Quantcast Troubleshooting Underground Circuits

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intensity. Repair by replacing this section of cable. If replacing this section of cable is not
practical and more precise location of the position of the ground(s) is required, refer to par. 6.4.2.
(2) If all of the lights are still dim or out, carefully check the relay and wiring in the
runway selector cabinet to make certain that only the proper circuit is being selected and is not
being shorted out in the selector cabinet. Also, check series plug cutouts, if used.
e) To determine if a fault is an open or an overload, de-energize the field circuit and
disconnect it from the output terminals of the runway selector cabinet or constant current
regulator. With an ohmmeter on a low-resistance range, measure the continuity of the field
circuit. If the circuit does not have continuity or it has a resistance of several thousand ohms, the
field circuit has an open fault. If the circuit has continuity, the regulator may be overloaded (due
to a ground or additional lighting load).
Locating Grounds in the Field Circuit
Troubleshooting Underground Circuits. When troubleshooting a field circuit, the
easiest procedure is to find and eliminate any grounds in the circuit, and then proceed to resolve
any other problems. Grounds are usually easier to locate, and often occur in conjunction with
opens, overloads, or shorts.
a) Symptoms. Dim and/or dark section or sections of lights on load circuit or low
megger readings on load circuit.
b) Troubleshooting. If there is a dim and/or dark section of lights, two grounds will
be found at the transition points between normal lights and the dim and/or dark lights. The
affected circuit should be driven at night to identify the lights that define the edges of the dim
section. The fault may not always be located at the last dim light, as it may be at the location of
last bright light, or in the cable between the two lights.
(1) As repairs are made, the dim section may clear up or shift position (frequently
the case with lighting damage). Remember to always repair both sides of a dim section. If the
dim section shifts, multiple grounds exist, and more grounds will be found at the edges of the
new dim section.
(2) When troubleshooting a single ground (no dim lights), use the "Intentional
Ground" method, "Ground Return" method, or "Sectional" method to locate the grounded point
in the cable. See the instructions and illustrations in the "Airfield Troubleshooting Methods"
section for further details.
(3) There are many devices and methods to assist you in locating grounds in load
circuits. The methods in this booklet are what the writer believes to be the easiest to use, which
do not require additional expensive equipment.
c) Causes. Grounds in airfield cables have many causes such as:
(1) Lightning (probably the largest single source). Lightning entering a load circuit
will exit in many locations, sometimes miles away.
(2) Animals such as gophers and groundhogs cause much damage to airfield cable.
(3) High resistance connections overheat and burn off insulation.


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