Security stuff: RS485 vs 220V

[Ranganok]

Hi all,

I have a question about electrical safety that I hope you can solve me.

I have a rail to connect spotlights like the photo rail:



Generally, 220V (L, N and PE) and DALI (DA + and DA-) circulate through this rail. I want to use the DALI connexions to transmit a MODBUS signal (over RS485 bus) to the lights, but I am concerned about the electrical safety of the controller and the lights (all consider the RS485 bus as SELV).

Is there a problem? Would I have to make a galvanic separation in the controller and the spotlights? Only in the controller?

Thank you

[mbs38]

Hi,

1. safety aside: where is your return path for the RS485 bus? You need three wires not two for that.

2. separation is good enough and that is all that's needed for SELV

3. BUT I would not do it, because you are laying a trap for the next person working on it...

[Ian.M]

If the spotlights have a grounded case and the DALI bus doesn't daisy chain to any other devices (other than through the rail), then all you really need is warning stickers on the lights and the rail that the bus should be treated as mains live. (which could happen at any time if conductive debris enters the track).

OTOH at the controller end, the risk of contact is significantly higher, so galvanic isolation is probably nessercery.

[mikeselectricstuff]

You can use passive receivers (e.g. IL610) for RS485 over 2 wires, as long as you don't have many devices on the bus