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; and 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 circuit will not damage the transformer, but the secondary voltage will rise above normal and distort the wave shape ofthe primary current. When enough lamps bum 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; for example, 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 need to look 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. That occurs when base maps are not kept up to date 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 at the time it occurs may not be great enough 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 effected 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 orss where they are close to each other.

GROUNDS. - When a string of lights bums 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. Underground Lighting Problems The care and craftsmanship of the original installation will, to a large extent, determine the life of the system. Still, no system lasts forever. Even the best installation and the most conscientious inspection and maintenance program cannot prevent the aging and gradual breakdown of a system. In almost all cases when an underground cable breaks down, it goes to ground. Where more than one conductor is enclosed in