Runway lighting is fed from a 20 amp constant current (which means variable voltage) transformer - aka "Tub" or "Regulator".
At each light you have a 6.6 amp transformer that insures if 1 light goes out, the rest remain on. From recollection the circuit voltage on the socket is around 300.
The FAA insists on this configuration because each light will always be the same intensity. There is no difference between the bulb you put in an hour ago, and the one that's been in there a year. You have shoulder lights (blue) every 200 feet - both sides, threshold lights, centerline lights, etc.. At Dulles there are 3 runways, including hold block, about 3 miles each, ya see how this gets out of hand?
You troubleshoot bad cable by grounding out the last known one, that "shoots around" on the ground and will pick up on the other side of the break. Neat huh???
You don't put the ground wire in the pipe with the energized conductor, or when a cable breaks, you'll never know it, AND you'll have an energized ground cable. You install a counterpoise ground in the trench with the conduit. It can be real confusing.
The next part is.... except for FAA documents, there is very little written material on this type of circuit and it's kinda "learn as ya go" thing.