Grounding control loops

Hard to say without a drawing. If I understand your circuit, the "main unit" is in the middle, remote sensor at one end and DCS at other. If so then the main unit has a 4-20ma input (from remote) and a 4-20ma output (to DCS). Looking at the diagram or spec sheet of the main unit will tell you if the input and outputs are isolated from each other as well as isolated from ground. Typically they will be isolated; meaning none of the terminals (+/- source/return) on main unit will have continuity between them and each cable shield could then have it's own ground. I prefer twisted pairs, and no grounded shields.

It is clear that you are talking about two seperate current loops one starts from gas deterctor and ends up with main control unit, the second loop is to retransmit the 4-20 ma to the DCS. you have to treat them as two different loops.

I stuck to the same philisophy.

Ground at central point and use galvanic isolating apmlifiers to disconnect all problem areas or wrong way round earths.

Ground loops are too difficult to find and once they are there you can pick up noise that is in the ground if the electrical earthing is suspect.

I cannot speak to your specific installation because I've learned the hard way that rules of thumb and generalities are only starting points. You must have detailed knowledge of all your components, the degree and type of isolation and how they use the earth ground and for what purpose. It's common to have a variety of components in the same cabinet that have adverse affects on one another with respect to a common earth ground. It is often the cause of erratic and intermittent faults.

In large factories the typical earth ground loop is about the last place you want to connect anything to. "Dirty" does not do justice. Use your own grounding rods or points.

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