The PLC input card counts phantom pulses. One tech says ground the shield at both ends. Another says one end. They’re both right, and they’re talking about different signals.
Low-frequency signals: ground one end
Analog signals like 4-20 mA and 0-10 V run at low frequency. Ground the shield at one end, usually the control cabinet. Ground both ends and you get a ground loop: a difference in potential between the cabinet and the field device pushes current through the shield, which couples noise back into the signal pair.
One end breaks that loop. The shield still works for electrostatic shielding; it just isn’t carrying current.
High-frequency signals: ground both ends
Encoders, fieldbus, and high-speed pulse trains need the shield grounded at both ends. At high frequency, a single-ended shield doesn’t reflect EMI; the shield itself acts as an antenna. You need 360-degree shield bonding at both ends to short the noise to ground.
The fine print: both ends requires the two ground points to be at nearly the same potential. If the cabinet and the field device are on distant grounds, bond them with a copper strap. Otherwise both-end grounding creates the loop you were trying to avoid.
The shield is not a safety earth
Don’t land the shield on the PE bar thinking it doubles as a protective earth. The shield wire is thin and can’t carry fault current. It also makes the noise problem worse. Use it for shielding only.
Don’t bundle input and output
Switching output loads induces spikes on adjacent input wires. Keep outputs and inputs in separate cables. If they have to share a tray, put a metal divider between them.
