In shielded data cables, what grounding practice helps avoid circulating currents?

• A. Ground shielding at both ends
• B. Ground shielding at one end to avoid circulating currents
• C. Do not ground shields at all
• D. Ground shielding at the power supply end

Answer: (B)

Grounding the shield at one end helps prevent circulating currents by eliminating a closed loop for current to travel along the shield. When the shield is grounded at both ends, any small difference in earth potential between those ends creates a loop, and current can flow around that loop through the shield. That circulating current can pick up or inject noise into the signal and waste power as heat, undermining the shield’s purpose.

By grounding the shield at a single point, you maintain the shield’s ability to block external electric fields and radio frequency interference, but there’s no closed conductive path along the length of the shield for current to circulate. The shield remains tied to ground where it’s most effective, while preventing unintended ground-loop currents.

Not grounding the shield at all leaves the shield ineffective against EMI. Grounding at only the power supply end is a specific setup that can still be problematic in some layouts, whereas the single-point grounding approach is the standard way to minimize circulating currents while preserving shielding.