What is an approach-light circuit?
A single series loop that runs out along the approach and back. One regulator pushes the same current through every light.
In ICAO practice an approach-light circuit is a series circuit. A constant current regulator (CCR) sends one fixed current out along the approach and back to itself. Each light’s isolating transformer has its primary connected in that loop, so the same current flows through all of them.
Brightness is set at the CCR by choosing a current step, and every light on the circuit follows it. A failed lamp darkens only its own light, but an open loop, such as a cable break, puts the whole circuit out.
Frequently Asked Questions
Q1What is an approach-light circuit?
In ICAO practice it is a series circuit: a constant current regulator (CCR) sends one fixed current out along the approach and back, and every light’s isolating transformer is connected in that loop.
Q2Why are approach lights on a series circuit?
In a series circuit the same current flows through every light, so all the lights get the same drive however long the cable is, within the rating of the regulator. Brightness is changed by changing that one current.
Q3How is brightness controlled?
The CCR provides fixed current steps. Selecting a step changes the loop current for the whole circuit, so every light on it dims or brightens together.
Q4What happens if one lamp fails?
Only that light goes dark. Its isolating transformer keeps the series loop closed, so the other lights stay lit.
Q5What happens if the cable breaks?
An open loop stops the current, so the whole circuit goes dark until the break is found and repaired. This is why cable records and testing matter.
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