The biggest threat to telephone wires (conductor pairs) is moisture. Many of the copper cables use paper or pulp as insulation between the individual conductor pairs. This type of cable offers excellent insulation characteristics as long as it is maintained dry. Pulp insulation has been around for a long time, and for many years it had essentially no competition. Eventually plastic insulated conductor (PIC) cable encroached upon its popularity and took over the number-one spot.

Unfortunately, the integrity of the protective cable sheath (which is made of lead or polyethylene) is compromised when cracks develop. The cracks in the sheath or associated splice cases allow water to enter and electrolysis to occur, which results in faulted pairs within the cable.

All of us are familiar with moisture damage when the morning paper is left in the rain. The characteristics of the paper change completely with the water damage. This is the same thing that happens inside the cable sheath when water permeates it. The first indication of moisture in a cable is noise on the line, followed by complete cable failure.

In underground pulp insulated cable, a special problem develops due to water pressure being applied to the outside of the protective sheath. And, since the cable is usually underground (often located several feet deep), a real disaster can occur when utility holes fill with storm water. As water rises above a cable, there is approximately 0.43 Pounds per Square Inch (PSI) of pressure applied for each foot of water level above the cable. So, for example, if a cable is 7 feet below the surface and the utility hole fills with water, there will be 7 x 0.43 (or 3 PSI) of water pressure bearing down on the cable. If there is a crack in the cable or splice case, water will permeate and cause conductor damage unless there is positive pressure within the cable that exceeds and counters the 3 PSI of external force caused by the water.

This is the basic premise of cable pressurization: Keep the pressure within the cable in excess of the pressure that could be applied by standing water. To help achieve this objective, telephone companies establish minimum air pressure standards for cables in different environments. For example, an underground cable (one that goes through conduit and utility holes underneath the street) might have a minimum air pressure standard of 5 PSI, enough to protect the cable from approximately 10 feet of water. A direct buried cable requires less air pressure protection (usually 3 PSI) because it is placed only a foot or so below the surface. Aerial cables typically require only 2 PSI because they are at less risk from water damage.

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.
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.

It is commonly thought that, once a cable leak has been found and fixed, the cable will be safe from moisture. After a repair, there is a natural tendency to conclude that everything is okay and that maintenance can be ignored until more leaks are found on the cable. This couldn't be further from the truth.

Maintaining a reliable cable plant is something like maintaining a large building. Deterioration of the building must be taken seriously, especially the roof, which should be frequently repaired to ensure extended use. Several parts of the country experience long periods of dry weather where structures are not subjected to water damage. There's no problem for buildings (and cables) when it doesn't rain, but after extended periods of neglect, they both require constant attention to fend off the water problems when the storms come. Maintenance is best performed when the weather's dry.

Today, this type of routine maintenance is often called proactive maintenance. Many telephone companies are shifting toward a proactive approach to the outside plant in order to prevent subscriber calls, Public Utilities Commission (PUC) complaints, and even catastrophic failure. It's preventive medicine for the outside plant. Rather than treating the outcome of poor health (big leaks and low delivery pressure), a good diet and exercise program (changing out old cable/devices and performing maintenance on the problematic routes first) will keep future problems from occurring.

Are Cable Leaks Hard to Find? When looking for leaks, an entire cable cannot be held under water (like a tire tube) to look for bubbles at the point of damage. You have to devise another method. Pressure readings will tell you that a cable is losing air, but they don't tell you where. Because there is an inherent amount of air flow in pressurized cables, the best scheme is to follow the air flow to the leak. By setting up check points and using devices to measure the air flow, the leak can be quickly located without a foot-by-foot examination of the cable.

To use another analogy, let's say we're transporting money between two points (Checkpoints A and B). We would want to count the amount leaving Checkpoint A and confirm that it arrived at Checkpoint B. If the two amounts were not the same, this would indicate that we had a problem somewhere along the way. By counting the amount of air flow at two designated checkpoints on a cable route, we can quickly determine if there's a leak between them.

By far, the most accurate way to measure air flow is to count individual molecules. To make this measurement more manageable, the molecules are measured by the box full. The box size is a standard cubic foot (12" x 12" x 12"), measured at sea level (a point of standard atmospheric pressure). The amount of standard cubic feet consumed is measured either per hour or per day. This measurement lets you measure high consumption rates (air dryer output) or relatively low consumption (air flow into a cable) in a meaningful way, using Standard Cubic Feet per Day (SCFD) or Standard Cubic Feet per Hour (SCFH). These measurements help in monitoring for system leaks and in cable maintenance.

Because of maintenance activity, cable splices, and the use of pneumatic fittings along the cable route, all cables tend to leak to some extent. For this reason, an allowable air flow rate, called Optimum Air Usage (OAU) has been established. OAU is the calculated air consumption rate that an air pressure system should use under normal operating conditions. It is based on a consumption rate of 1.25 SCFH per sheath mile of cable.

Like the minimum air pressure standards, OAU is important in dispatching maintenance technicians. A flow increase at an air source is not valuable information unless you also know the OAU, or what the air source should be flowing. OAU is also important in the design of an air pressure system and for evaluating system quality.

