How Air Pressure Is Needed to Keep Water out of a Sealed Enclosure

Moisture Penetration

• Buried telephone cables become subject to pressure from the surrounding soil. This soil, especially the soil on top of the cable, exerts a continual downward force. Soil saturated or filled with water or moisture creates pressure that will eventually crack or break the cables. Sudden increases in pressure due to water runoff on top of cables present challenges in protecting the integrity of the cable.

Equal Pressure

• The premise behind the use of air pressure is to create an equal pressure force of air to counteract the water pressure. This air pressure pushing up against the downward water pressure equalizes the force and keeps the cable dry. Cables laid in different locations are subject to varying degrees of water pressure. Thus, the necessary air pressure changes in different locales.

Pressure Levels

• Pounds per square inch, or PSI, is the unit of measurement for pressure. One foot of water above a cable exerts 0.43 PSI, according to System Studies Incorporated, a supplier of pressurized systems for telephone companies. Multiply that number by the depth of the cable in feet and you have the water pressure. For example, a cable buried 7 feet would be subject to 3 PSI. The air pressure level needs to exceed the water pressure with a margin of error for safety. Telephone companies develop different standards, depending on location. A cable under a street may have 5 PSI while one buried just a few feet underground would be 3 PSI.

Air Pressure System

• Air compressors supply the air pressure to the cables. Telephone companies use dryers to produce air with low humidity. They monitor the system to ensure that adequate levels are maintained to protect the cables from moisture damage.