Dimmer protection

I guess this question relates to any of the domestic Cbus dimmers but specifically to the L5508D1A 8 channel 1A dimmers.

I note that there is a requirement for an 8A (pragmatically 10A) MCB on the input side and a 1A MCB on each channel's ouput. As a type C MCB will typically only trip when overloaded by 3-5 times rated current, my question is: does the dimmer have internal protection against overcurrent and if not, how is it normally protected?

Would seem a shame to fry the internals every time a bulb goes, and I can't believe that is what happens but they are not cheap units. Any advice about how to properly protect the product would be gratefully received.

Many thanks

 

To comply with IEC60669-2-1, it is a requirement to install an MCB on each dimmer channel, and I understand that you are required to meed this standard in the UK. It's recommended but not required elsewhere as it provides some extra protection to the dimmer channels.

The issue of protection largely revolves around the failure mode of incandescents when they fail short-circuit. When this happens, the impedance of the bulb can be quite variable based on how the filament collapses. If the filament failure results in a dead-short inside the bulb, the fault current could be hundreds of amps for a fraction of a second, not just 3-5 times the rated current. The exact current depends on lots of factors such as the supply impedance to the property, the length and size of all the cabling involved, the brand/type of bulb, etc.

Incandescents are supposed to have a fuseable section within the bulb that fails under a fault however, being a fuse, they take a few moments to pop and the fault current during this time is significant. The leading-edge dimmer circuitry is generally robust enough to handle these short circuits for a brief period of time, i.e. the triac will survive long enough to either let the MCB trip, blow the fuse in the bulb or burn off the bit of metal inside the bulb causing the short. We're talking fractions of a second here.

The issue of incandescent failure modes is slowly going to fade away, as they are phased out by other forms of lighting which have different failure modes.

Whilst leading edge dimmers typically use Triacs/SCR's, a trailing edge or universal dimmer will typically use mosfets, which are far less robust. The 4-channel DIN Universal Dimmer has built-in short circuit protection circuitry to deal with this.

 

Newman

Thank you for your thoughtful reply.

Would I be right in paraphrasing to say that the 8 channel dimmer should be able to handle a short circuit caused by bulb failure, whilst a 4 channel dimmer will be able to handle the same failure?

 

The robustness of the 4 and 8 channel leading edge dimmers is very similar. Unfortunately there's too many variables to give definitive answers however the 4-Channel Universal should perform better than the leading edge dimmers in this regard.