What happens in a series circuit when a lamp goes out?
One lamp fails and the rest of the circuit carries on. The reason is the isolating transformer, and it is also why a lamp-out is a very different fault from an open circuit.
In an AGL series circuit each lamp sits behind an isolating transformer. When a lamp fails, only that transformer’s secondary opens. Its primary stays in the loop, so the circuit does not open and the other lamps keep burning at the same current.
1Why the loop stays closed
The lamp is on the secondary side. A failed filament opens the secondary only, and the primary winding still carries the loop current. This is the whole point of the isolating transformer. Older arrangements with lamps wired directly in series had no such path, so a failed lamp broke the loop and a film cutout was needed to bridge it, covered in a separate entry.
2What the rest of the circuit sees
The constant current regulator holds the set current, so the remaining lamps stay at the same brightness. The failed lamp no longer draws power, so the circuit load drops a little and the CCR output voltage falls with it. As more lamps fail the load falls further, and how well the CCR regulates at low load comes from its specification.
3The unit with the open secondary
With nothing on its secondary, the transformer is driven towards core saturation by the loop current. The waveform distorts, the core runs hotter, and voltage peaks can appear on the open secondary terminals. Transformers are designed to tolerate this for a time, but it is not a state to leave, and the secondary side is treated as live.
4What goes wrong
Many lamps out can leave the CCR lightly loaded, with poor regulation or an alarm. Open-secondary heating shortens transformer life. A lamp-out is easy to confuse with an open circuit, but an open circuit breaks the loop, puts every lamp out and trips the CCR. And a lamp that is out is not proof the circuit is dead, so isolate and prove dead before changing it.
| Condition | Secondary | Loop | Result |
|---|---|---|---|
| Normal | Lamp lit | Closed, set current | All lamps at the step brightness |
| One lamp out | Open | Closed through the primary | Other lamps unchanged; load a little lower |
| Several lamps out | Several open | Closed | Load falls further; maintenance limits apply |
| Open circuit | Not the cause | Broken | All lamps out; CCR trips (open-circuit protection) |
Frequently Asked Questions
Q1Does one failed lamp put the other lamps out?
No. Each lamp is on the secondary of an isolating transformer whose primary stays in the series loop. When the lamp fails, the loop is still closed through the primary, the constant current regulator keeps the set current flowing, and the other lamps stay at the same brightness.
Q2Why does the CCR not trip when a lamp fails?
Because the loop has not opened. The CCR sees a slightly lower load, since the failed lamp no longer draws power, and keeps regulating. Open-circuit protection acts only when the loop itself breaks.
Q3What is the difference between a lamp-out and an open circuit?
In a lamp-out only the secondary side of one transformer is open, so the loop stays closed and the rest of the circuit carries on. In an open circuit the loop itself is broken, for example by a cable, a connector or a transformer primary, so every lamp on the circuit goes out and the CCR trips.
Q4Is a transformer with an open secondary dangerous?
It can be. With the secondary open the loop current drives the core into saturation, which distorts the waveform and can put high voltage peaks on the open secondary terminals. Transformers are designed to tolerate this for a time, but the secondary side must be treated as live. Isolate and prove dead before changing a lamp, and follow the airport's procedures.
Q5How does anyone know a lamp is out?
By inspection, and on some systems by monitoring that picks up a change in load or senses individual fixtures. What is available depends on the system. How many lamps may be out, and which combinations are not allowed, is covered in later entries on serviceable lamp percentages and the adjacent-lamp-out rule.
Q6Why should a failed lamp not be left?
A transformer left with an open secondary runs with extra core heating, and each failed lamp lowers the circuit load and the lighting available to the pilot. Maintenance limits on unserviceable lamps are set by the airport's rules, so lamps are replaced within the time those rules allow.
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