I am working on my wiring diagram for my analog outputs. I am going to use a signal conditioner/ barrier to send intrinsically safe 4-20 mA to my control valves in the field. I am using two pair shield cable to wire the control valve to the terminal blocks in the control panel. Inside the panel, I will use another cable to connect the terminal blocks to the barrier IS terminals (see attached sketch). Then on the Non-IS side of the barrier I will use another cable to connect the barrier to my PLC analog output card. My question is, since I am using shielded cable, where should I connect the shield/drain wire from the cable that is between the PLC analog output and the Non-IS side of the barrier. The reason for this question is that I do not want to create any ground loops, but I am not sure if the non-IS cable shield should be landed on the ground dedicated to IS signals. I hope the sketch make it easier to understand what I am talking about. I would appreciate your feedback on this matter
The NIS ground should be at the cabinet's electrical ground point. Typically, you'll put your 120V grounds on a pad, your 24V neutrals on a shorted TB connected to the 24V Power Supply 0V/Return, and lastly, your shields to a separate pad.
A typical installation will have three terminals dedicated to each T/S pair. The first is signal, second is return, third is shield. Some folks use 5 terminals for Sig-Ret-Sh+Sh-Sig-Ret, then repeat for the next two pairs. This saves one TB for each two pairs. Lastly, all of the pairs are wired to the shield pad.
Each of the separate pads or return TB's are then routed to the system ground pad through a single large wire depending on the size and number of conductors. Three pads means three wires.
The same can be done on the IS side, but the TB's need to be dedicated to the IS circuits - no mixing IS and NIS shields or returns on the same TB/pad; although everything eventually ends up on the ground block going directly to the ground pit/central point.
The IS wiring needs to be light blue in coloring outside the cabinet. I don't know if this is just standard practice or by code, but I follow it, so I never had to look it up. All of the IS grounds need to be done in accordance with the classification of the area where they go. The shield grounds are usually at the cabinet, inside, on the IS "side." Don't use light blue for any NIS circuits, ever. Experienced folks will see the light blue and assume the circuit is IS.
Lastly, the glands for the IS circuits in the field have to be rated for IS, and stamped. Keep ALL of the IS certification paperwork in a single binder. Keep a copy, too, but only the originals are acceptable for inspections. Get a signature for any paperwork you turn over, so when it gets lost and everything has to be replaced (you can't recertify a system in the field for all intents and purposes), you'll be covered. Do not remove any nameplates, obscure any serial numbers, or otherwise modify any fittings. You are allowed ONE certified reducer at each gland, if size change is needed.
Lastly, if you don't really know IS, find someone who does, or you're going to make a lot of mistakes. I hire IS engineers for consulting and panel building, to make sure it is done right, most of the time.
