4-20mA PLC analog output ground referencing for other devices

Question

I'm currently using a PLC to control an induction heating coil.

I'm using a chinese induction driver that allows 4-20mA PID input. This device runs off standard australian 230v AC 50hz.

I've successfully operated the device using a standalone PID controller that also runs off mains power.

My PLC runs of 24v DC and has it's own din rail mounted power supply.

The output 4-20mA signal is referenced to the chassis of the PLC.

When I connect my PLC PID output to the chinese induction driver PID input, the driver short circuits catastrophically.

I believe this is due to the ground reference of the driver unit and my PLC being different. I confirmed this by measuring the potential difference between the two PID signal negatives and found that there was a 109V DC difference, which explains why the driver overloaded.

My question is, how can I reference the ground of my PLC to the induction driver unit?

I've gone through the circuitry of the induction driver and believe it does not have any features for sharing a ground reference. It receives it's power by L and N wires from a mains outlet. The control board of the induction driver has no grounds to it's chassis enclosure as such, I believe connecting the two chassis would provide no solution.

The standalone PID controller that worked perfectly also operates off mains power (L and N wires with no earth) and provides a 4-20mA signal that the induction driver seems to agree with.

Any information would be greatly appreciated, thank you.

Answer 1

Hard to tell without datasheets for the relevant products, but it sounds like the driver lacks isolation between the 4-20mA input and the mains. If so, this is a rather sub-standard design.

You can use it if you add a 4~20mA:4~20mA isolator between the PLC and the driver. Make sure the isolator is rated to continuously withstand mains voltage.