Back EMF from contactor

Currently doing some alterations to my workshop and have been controlling lights and power from the local override buttons on my relay module due to wall switches being temporarily removed. My 'man cave' part of the shed is powered through a single phase din rail contactor, so everything is turned off when not in use and when I switched off the channel that controls the contactor, I noticed that some of the other channels flashed red briefly which I can only assume is caused from back EMF from the contactor coil. I tested other channels in this relay unit and another that are controlling contactors also and and I get the same results.

Assuming that it is back EMF, does anyone use an AC snubber across the coil to prevent this? Anything else that could be the cause?

 

I have not seen that.
Could you send a command from Toolkit or something to see if it still does the same?

 

Good call DarylMc. Will give that a test later this afternoon.

 

Interesting results: other channels would flash red when done via toolkit also so thought I would remove all load from the circuit and test again - no red flash on other channels. Starting to wonder if the load being switched off is causing a spike rather than the coil like I first thought. Don't think my oscilloscope is mains rated otherwise I would test and see what is going on.

 

Much appreciated Nick. It probably happens all the time, but who is standing in front of the relay module when an inductive load switches! My first thought was that I was about to end up with a dead relay module. Thanks for the clarification.

 

Hi Damax
Originally I thought maybe there could be a problem with the relay itself or the override buttons or the act of pushing them might have been physically doing something.

So you removed the load from the contactors and switched the contactors with the CBus relay and the problem went away?
If that is the case I think it would be a good idea to check the integrity of the supply and all the wiring terminations.
Maybe also consider if your install segregation has a part.

It's hard to imagine the man cave has so much going on to upset the CBus relay.
Obviously Nick has come across this before.
I haven't and the information is really valuable.

 

Thanks for the extra comments Darryl - My man cave has a few items that would be out of the norm including a few large computer power supplies. Starting to think that they may be causing a spike across the contacts when switched.

Will give the snubbers a go as Nick suggested but good call on the thought of segregation. Might look at relocating the contactors either further away from the relay module or even in an external enclosure from the C-Bus board altogether.

Ben.

 

Computer PSU's are something which does cause a large power spike when they get turned on.
Much more than most lighting applications.
I've had a few customers concerned about the crack when they plug them into a live socket outlet.