Underground Distribution Systems

Normally speaking, underground systems that are properly installed require little maintenance of the routine type. Since both the equipment and the cable are well protected from man and the elements, the system normally is not subject to the same problems that overhead systems experience.

In some areas, groundwater or dampness may create some problems for underground systems by increasing rust or corrosion. Racks and splice boxes may require more painting and other rust or corrosion maintenance. Look especially for rusted nuts on boxes and rack hangers. They should be cleaned and painted. The manholes and vaults should be cleaned. These areas should not be used for storage nor should trash be allowed to collect in them.

Check the manhole walls for evidence of cracks, breaks, or other evidence of water seepage or leakage. Check empty ducts for plugs and evidence of water seepage.

You will find manholes with enough water in them to hamper or prohibit work operations. In such cases, bail the water out with a bucket and rope or pump it out with a manhole pump. Sometimes sump holes are built into the floor of manholes, and these provide places to bail from or to pump from the lowest places in the manhole. When water runs into a manhole from unoccupied ducts, hard rubber plugs are provided to stop or slow the water. When the manhole pump is used, place it in a position so the flow of water will be away from the manhole. This would be on the downhill side. Place the pump at least 10 feet from the manhole opening. The pump has a hose to be inserted in the manhole and an outlet hose to carry the water away from the manhole. Check cables for proper racking, making sure that they are trained in the proper direction and positioned so an ample cable radius is left for bends and expansion/contraction. This radius is basically 5 to 12 times the cable diameter, depending on the sheath type and the number of conductors. Make sure that 6 inches of straight cable exists for racking on each side of the splice. Check splices for evidence of leakage or tracking. Look for scrapes, burns, cracks, corrosion, or any other evidence of cable insulation deterioration. See that all cables are properly tagged for identification.

Check potheads and terminations that are attached to risers for leakage, tracking, and evidence of overheating or an overvoltage. Also check the security of the mounting of the pothead and conduit.

TROUBLESHOOTING CIRCUITS

Troubleshooting of cable systems is much the same as any other type of electrical troubleshooting. You need a thorough knowledge of the system as well as the ability to analyze troubles. A review of the history of the system provides

clues to present or future troubles. Simply using your eyes and head is sometimes the most effective method of locating the trouble. A knowledge of test equipment, an ability to read drawings or schematics, and an understanding of electricity are the key factors in locating electrical troubles.

Types of Trouble

The same basic types of trouble can occur in the airfield lighting cable system whether that system is in series or in multiple; however, the results of these circuit troubles can cause dramatic differences. For instance, a short circuit across the terminals of a distribution transformer supplying a multiple system is a dangerous overload; while the same short circuit across the output terminals of a CCR and series transformer is a no-load condition. An open in the output circuit of a CCR, on the other hand, creates a dangerous overload. Burned-out lamps in the secondary of a series IT will not damage the transformer, but the secondary voltage will rise above normal and distort the wave shape of the primary current. When enough lamps burn out, the primary current may rise high enough to shorten lamp life and possibly damage the regulator. These critical factors should tell you why you need to know the circuit.

In the discussion above, all types of electrical trouble were mentioned. They are opens, shorts, grounds, and improper power.

OPENS.\An open circuit is an incomplete circuit. Somewhere the circuit has a break; therefore, there is not a complete path for current flow throughout the circuit. Because there is no current flow, the circuit cannot operate. In analyzing circuit trouble, if the lights are not burning, the motor is not running, and so forth, you know that you will be looking for a break in the circuit. Usually this break will be at the unit(s) of resistance (burned-out lamp, broken resistor, motor burned out), but sometimes the break will happen in the cable. When the cable breaks, this break is most likely to happen at a splice or connection. Other cable breaks may be caused by digging operations being done in the wrong place. This occurs when base maps are not kept up and when unauthorized digging operations take place. It is an excellent reason for installing and maintaining direct burial cable markers.

Improper installation of cables can cause them to fail. Cables may be damaged by kinking, bruised by rocks, crushed by wheels, or cut by shovels when proper care is not exercised during handling and installation. While the damage may not be great enough at the time it occurs to take the cable out of service, it may be the starting point for a cable failure at a later date. This failure may be either in the form of a broken cable (open), cross type of short (two cables touching), or a short to ground (cable in contact with earth ground). Any of these troubles can render the circuit inoperative. The indication of the type of trouble that you have in the circuit and the point in the circuit where this indication appears should assist you in locating and repairing the circuit.

With an open circuit, that portion of the lighting system being supplied by the affected cable will not operate. A string of lamps that do not light, then, would indicate an open cable.

SHORTS.\If lamps are lit when they are not supposed to be or if a circuit is affected by another circuit, you most likely have a cross type of short between the two circuits. The logical point to start looking for this trouble is where the two cables cross or where they are close to each other.

GROUNDS. \When a string of lights burns dim or when fuses blow on a circuit, you have a short to ground. The insulation on the supply cable is damaged. This defect lets current pass directly from the conductor to the earth and prevents the lamps from receiving enough power to operate correctly; that is, some of the resistance of the circuit is being bypassed. The amount of resistance being bypassed in the circuit governs the effect of the short to ground. If enough resistance is removed (bypassed), then the current rises to a point that is sufficient to blow the fuses and thus disconnect the circuit.

IMPROPER POWER.\Improper power can result when regulators or distribution transformers are not connected properly. If the incorrect input voltage is connected or if the regulator has been purposely connected for an unusual load requirement, improper power can be applied to the system and serious damage may result.