I have a situation where I would like to connect several 2, 3, and 4 wire current loops to the same PLC input card (current sinking). The PLC card has one common, and it's isolated internally. I've got it drawn up as follows:

The two wire devices are in series with my 24 VDC power supply (parallel with each other), with the negative terminal of the P/S connected to PLC common. The output of the devices are connected to the PLC input channels. Standard wiring for a loop powered device connected to a sinking input card.

The 3 wire devices are in parallel with the 2 wire devices (using the same power supply), with the only difference being the negative terminal of the 3 wire device (third wire) is connected to PLC common (which is connected to the negative terminal of the P/S).

The 4 wire devices are all powered through 120 VAC. The positive terminal of the devices for the current loop are connected to the PLC input channels, and the negative terminal of the devices are connected to PLC common (which is, again, connected to the negative terminal of the P/S).

I wish I could upload a drawing. Basically, The negative terminals of the 3 and 4 wire devices are connected to the (-) 24 VDC which is connected to PLC common, while the 2 wire loop devices are wired using the 24 VDC supply as their excitation source. 24 VDC supply is also the excitation source for the 3 wire devices.

This is the proper configuration, correct? I shouldn't have any issues with ground loops?

Thank you guys in advance.

 

All looks fine except the three wire wiring. wherein the third wire should be connected to current sink in the PLC instead of ground, is it not?

Regards,
Manoj

 

It sounds like a single ended PLC AI card. Good luck.

I assume that the 'PLC common' is the (-) side of all the analog inputs?

Whether there's a ground loop will depend on the isolation provided by the 3 or 4 wire devices; most all loop powered 2 wire devices are isolated.

If it were me, I'd connect the 2 wire devices first, expecting no problem.

Then I'd connect the 3 wire devices and see whether each has a valid reading.

If everything is fine, then I would connect the 4 wire device outputs (expecting a problem) and see whether either affects any of the other already connected channels, like offseting a value or driving the values offscale.

Let us know what you find.