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Safety contact on outputs

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Looking for a piece of advice here. I put safety contact on wire that brings power to outputs, and noticed while testing that sometimes PLC loses serial connection with laptop when safety is activated. This made me wonder is any circuitry in controller powered from this side and is this good practice or should I change it?

Jazz controller is on the picture but I also noticed this on Vision.


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Hi Isakovic

I have found that this can happen when loads are switched on or off.

In your case multiple loads would come on and off with the KZ contact.

All relays should have flyback diodes and all AC loads should have RC snubbers.

Thers are some very good topics on this in the forum and I have got a lot of help from Joe, Keith others on this at the following forum.



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I would make a couple of comments:

  • This is not good practice for safety engineering.  The way you have it, the PLC is in between the safety contacts and the downstream equipment.  For example, if the PLC failed (for any reason) and put 24V on the outputs (say from an internal short circuit), the safety contact would have no effect.  A better circuit would place  the safety contacts *after* the PLC outputs, and yes you would then need 3 contacts.  I say this is a "better" circuit, but it still may not be the correct circuit for your application.  You would need to check your local safety regulations and follow all requirements of those regulations.
  • With the problem you are experiencing with comms loss, I suggest it would be an EMC disturbance resulting from the sudden loss of power to the PLC output circuit and/or the drive it is controlling.
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Thanks for input guys, that's a good thread sgull, very informative.

There's always one safety contact for contactor command voltage for motors and in run command circuit of VFDs. Maybe cutting power to outputs has sense only for relay models, especially with disturbance it is causing.

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