What Is the Best Design for Protecting 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).   (continued...)