Issues with Screening

TIA/EIA 568-B.1 clause 4.6 requires that the drain wire of all screened cables (ScTP) be terminated at both ends. In the equipment room the drain wire is terminated to the telecommunications grounding bus bar. In the work area the drain wire is grounded to the chassis of the local equipment, which connects to the local power grounding system.

Unfortunately, as explained in Section 6.12.2, "Immunity to Large Ground Shifts," it is never a good idea to make direct ground connections between systems with separate AC power inputs.

To the credit of TIA/EIA 568-B.1, the standard does stipulate that prior to connection, the AC voltage between the two ends of each cable shall not exceed 1 V rms. That's a nice-sounding number, but in the field you can't expect technicians to check this specification. Even if your installation people do check, they aren't likely to observe the worst case because currents circulating in the ground fluctuate wildly during the day corresponding to the amount of electrical power consumed by major metropolitan areas. [78]

[78] For example, a surge of ground noise occurs during the Super Bowl at halftime when viewers all over America activate their blenders and refrigerators.

POINT TO REMEMBER

• Even though screened cables are heavily favored in Europe, this author does not recommend their use.