Specific attention must be paid to instrument grounding, especially where field instruments are connected to a computer or microprocessor based control system. Where cable screens are used, ground continuity of screens must be maintained throughout the installation with the grounding at one point only such as that in the control room. At the field end the cable screen should cut back and taped so that it is independent from the ground. Intrinsically safe (IS) systems should be grounded through their own ground bar in the control room. Static grounding of instrument cases, panel frames and so forth, should be connected to the electrical common plant ground as illustrated in the figure below:
Instrument grounds should be wired to a common bus bar within the control center, and this should be connected to a remote ground electrode via an independent cable (preferably duplicated for security and test purposes). The resistance to ground, measured in the control room should typically not exceed 1 𝝮 unless otherwise specified by a system manufacturer or by a certifying authority.
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