I have checked it with the new cable and it has continuity, 0.1ohmThe USB cable seems very thin? You might check the shield continuity from one end to the other (should be less than 1 ohm)
From computer to the USB Isolator it has continuity with the ground Earth, then, from de USB Isolator to KFLOP it has continuity with DC Ground hole of KFLOP but does not have continuity with earth ground.You might check the USB shield connections as it goes through your panel. The shield might not have connectivity from both sides of the panel and/or it may be connected to the panel which may not be desirable.
Ok, I have UF5408 and NTE569, wich of them do you recommend? Or another model?The diode across the Relay coils must be fast switching
I have a 24V Delta power supply, it is used only for the external buttons and the coils.What power supply are you using for the relay coils? Is it is isolated from everything else?
Ok, I will to replace them.Consider using Solid State Relays to possibly avoid the issue.
After changing the USB cable the problem is still present.Try disconnecting whatever the relays are controlling to determine if it is the coils causing the problem or the device the relay is controlling.
The three contactors that you can see in the picture are controlling:
1.- A trifasic async motor of 1/8HP (Screw conveyor transporter)
2.- A monofasic 120V motor of 1/8HP (Coolant pump)
3.- A monofasic 120V motor of 80W motor (Oil pump)
I have disconnected them, and the problem is gone, so it is not problem of peak voltaje in the coil, right?
Yes, it is connected to earth ground.Is the PC earth grounded?