Communications systems protection

Communications systems protection

The protection of wire and optical communications systems equipment and service from electrical disturbances. This includes the electrical protection of lines, terminal equipment, and switching centers, and inductive coordination, or the protection against interference from nearby electric power lines.

The principal sources of destructive electrical disturbances on wire communications systems are lightning and accidental energization by power lines. Lightning that directly strikes aerial or buried communication cable may cause localized thermal damage and crushing. Simultaneously, it energizes the communication line with a high-level voltage transient that is conducted to terminal equipment. Indirect strikes are more common than direct strikes and, although they normally do not cause mechanical damage, they propagate electrical transients along the line.

During fault conditions on commercial power lines, high voltages may occur in nearby communication lines by several mechanisms. The most common is magnetically induced voltage caused by the high unbalanced currents of a phase-to-ground power-line fault. During such an event, an aerial or buried communication line that is parallel to the faulted power line intercepts its time-varying magnetic field, incurring a high longitudinal voltage.

In city centers, the diversion of lightning strikes by steel-framed buildings, and the shielding effect of the many underground metallic utility systems, considerably reduce the probability of high-voltage transients from lightning or induction from power lines. Since communication and power facilities are routed in separate conduits, the possibility of a power contact is remote.

Metallic communication lines often are made up of many closely spaced pairs of wires arranged as a cable. A grounded circumferential metallic shield on the cable reduces the magnitude of electrical transients from nearby lightning strikes, and also can intercept a low-current direct strike, minimizing damage to the internal conductors. The effectiveness of the shield is improved if its resistance per unit length is low and if the dielectric strength of the insulation between the shield and the internal conductors is high.

Damage to cable plant from a power-line contact is minimized by providing frequent bonds between the cable shield or aerial support strand and the neutral conductor of the power line. These bonds create low-resistance paths for fault currents returning to the neutral, and hasten deenergization by power-line fault-clearing devices. Closely spacing the bonds limits the length of communication cable that is damaged by the contact.

The outer metallic shields of belowground cables may be attacked by electrolytic action and require protective measures against corrosion. See Corrosion

Optical-fiber communication lines are enclosed in cables that may contain metallic components to provide mechanical strength, water-proofing, local communications, or a rodent barrier. Though the cables otherwise would be immune to the effects of lightning or nearby power lines, such metallic components introduce a measure of susceptibility that is made all the more important by the high information rates carried by the fibers. A direct lightning strike to a metallic component of buried optical-fiber cable can cause localized thermal damage, arcing, and crushing that together may damage the fibers. Electrical protection is provided by cable designs that withstand these effects. As with metallic lines, the probability of this damage can be reduced by burying one or more shield wires at least 1 ft (0.3 m) above the cable.

Alternating currents may be conducted on the metallic sheath components of optical-fiber cables during an accidental contact with power-line conductors. Cable damage is minimized in extent by bonding the sheath to the neutral conductor of the power line, and by providing enough conductivity to carry the currents without damage to the fibers. See Optical communications

To protect the users, their premises, and terminal equipment, communication lines that are exposed to lightning or contacts with power lines are usually provided with surge protectors. Article 800 of the National Electrical Code requires that communication lines exposed to contact with power lines of voltages greater than 300 V be equipped with a protector at the entrance to the served premises. Interbuilding lines that are exposed to lightning also have protectors. Although not required, it is common practice to minimize equipment damage by so equipping communication lines in an area of significant lightning exposure even if there is no power-contact hazard. Article 830 extends similar requirements to coaxial circuits that provide network-powered broadband communications.

Switching centers contain electronic equipment that routes communications to their proper destinations. This equipment is protected from the effects of electrical transients appearing at interfaces with external communication lines in a similar way as for terminal equipment.

Inductive coordination refers to measures that reduce the magnitudes and effects of steady-state potentials and currents induced in metallic communication lines from paralleling power facilities. See Inductive coordination