What is an open circuit?
A break in the series loop that leaves current nowhere to flow — and, because every fixture shares that one path, switches off the whole circuit.
A series lighting circuit is one continuous loop: the CCR drives a single regulated current through the primary cable and every isolation transformer primary in turn. An open circuit is a break anywhere in that loop — a cut or damaged cable, a failed connector, a failed transformer primary or an open CCR output — so the current has no path to complete.
With no current, every lamp on the circuit goes out, not just the ones near the break, and the CCR cannot hold its output: its open-circuit protection shuts the output down and raises an alarm. A failed lamp is different — it sits on the secondary side, and the isolation transformer is designed so that the primary loop stays closed, and a film cutout shorts the secondary so the transformer is not left running with an open secondary. The cable ends at a break can still be hazardous if the CCR is energised, so the circuit is isolated and proved dead before anyone works on it.
- Break. The primary loop opens at a cable, connector or transformer primary.
- Current stops. The single series path is interrupted for the whole circuit.
- Lights out. Every fixture on the circuit goes dark together.
- CCR trips. Open-circuit protection shuts the output down and alarms.
- Isolate. CCR off, locked out and tagged; prove the circuit dead and earth it at the work point (see Entry 10, Circuit Isolation).
- Locate & repair. Split the loop, test each section for continuity, repair, then re-test.
Frequently Asked Questions
Q1What is an open circuit in airfield lighting?
A break in the series loop that leaves no complete path for current. It can be a cut or damaged cable, a failed connector, a failed isolation transformer primary winding or an open CCR output, and it stops current in the whole circuit.
Q2Why does one break turn off every light on the circuit?
Because all the isolation transformer primaries are connected in series with the same cable. There is only one current path, so a break anywhere interrupts the current for every fixture on that circuit, not just the ones near the break.
Q3Does a failed lamp cause an open circuit?
No. The lamp is on the secondary side of an isolation transformer, which is designed so that an open secondary does not open the primary loop. A film cutout then shorts the secondary, because an open secondary on an energised transformer would otherwise saturate the core, overheat and develop a high voltage. One lamp or LED failure darkens that fixture only; the rest of the circuit keeps working.
Q4What does the CCR do when the circuit opens?
The constant current regulator cannot push current through a break, so its open-circuit protection shuts the output down and normally raises an alarm. This limits the voltage the regulator would otherwise develop trying to force current through the gap.
Q5Is an open circuit dangerous?
Yes. The break and the cable ends around it can carry high voltage if the CCR is energised, and the circuit is never safe to handle just because the lights are out. The CCR is switched off, locked out and tagged and the circuit is proved dead before anyone touches a cable end, connector or pit.
Q6How is an open circuit found?
With the loop de-energised, a low-resistance tester is connected across the cable ends and reads over-range (OL). The loop is then split at transformer pits and connectors, and each section is tested until the section that reads open is isolated and the break located. See Continuity Testing.
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