Grounding and Shielding Cable
Although the primary purpose of requiring Screened Twisted Pair (ScTP) or Shielded Twisted Pair (STP) throughout most of Europe is to prevent network signals from leaking out of the cable, most people think of shielding as a way to prevent signals from leaking into the cable. Although the use of shielding is a good way to meet both requirements, there are some potential problems.
Figure 2-28 shows one style of shielded metallic connection box. Part A shows the parts of an ScTP cable. Part B shows one type of wall jack where the shielding fully encloses the end of the cable, with an arrow indicating the point where the cable shield connects to the jack shield. Part C shows a cutaway of a correctly terminated fully enclosed ScTP 8-pin modular plug (RJ45), with an arrow indicating the point that the cable shield connects to the plug shield over 360 degrees (full circle). Below that is an incorrectly terminated ScTP cable, where the cable shield does not enclose the wire pairs completely into the shield of the plug.
Figure 2-28 One style of Screened (ScTP) connection
The fundamental purpose of a shield is to fully enclose a signal so that no radiated field can enter the cable and disturb the signal lines, and equally important, so that no field is radiated out of the cable, where it could interfere with other electronic devices. Note that it is absolutely essential that shields fully enclose the signals in every regard. Extending a drain wire even a short distance past the shield of a cable to make a connection defeats the quality of the shield significantly (see part C in Figure 2-28). Proper installation requires mounting clamps that are located inside enclosed metal spaces, so that openings are absolutely minimal. Coaxial cabling systems and connecting hardware lend themselves well toward this goal.
Generally speaking, a connection to ground is made for personal safety reasons. To meet current safety requirements, almost all powered equipment must have a third wire safety connection to ground. The issue then becomes where (at what locations) connections have to be made between the earth ground (chassis) and the shield.
All earth ground connections eventually lead to a building ground location. Voltage potentials in the earth ground lead are caused by leakage currents in the various pieces of electrical equipment. The leakage current times the resistance of the ground wires cause voltage potentials, which easily can exceed several volts. Voltage potentials between buildings are generally very significant. Lightning is another important consideration when connecting buildings. For data communication between buildings, fiber optic connections are the only practical and safe solution.
You do not want to have a cable shield become a ground return path. This can be avoided in one of two ways:
- Permit only a single connection between earth ground and the shield. The recommended end for grounding an ScTP or STP permanent link is at the wiring closet end, where the ground should be bonded to the building earth ground.
- Make certain that there is no substantial voltage potential between the earth ground connections of the equipment and any connection to data communications systems. If there is no voltage, there will be no current, and therefore no problem. This is the solution that is followed for shielded twisted-pair cabling systems (STP and ScTP).
In coaxial cable systems (10BASE2 and 10BASE5), the connection between earth ground and the shield is made at one location in the cable system. This is typically done at one end of the coax run. At all other locations there is isolation between tap connections and any earth ground source. For 10BASE2, protective plastic caps are often used to prevent accidental contact between the BNC "Tee" connector and the PC chassis. Most WAN and wireless coax use is point-to-point with little opportunity for shorts, unlike coaxial Ethernet.
Coax may be tested by measuring the current flow between the shield of the cable and the shield mating connection on the end equipment. Disconnect the cable and measure between the shield connections with a digital multimeter. Less than 20mA is unlikely to disrupt data.
When using shields with twisted-pair cabling systems (ScTP, STP), you can verify the absence of ground loop potentials by testing for them after all non-LAN electrical equipment has been installed and is operational. Then activate the LAN equipment and measure the voltage potential between the shield of the other end and the chassis of the equipment to be connected (see Figure 2-29). If the voltage is less than 1 volt AC, you may be reasonably assured that there will be no ground loop effect.
Figure 2-29 Measuring a cable shield for AC voltage with a digital multimeter
If the voltage is substantially higher, you must locate the source of the leakage. This normally involves working with a qualified electrician to correct the problem that is creating the voltage potential. This is not always easy to do, and if not possible, you should convert the connection from copper to fiber optic cable.