we work to where the load is all halogen. Also for some other lighting types too. We also tend to use C type Breakers on lighting circuits too.... In fact come to think of it ''B'' type breakers tend to be used in quite limited areas, but this is very much more of a larger commercial project than your usual small commercial set-up... ie, longer/larger lighting circuits than you would find normally...
 
A dimmer won't normally cause the load current to increase above that which is measured with a standard switch fitted.
BUT
For conventional wound transformers, if the waveform out of the dimmer is not symmetrical, it may cause the transformer core to saturate, the current then effectively being limited only by the resistance of the copper windings.
Either changing to an electronic transformer (or multiples), or changing to 230V dimmable GU10 LED lamps would remove the problem.
Sometimes though, it is false triggering of the dimmer caused by mains interference that results in the output waveform being distorted.

The practice of derating for certain load types is to protect the switching element (triac) within the dimmer itself.

Simon.
 

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