How should you ground your shielded Ethernet cable?

The advent of Category 7 cable, which includes four individually shielded twisted pairs, has introduced grounding to the concerns of the network engineer. The general advice of wire manufacturers, and the general configuration of pre-manufactured cables, is for both ends to be grounded. This is contrary to common knowledge in electrical engineering regarding ground loops and the potential for lightning-induced current.

Some purported electrical engineers assert that a shield grounded at only one end is “not a shield at all”, but an RF filter resonant to RF frequencies at its quarter wavelength, and will induce voltage to internal conductors at that frequency. They neglect to mention that a shield grounded at both ends also has resonant frequencies and can induce voltage to its internal conductors.

No configuration of grounding results in a shield always being a low-impedance path to ground across its entire length, for anything but DC and low frequencies. There will always be high-voltage points and null points across its length, for any frequency with a wavelength approaching or smaller than 4 times the circuit length, where the circuit includes the ground path between both ends of the shield if the shield is connected at both ends.

Grounding both ends of the shield simply increases the circuit length, creating a loop with external conductors, including conductive soil. This makes the circuit more vulnerable to lower frequencies than it would be otherwise, and admits additional possibilities for lightning currents to be introduced onto the shield. And you can’t count on the conductors being balanced such that induced currents oppose each other across the length of the circuit.

You must also consider that conductive soil is not a perfect sink to RF currents. Radio Amateurs learn that soil, unaugmented by radial wires, generally makes a poor counterpoise. The ground system in your building is anything but a theoretically perfect ground plane, and will in general present a high impedance at RF. Induced lightning voltage across the soil is a possibility with nearby strikes.

This is one reason for use of optical fiber, especially in areas where lightning-induced currents or RF interference (emitted or received) is a problem. No problems with electrostatic or magnetic induction when you use optical fiber. But we eventually transition to copper or aluminum at the ends of fiber runs. We must then consider lightning-induced currents and how to shield them.

Lightning frequencies are generally low, and energy is generally distributed across frequencies with a 1/f characteristic: more energy at lower frequencies. In this case, grounding the shield at both ends can indeed cause a shield that would be unresonant if single-point grounded to conduct significant energy. Ferrite chokes and other inductive means of increasing the external shield impedance are ineffective at blocking lightning-induced current, because they saturate.

So, I’ll be using single-ended grounding for my application, if for no other reason than that it removes so many unknowns from the equation. I suggest that those who wish to do otherwise actually attempt to model the circuit for its RF resonance and potential for induction of lightning current, using NEC or similar software.