Ceiling fan interference

I am currently experiencing interference from cieling fans being switched to the "off" position. They seem to be interfering with some outdoor PIR units. The PIR triggers its controlled load as if there has been movement. Only happens at night as the PIR's have been programmed for night movement.
Has anyone come across this? I have tried filters on the fan switch but no joy.
Any help would be appreciated.

 

Is the fan switch located directly above or below a CBus switch?
Do you have other fans and do they have the same effect on your motion sensor when you turn them off?

 

What kind of switch is used on the fan?

Is it a rotary mechanism or is the fan switched with C-Bus relays?

What kind of filtering have you tried?

 

I dont think filtering is the problem. I think the problem is back-EMF from the motors spinning down or a disconnect surge.

I would be more inclined to install a draining capacitor than a filter. Then again ... in some cases those are the same thing.

 

If the PIR is a C-Bus PIR then the most likely source of false tripping will be radiated interference. This could be due back-emf radiating off the supply wires running to the fan or it could be radiated off the switch contacts.

The easiest thing to test is probably swapping the motion sensor for another one, if one is on-hand.

An interesting test to do would be to surround the sensor with an earthed shield (such as aluminium foil wrapped around a yogurt container and then earthed) and then re-test it. If this fixes the problem then you'll know that it's a radiated emissions problem. If this works and the PIR is a 5753-series one then you may be able to leave a shield installed over the rear of the unit in the ceiling.

 

Many thanks for the feedback. (excuss the pun)
The switches are the ones that come with the fans.
Filtering to-date has been the addition of a capacitor.
It is not just one PIR being effected and not just one switch causing the problem.
The problem would appear to be EMF which the PIR is seeing as movement.
So the problem may be in the PIR movement circuit not the C-bus component.

 

I've seen some rotary fan switches interfere with things like touch lamps in the past so I figure they are a fairly noisy switch.
One problem I could imagine happening is after the CBus key inputs are installed then someone comes along and decides that the rotary fan switch would look nice above it.
And perhaps even run the 240V cables for the fan down the same hole as the pink CBus cable.
Therefore giving it maximum chance of being a problem.
Could that be the arrangement?
Maybe it's also worth logging the CBus to see if there are any commands going through when this happens.