Protecting Telco Lines
Communication lines that travel from the telephone company (telco) central office (CO) to customer sites require transient protection. Furthermore, lines that travel from one building to another, or from one floor to another, also may be exposed to a transient threat. Problems can result whether the lines are twisted pairs or coaxial cable.
The gas tube is the traditional protection element installed at the telco central office. The primary purpose of the gas tube (and its carbon predecessor) is to protect CO personnel from injury and CO equipment from damage in the event of a lightning flash to exposed lines or accidental contact with high- voltage utility company cables. Protection devices usually are included at the telco service entrance point on the customer’s premises. Telco providers do their best to ensure that disturbances do not reach customers, but the final responsibility for transient protection lies with the equipment user.
Solid-state voltage-clamping devices generally are used to protect audio and data lines. The transient-clipping devices shown in Figure 17.9 are selected based on the typical voltage levels (including head- room) used on the loop. Figure 17.10 illustrates another protection arrangement, which prevents the introduction of noise into audio or data lines because of a common-mode imbalance that may result from transient suppressors being tied to ground. The use of a low-capacitance suppressor ensures minimum capacitive load- ing on the circuit.
For balanced telco lines, critical transient considerations include both the above-ground voltage of the two conductors (the common-mode voltage) and the voltage between the two conductors (the normal-mode voltage). When individual clamping devices are used on each conductor, as shown in Figure 17.9, one device will inevitably clamp before the other. This action can create a significant voltage differential that can damage sensitive equip- ment on the line. A common solution is the three-element gas tube. The device has a single gas chamber
with two gaps, one for each side of the line. When one side reaches the ionization potential, both sides fire simultaneously to ground.
Transient-suppression devices for low-voltage telco and data circuits are available from a number of manufacturers in a variety of configurations. Figure 17.11 illustrates some of the more common implementations.