Dual feed systems prevent this from happening. They introduce air into the system at different points along the cable route. As described previously, the most efficient way to do this is with an air pipe. In a dual feed system, pressurized air converges on a leak from opposite directions, supplying positive pressure protection to the sections of cable between the leak and the two air sources (see figure below).

What Makes Up an Air Pressure System?
We've already mentioned some of the key system components: air compressors, air pipe and air pipe manifolds. In the central office, where air is introduced into the system, there are a number of other important system components. Distribution or meter panels (referred to by either name) are rack-mountable equipment panels that regulate pressure from the air compressor and distribute it to the cables in the vault. Pipe alarm panels provide the same function for air pipe leaving the central office. Both panels are equipped with flow raters for physically checking outgoing flow rates.

In well-designed systems, the central office panels are also equipped with pressure and flow monitoring devices. These sensors, called transducers, monitor delivery pressure and flow rates at the point of installation, but they cannot perform this function by themselves. They must be wired to a central office monitor so that continual device readings can be taken throughout the day and night.

In the field, pressure transducers are typically installed at the ends of cables and at designated points along the cable route. The placement of these devices is critical in the leak locating process. Flow transducers are also installed wherever air is introduced in to the system. The most obvious field location is at an air pipe manifold. Some systems use remote air dyers in the field to provide a boost in cable pressure. These air sources are also monitored for pressure and for flow.   (continued...)