Most intermittent weighing faults in industrial plants are grounding problems. The symptoms are familiar: the reading jumps when a motor starts, drifts when the cabinet warms up, or changes when someone touches the frame.
One ground, one shield connection
Connect the cable shield at the instrument end only. Connecting both ends creates a loop, and any potential difference between the two earth points drives current through the shield, which then couples into the signal pair. If the cell body must be bonded for safety, bond it to the machine frame with a separate conductor, not through the shield.
Isolate the cell from the frame when needed
Where the frame carries welding current or drives, mount the cell on insulating pads and provide a clean signal ground at the instrument. This is standard on welding equipment and on large drives. Check isolation with a megohmmeter before commissioning; a reading of a few ohms between cell body and frame means the isolation has failed.
Routing
Keep signal cable away from motor cables, never in the same trunking, and cross power cables at right angles where they must cross. Twisted pair with an overall shield, terminated into the connector shell, is adequate for almost all plant environments.
Kevin Huang