The Canadian Electrical Code, Section 60, Electrical Communications Systems covers the requirements for communications systems wiring entering into buildings and installed throughout buildings. The code considers a communications system, any system that carries voice, sound or data signals. In addition to communications services such as telephone systems, radio, television, remote control and fire alarm systems are all deemed to be communications circuits when their signals are carried through communications lines. Although Section 60 focuses mainly on the electrical safety requirements, it also addresses a number of technical issues.
Rule 60-700 specifies that the metal sheath or shielding of communications circuits entering a building must be grounded or interrupted by means of an insulating sleeve at the point where they enter buildings. The reason — to minimize the probability of introducing electric fire or shock hazards into a building in the event of induced voltages, accidental contact with power circuits or lightning via the shielding. This requirement applies to both aerial and underground communications circuits, since either may have an electrical system nearby.
Rule 60-306 also requires that communication cable metal sheaths or shielding be grounded when installed within buildings. The reason — to head off any possible electrical fire or shock hazards, should communications wiring become exposed to an induced voltage, or make accidental contact with an electrical circuit. But Rule 60-306 does not specify that metallic sheaths or shielding be grounded at both ends (bonded to equipment enclosures), or whether grounding only one end is sufficient. Grounding both ends of cable shields may result in unwanted current flow on the shielding that may interfere with communications signals, a common problem in communications wiring.
“Ground loops” are caused by potential differences between the points in a grounding system. Voltage differences at the ends of cable shields or metallic sheaths bonded to equipment may result in unwanted current flow and electrical interference to communication signals. High frequency leakage current flowing in bonding conductors often increases the inductive reactance of the bonding conductors and their overall impedance with a resultant voltage drop. Voltage differences can increase greatly when bonding conductors have widely differing lengths. A common remedy for ground loops is isolation, bonding communications cable shielding at only one end. But this method of installation also exposes communications systems to a different problem, electromagnetic interference.
Low voltage communications signals also need protection from electromagnetic interference (EMI), produced by electrical power and lighting circuits, lightning, etc. A grounded shield or metallic sheath can provide some EMI protection. Grounded metallic enclosures, metallic conduit and cable armours can have the same beneficial effect. But to provide effective protection, induced current must be permitted to flow in the cable shielding to oppose EMI. Sheath current flow produces an electromagnetic field that “bucks” and opposes the problem EMI, thereby minimizing its harmful effects and thereby avoiding interference of communications signals. Therefore, by leaving one end of the shielding ungrounded to fix “ground loops,” a new problem rears its ugly head. When the shielding is bonded to equipment at only one end, induced currents are prevented, eliminating the EMI protection and preventing the cable shielding from doing its job. It seems that either plan comes with its own problems. The trick will be how to fix both ground loops and EMI.
There may also be an added safety problem. With cable shielding grounded at one end only, the opposite end may develop sufficient voltage, to cause an electrical fire or shock hazard in case of a lightning strike, power system fault or cable contact with a higher voltage. Any of these can create a shock hazard at the ungrounded end, should anyone make contact with the shielding. Arcing may also ignite nearby flammable materials or cause damage to sensitive electronic components.
DC and low frequency currents may be blocked or reduced by connecting a blocking device such as zener diodes or a capacitor between the cable shielding and ground. Where communications wiring enters a building, surge protection between cable shielding and ground provides the same result. This allows current to flow in the shielding when there is an unusual event such as a serious overvoltage or lightning strike and ground loops are prevented under normal conditions.
As with past articles, you should always check with the local electrical inspection authority for a more precise interpretation of any of the above in each province or territory as applicable.