4-20mA loop grounding

[Jenny8711]

Hi all,

I'm connecting several 4-20mA sensors together in parallel (only one shown below) The enclosure is ABS plastic with metal backplate

DAQ could be powered from 24 rail or separate PSU (optional)

I am wondering where exactly to ground the shield of the sensors? (only grounding at instrumentation end, not sensor end)

Many posts have talked about grounding at common/earth etc, would this be at the 0V rail? For all sensors? Can sensors and power supply be grounded together?

Any help and advice gratefully received

Thank you

 

[robertmee]

Always ground at the supply

 

[nicherbert]

Your diagram shows 0V, but no earth ground (⏚) location. The backplate would normally be bonded with a wire to an earth ground (green or bare copper wire), mostly for safety reasons. The shields are normally then tied to this ground. They can also just be bonded together and left floating as well, but typical to bond it to the panel ground. That being said, if the panel has a power feed coming in, then hopefully the backplate is bonded to an earth grounding conductor, otherwise, a ground rod or similar grounding path should be established. The whole point is to dissipate any induced voltage on the shield to ground (earth), and in one place only. If not, you create a ground loop and the current moving through it could induce a voltage on the wires inside. Now, sometimes you will see engineers bond the 0V and ground, but that would typically be done at the power supply. If this is done, then tying it to 0V is essentially the same thing, but you need to understand if that has been done or not.

 

[Jenny8711]

OK thank you

So I understand for my own learning, do you mean:

Power feed live and neutral to PSU
Power feed earth to backplate stud
Backplate stud connects to shield cables and also PSU earth

As attached diagram

Your help is much appreciated!

Thanks

 

[nicherbert]

OK thank you

So I understand for my own learning, do you mean:

Power feed live and neutral to PSU
Power feed earth to backplate stud
Backplate stud connects to shield cables and also PSU earth

As attached diagram

Your help is much appreciated!

Thanks

Yes, exactly like you have it drawn. The entire point is to dissipate any voltage you get on the shield to earth ground. Now, keep in mind, only do this on the Control Panel side. On the sensor side, cut and tape the shield. If you stick to this method, you likely will never run into any issues.

 

[Jenny8711]

Yes, exactly like you have it drawn. The entire point is to dissipate any voltage you get on the shield to earth ground. Now, keep in mind, only do this on the Control Panel side. On the sensor side, cut and tape the shield. If you stick to this method, you likely will never run into any issues.

Thank you