Dimmer switch interference?

It sounds unlikely, but not totally incredible. some designs of dimmer, and some designs of lamp with switch more power supplies for that matter, manage to put several volts peak to peak of spikes onto the mains, high but of short duration, so a meter with a long averaging time does not see them.

Are these dimmers with a loop through neutral or just in series with the lamps like a traditional switch would be ? What sort of lamps are being dimmed ?  Be very wary of what meters read on the sliced wave-forms out of dimmers - it is not like measuring a sine wave and that can confuse the meter internals.

Has it always done it ? Filter capacitors in one or other dimmer  may be missing or defective. (foil type capacitors can reduce in value spectacularly over time a mains surges blow small holes in the foils, and then 'self heal' by burning back - I have cut open a few where the foils look more like a paper doily than a continuous conductive sheet.) Ideally if neutral is available at the dimmer some filtering capacitance is in order, but that may not be practical if there is no neutral or no space. If you swap the dimmers does the effect swap with them ? - i.e is it the dimmers that are source and victim, or the wiring layout...

More info needed really.

Mike

Hello Mike

Thanks for your reply, as far as I know it has been a problem for quite a while, it looks like a quite old installation with halogen down lights changed to LED at some point. The dimmer switches in the lounge are standard MK grid dimmers not the LED version. Not sure what the dimmer switch in the dining room is looks like it might be BG or something like that. They are all standard traditional dimmers with no neutral connection.

I agree about the meter and inaccurate readings but I was only really using it for comparisons and not looking for an accurate reading, the clip on ammeter is designed to do pretty much what it was using it for so I expect that to be pretty good (megger actually suggested using it in this way to identify circuits with high harmonic content). 

It could well be spikes caused by failing smoothing/filtering capacitors that would have an effect on the entire circuit in a similar way to harmonics and the ammeter might have filtered the spikes when set to only look at 50/60Hz.

I have agreed with the client to replace the lights with new KSR fittings as the existing ones look like they need replacing anyway (I’m not confident they are fire rated so will be happier with new ones!) also I have warned the customer that this may not fix the problem and to expect to replace the dimmer switches as well, this should hopefully cove it!

Hi Paul, it really sounds to me that something is miswired, have you confirmed the wiring is correct? 

That was what i thought, but its all wired correctly and the supply is comming from the dining room then to the lounge, disconnecting the feed to the lounge switches results in all the lounge lights off but leaves the dining room lights on. whatever it is its affecting the dimmer switch/light upstream!

Just a thought but some old Dimmer units cannot cope with dimming LEDs.  They work fine on Halogens with the larger load.
A simple test is to switch over some the LED bulbs for Halogen (you can often get these in charity shops almost for free)

Thanks for the reply Paul.

Odd isn't it do let us know how you get on please.

Mike

not that simple, they all dim individualy OK it is interference when both switches are on that is the problem.

I have just replaced 8off lights, they were all very old MR16 lamps with JCC electronic transformers. The new fittings are KSR QR10 fire rated downlight so much better than the open backed MR16 fittings!

This has improved the flickering by probably 95% with everything on minimum there is still a small flicker but with any of the dimmers adjusted to 25% there is no perceivable flicker. The customer is seeing how it works in real life and the next thing is to try replacing the dimmer switches with something more modern/new. As suggested it is quite likely that the old dimmers have failing or failed capacitors in the noise filtering.

(with the living room dimmer set to anything over 25% the dining room dimmer works over its full range, so its not just over dimming the lamps)