mcb chatter

[Clyde]

We found on one job that a mcb rated at 16A was used to supply a marshalling box for c-bus lighting (located approx 20m away), a buzz or chatter could be heared from the mcb when the lights were dimmed using a L5508D1A.
The load was a number of low voltage downlights supplied by individual transformers, split over several of the channels.

The supply to the Dimmer module and the lighting channels were then taken from a 6A mcb via a 100A DP isolator.

The supply cable for the marshalling box was of sufficient csa to allow us to increase the from 16A to 20A however the chatter still remained. It was only when we increased the supply mcb too 30A (still OK fro csa) that the chatter stopped.
The buzz still remained on the 6A mcb, when the lights were dimmed down low.
Can anyone please explain why the dimming action should effect devices up-stream from the dimming module, and if anyone has experienced this before

[Guru]

See links:

http://www.epanorama.net/documents/lights/lightdimmer.html
http://www.ies.nl/pages/white_papers/dimming.htm
http://www.lighting-association.com/Newsletters/September%20Newsletter.pdf
http://www.ies.nl/pages/white_papers/sinewave.htm

[UncleDick]

As the graphs from Mr Gnu shows Phase angle control gives rapid rise times when the TRIAC is turned on these typically dont give problems with domestic incandescent lights but can be noisy with ceiling sweep fans where the step change in voltage (and therfore current flow which) can cause the motor windings and/or laminations to vibrate. In the case of the MCB's you detail I suspect that like many MCB's the ones you were using have a thermal trip (for overloads from x 1.1 up to x 3 or x 5 rated current and then a magnetic trip for short circuits and overloads > x 5 rated current. The magnetic trip usually consists of a fairly basic electro magnet conected to a trip mechanisum, the coil in this electro magnet (which typically continuously carries the full load current) is as likely to rattle and hum as the windings in a motor I supose (the step change in current flow through the load will be passing through the MCB). However the missing bit in this puzzle is why this is the first time this has croped up (to my knowledge). Changing the MCB will have changed the electrical charicteristics of the electro magnet or perhaps the coil was better wound or supported. All of this is a guess on my part, as I say I havn't come across this before and have been dealing with Phase Angle dimming and MCB's for twenty years.

[Clyde]

Thanks for the help guys, I am still reading all the info 'Guru' provided, some interesting stuff.

I still cannot understand though how this can effect an mcb placed up-stream from the dimmer unit, mabye some of the bedtime reading Guru provided might enlighten.

Thanks again
Clyde :)