How are approach-light transformers arranged?
One isolating transformer per light, in a can below ground beside the stem. The series loop runs through each can, and a short lead goes up to the lamp.
Approach lights are fed from a series circuit, and every light has its own isolating transformer. The transformer sits below ground in a can beside the base of the stem. The series loop cable passes through the can and into the transformer’s primary.
A short secondary lead then runs up the frangible stem to the lamp. The primary stays in the loop whether or not the lamp is lit, so a failed lamp darkens only that light and the rest of the circuit stays lit. The lamp side is also electrically separate from the loop cable.
Frequently Asked Questions
Q1How are transformers arranged in an approach lighting system?
Each approach light has its own isolating transformer. The series loop cable passes through the transformer’s primary, and a short secondary lead carries the low-voltage lamp supply up to the light.
Q2Where is the transformer installed?
Below ground, in a can or pit beside the base of the light’s stem, as close to the light as practical. This keeps the frangible stem light and protects the transformer from weather and traffic.
Q3Why does every light need its own transformer?
A series circuit carries one fixed current through every light, so each lamp needs a matching supply. The isolating transformer feeds its lamp from that loop current, keeps the lamp side electrically separate from the loop cable, and keeps the loop closed if the lamp fails.
Q4What happens if a lamp fails?
Only that light goes dark. The transformer keeps the series loop closed, so the rest of the lights on the circuit stay lit. A failed lamp still leaves its transformer on the loop, so failures are replaced promptly: several on one circuit change the load the CCR sees.
Q5How is the transformer rating chosen?
It is matched to the load of the lamp or lamps it feeds, as set out in the design drawings and the applicable standard.
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