Where should cable shielding be grounded?

Where should cable shielding be grounded?

• A. At both ends
• B. At the source end only
• C. At the supply ground
• D. At one end only, typically the PLC/DCS side

Answer: (D)

Grounding the cable shield at a single point provides a clean path for interference to escape without creating circulating currents between different grounds. In a 4–20 mA loop and HART wiring, connect the shield to chassis/earth ground at the controller side (PLC/DCS side). This gives the shield a defined reference and a low-impedance drain for EMI, while keeping the signal conductors free from a loop that could pick up noise. Grounding the shield at both ends can introduce ground-loop currents if there’s any voltage difference between the end grounds, turning the shield into a conduit for noise rather than a shield. Leaving the far end ungrounded avoids that loop while still protecting the signal path, as the shield will shunt interference to the grounded controller. Ensure a solid, low-impedance bond at the controller’s shield connection to the plant ground.

Grounding the cable shield at a single point provides a clean path for interference to escape without creating circulating currents between different grounds. In a 4–20 mA loop and HART wiring, connect the shield to chassis/earth ground at the controller side (PLC/DCS side). This gives the shield a defined reference and a low-impedance drain for EMI, while keeping the signal conductors free from a loop that could pick up noise. Grounding the shield at both ends can introduce ground-loop currents if there’s any voltage difference between the end grounds, turning the shield into a conduit for noise rather than a shield. Leaving the far end ungrounded avoids that loop while still protecting the signal path, as the shield will shunt interference to the grounded controller. Ensure a solid, low-impedance bond at the controller’s shield connection to the plant ground.