Selecting and terminating data cabling in an electrically noisy plant room

Sharing a tray with motor cables couples electromagnetic noise into the data cable; the varying currents in VSD output cables are a particularly strong source. Twisting rejects common-mode noise but does not eliminate it, so the data cable should be separated from power cables by the recommended distance or run in a separate tray or conduit, crossing at right angles where it must cross. On cable choice, Cat 5e supports 1000BASE-T to 100 m but has less headroom in a noisy environment; Cat 6 shielded cable with proper bonding offers better immunity, and Cat 6A is the best option for the 85 m run if the customer wants the highest certified category. The recommendation is Cat 6A shielded, installed with separation from power cables, with the shield bonded at both ends to the equipotential bonding system, and the link certified with a suitable tester.

The shield is only effective if it is bonded to the equipment earthing or equipotential bonding system at both ends, using the connector's bonding facility, so that induced currents have a low-impedance path. Terminations must maintain the twist right up to the point of connection, with no more than the permitted untwisted length, and the cable jacket should be stripped only as far as necessary. The patch panel and outlets must be bonded to the same earthing system, and the cable should not be run alongside or parallel to power cables for any distance. The contractor should verify continuity of the shield and the bonding path, then certify the link with a tester that reports the category achieved, including crosstalk and return loss results. Poor termination is the most common reason a shielded link fails certification.

Start with the physical layer: run a certification test on the link and compare results with the category limits, checking for marginal crosstalk or return loss that indicates a poor termination. Inspect the patch cords and connectors for damage or untwisted pairs. Check the shield bonding continuity at both ends. Then check for noise sources: confirm the separation from motor cables, look for a VSD output cable running parallel, and check whether the drops correlate with motor starts. Use the switch's error counters and the PC's event log to see whether the link drops or the errors are at the application layer. If the physical test passes, check the switch port, duplex settings, and power. Document each step and its result so the cause can be proven rather than guessed.