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.

As mentioned previously, most of the air distributed into a cable system is provided by a mechanical air compressor/dryer. This central office equipment compresses or squeezes air molecules together within a given area to create greater air pressure. It's kind of like a garbage compactor in some ways, but with a more useful end result. The amount of compression is measured in Pounds per Square Inch or PSI. For years, PSI readings were the only measurements being used in air pressure maintenance. They're still very important, but nowadays people have come to rely on both pressure and air flow measurements.

Another important concept in cable maintenance is pneumatic resistance. This is the amount of resistance that air flow meets as it moves along inside the cable sheath between conductors. What determines the amount of pneumatic resistance in a particular section of cable are its length, the gauge of the conductors, the type of insulation (PIC or pulp), and the number of pairs in the cable.

Pneumatic resistance must be taken into account when using pressure and flow measurements when leak locating along a route. Together these three components comprise the important information needed to perform many of the successful leak locating formulas being used today.

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 figure below).

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.

Over the years there have been a number of changes in the management of air pressure systems. In order to be successful today, air pressure managers need to know the quality of the cable pressurization system and what it costs to maintain that particular level of quality. With this information they can make intelligent decisions regarding where to dispatch technicians and, just as importantly, where not to dispatch them.

Keeping track of the labor hours spent on cable maintenance per sheath mile of cable is a key cost analysis control. It may take some effort initially to obtain sheath mileage data, but once this information is known maintenance tasks can be evaluated to determine labor hour efficiency. When this information is compared with a rating of the quality of the system, effective management decisions can be made.

New instrumentation and computer programs, such as the PressureMAP Management Analysis Program, have simplified cable maintenance and given management much greater control over dispatching and labor hours. This software program obtains monitoring device data from office monitors, analyzes it, and provides valuable management and maintenance reports.

One of PressureMAP's most valuable tools is its "high five" report. In the past managers would have to manually pour over pages and pages of monitoring system printouts each morning in order to determine where to dispatch technicians. PressureMAP eliminates this function by carefully analyzing pressure and flow conditions and prioritizing the five most important leak locating tasks in each office. This capability frees up management time and eliminates the stress and confusion of "on-the-spot" dispatching.

PressureMAP also provides an early warning alarm system wherein damaged cable problems can be detected before conductors get wet. Alarms are received from office monitors, evaluated to determine if they need immediate response, automatically verified if determined to be of alarm status, and distributed to assigned centers or personnel for immediate response. This type of reactive capability is a vast improvement from the monitoring systems of old, where an alarm status represented a drop below or a step above a programmed device threshold.

PressureMAP's System Quality Index (SQI) uses pressure readings and air flow rates per sheath mile of cable to provide an accurate measurement of the status of a system—both by office and by route. The standard SQI rating is between 80 and 85. Indexes above the optimum represent excessive maintenance ("goldplating"). Low indexes are the result of one or more factors: too few labor hours being spent in an office/route, inadequate/inaccurate engineering, poor dispatching, and/or a lack of proper leak locating skills.

But what's even more important for today's telephone operations is the software's proactive capabilities. Not only does PressureMAP identify system-threatening conditions and dispatch technicians accordingly, it also offers the tools to systematically improve the cable pressurization system. This proactive function helps eliminate expensive after-hour alarm response time and makes it possible to schedule key maintenance activities. To use an earlier analogy, this is the difference between making needed repairs to your roof at your convenience, or having to do the work in the pouring rain.

The reasons behind cable pressurization are basically easy to understand. The primary goal of pressurization is to protect the cables from water damage and keep cable pressures above the minimum standards established for the various cable environments. When cable pressure is low, it's typically the result of low delivery pressure or actual cable leaks.

Monitoring devices in the central office and field help make it possible to identify both types of problems and determine the cause of low cable pressure. In the case of cable leaks, air flow analysis makes it possible to prioritize which leaks are doing the most damage to the system. With a knowledge of Optimum Air Usage, you can easily distinguish between a good flow and a bad flow. And once you know this, air flow leak locating techniques can be used to help locate the large, system-damaging leaks.

Air pressure design is an important factor in the success of an air pressure operation. Certain designs, such as dual feed, offer better cable protection than others. An air pipe system is one of the best means for providing dual feed protection to cables, but it is not cost effective for every type of air pressure operation. For this reason, there are alternative dual feed design systems available which provide many of the same protection and monitoring capabilities of the more expensive systems.

Improvements in air pressure monitoring have made it easier to analyze system conditions and perform important management and maintenance functions. These software programs offer greater tools and controls for today's cable pressurization managers and maintenance technicians. They emphasize a proactive approach to cable maintenance while, at the same time, providing superior alarming and dispatching capabilities.

Cable pressurization is an important part of today's telephone operations. The technology being used in this field has evolved rapidly in the past decade, and improvements are continually being made. For more information on basic cable pressurization, system design components, proactive maintenance and alarming, please contact System Studies Incorporated (email:sales@airtalk.com; phone: 800-247-8255 or 831-475-5777).