Why are shields used in instrumentation cables and how should shields be terminated to prevent ground loops?

Why are shields used in instrumentation cables and how should shields be terminated to prevent ground loops?

• A. Shields do not affect signal integrity and can be ignored in most installations.
• B. Shields should always be connected to earth at both ends.
• C. Shields protect against EMI; ground shields at one end to prevent ground loops; connect to instrument common or earth at a single point.
• D. Shields are only used for shielding against moisture.

Answer: (C)

Shields around instrumentation cables are used to block external electromagnetic interference so the low-level signals being carried remain clean. The shield intercepts noise and gives it a path to ground, helping protect signal integrity. To prevent ground loops, the shield should be terminated at a single point. Grounding the shield at both ends can create a loop where current flows along the shield due to differences in ground potential, introducing noise into the signal. By tying the shield to a single reference point—typically the instrument common (signal ground) or a single earth connection—you keep the shield at a defined potential and avoid circulating currents along it. This single-point termination makes shielding effective without creating ground loops. In practice, you usually connect the shield to the instrument’s ground at one end and avoid grounding it at multiple points unless a specific system design calls for it. Shields aren’t about moisture protection; their purpose is EMI protection, with grounding chosen to minimize loop currents.

Shields around instrumentation cables are used to block external electromagnetic interference so the low-level signals being carried remain clean. The shield intercepts noise and gives it a path to ground, helping protect signal integrity.

To prevent ground loops, the shield should be terminated at a single point. Grounding the shield at both ends can create a loop where current flows along the shield due to differences in ground potential, introducing noise into the signal. By tying the shield to a single reference point—typically the instrument common (signal ground) or a single earth connection—you keep the shield at a defined potential and avoid circulating currents along it. This single-point termination makes shielding effective without creating ground loops.

In practice, you usually connect the shield to the instrument’s ground at one end and avoid grounding it at multiple points unless a specific system design calls for it. Shields aren’t about moisture protection; their purpose is EMI protection, with grounding chosen to minimize loop currents.