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Galvanic isolation means there is no direct electrical path for current between two circuits, so differences in ground or noise on one side cannot be transmitted to the other. In instrumentation, this isolation protects equipment and signals from ground potentials and interference while still allowing the signal to pass through. The typical way to achieve this is by placing an isolator between the transmitter and the PLC. The isolator transfers the signal across a barrier using non-conductive coupling, such as an opto-isolator or a transformer, so there’s no DC path linking the circuits. The other options rely on shared conductors or grounding, which defeats isolation: a shielded cable with a common ground provides a ground reference; a direct copper link ties the circuits together; a ground loop creates circulating currents. An isolator is the standard means to maintain galvanic isolation while conveying the signal.