Where Does the Air Pressure Come From?
The cable, although filled with individual conductors and associated insulation, is much like a long garden hose carrying air pressure instead of water. The pressure comes from a mechanical air compressor and dryer, located somewhere near the telephone company cable vault. The compressor supplies the air at approximately 10 PSI, while the dryer removes the residual moisture. Essentially, this means that air with very low humidity is forced into the cables.

How Is the Air Pressure Distributed?
Imagine once again that the cable route is like a garden hose. This hose is made up of different sections that are many thousands of feet long. Depending upon its age and a number of other variables, there will typically be leaks at various points along the cable. Like a hose, the pressure in the cable diminishes as the length increases. The pressure must be re-established in the system along the route, or the cable will be unprotected at substantial distances from the central office. Obviously, finding and fixing all of the leaks in the cable will help considerably. But, with technicians constantly working on cables (opening splices, etc.) and with electrolysis being a constant threat, air pressure must also be raised.

One popular way of raising air pressure in the system is by using air pipe that follows the cable route and introduces pressure at various fixed points (see figure below). Because the air pipe does not include conductors (which restrict the passage of air), it is a far more efficient method of transporting air to the needed areas. The air pipe is connected to a manifold which distributes air to the cables in the utility hole.

To make sure that adequate delivery pressure is supplied to the cables in the field, most telephone companies set a minimum pipe endpoint pressure standard of 7.5 PSI. If the pipe pressure falls below this standard, cable protection is jeopardized. For example, it's impossible to maintain 5 PSI in an underground cable when the delivery pressure from the air pipe is only 4 PSI. For this reason air pipe delivery pressure must be carefully monitored. Think of it as a main artery in the system. If pressure in the air pipe is low, the whole system is low.   (continued...)