Screened and Shielded Cabling - Noise Immunity, Grounding, and the Antenna Myth
Grounding and Cabling Systems
ANSI-J-STD-607-A-2002 defines the building telecommunications grounding and bonding infrastructure that originates at the service equipment (power) ground and extends throughout the building. It is important to recognize that the infrastructure applies to both UTP and screened/fully-shielded cabling systems. The Standard mandates that:
- The telecommunications main grounding busbar (TMGB) is bonded to the main building service ground. Actual methods, materials and appropriate specifications for each of the components in the telecommunications grounding and bonding system vary according to system and network size, capacity and local codes.
- If used, telecommunications grounding busbars (TGB's) are bonded to the TMGB via the telecommunications bonding backbone.
- All racks and metallic pathways are connected to the TMGB or TGB.
- The cabling plant and telecommunications equipment are grounded to equipment racks or adjacent metallic pathways.
TIA and ISO standards provide one additional step for the grounding of screened and shield cabling systems. Specifically, clause 4.6 of ANSI/TIA-568-B.1 and clause 11.3 of ISO/IEC 11801:2002 state that the cable shield shall be bonded to the TGB in the telecommunications room and that grounding at the work area may be accomplished through the equipment power connection. This procedure is intended to support the optimum configuration of one ground connection to minimize the appearance of ground loops, but recognizes that multiple ground connections may be present along the cabling. Since the possibility that grounding at the work area through the equipment may occur was considered when the grounding and bonding recommendations specified in ANSI-J-STD-607-A-2002 were developed, there is no need to specifically avoid grounding the screened/shielded system at the end user's PC or device.
It is important to note the difference between a ground connection and a screen/shield connection. A ground connection bonds the screened/shielded cabling system to the TGB or TMGB, while a screened/shield connection maintains electrical continuation of the cable screen/shield through the screened/shielded telecommunication connectors along the full length of cabling. Part of the function of the screen or shield is to provide a low impedance ground path for noise currents that are induced on the shielding material. Compliance to the TIA and ISO specifications for the parameters of cable and connecting hardware transfer impedance and coupling attenuation ensures that a low impedance path is maintained through all screened/shielded connection points in the cabling system. For optimum alien crosstalk and noise immunity performance, shield continuity should be maintained throughout the end to end cabling system The use of UTP patch cords in screened/shielded cabling systems should be avoided.
It is suggested that building end-users perform a validation to ensure that screened and shielded cabling systems are properly ground to the TGB or TMGB. A recommended inspection plan is to:
- Visually inspect to verify that all equipment racks/cabinets/metallic pathways are bonded to the TGB or TGMB using a 6 AWG conductor.
- Visually inspect to verify that all screened/shielded patch panels are bonded to the TGB or TGMB using a minimum of 12 AWG conductor, if not specified by the manufacturer instructions.
- Perform a DC resistance test to ensure that each panel and rack/cabinet grounding connection exhibits a DC resistance measurement of <1 Ohms between the bonding point of the panel/rack and the TGB or TMGB. (Note: some local/regional standards specify a maximum DC resistance of <5 Ohms at this location.)
- Document the visual inspection, DC test results, and all other applicable copper/fiber test results.