What Is the Best Design for Protecting the Cables?

In some ways, cable pressure systems are similar to municipal water systems. They have main feeder routes and lateral sections of cable which branch off and serve customers along the way. If you're in an outlying residential area, far from the pumping station, your water pressure is not going to be as great as the pressure on the main feeder route. The water pipe serving your house is going to be much smaller than the water main leaving the main station.

Pressurized air is not supplied to the cables in an air pressure system by different size pipes. Still, the pressure at the end of a cable route is always going to be less than at the delivery source due to cable leaks and the pneumatic resistance of the cable. When a leak occurs in a cable, it can result in a section of cable being totally unprotected, depending upon the size of the leak and whether or not there is additional air being supplied to the cable.

There are three basic engineering designs for protecting cables: a static system, a single feed system and a dual feed system. Static systems were used before there was a good method of supplying a continuous source of air to a cable (before air compressors/dryers). They're like bicycle tires; they hold pressure for a while, but eventually they will go flat.

Single feed systems pump air into the cables from one direction (one air source, such as a central office air dryer). They provide adequate cable protection as long as there are no serious leaks in the system. If you have a big leak, one that drops the cable pressure to 0 PSI, the entire section of cable beyond the leak (on the side opposite the air source) will have no air protection at all (see figure below).


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 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.