What is series-circuit safety?
Why one live series loop is hazardous everywhere along its length, and what must happen before anyone touches it.
An AGL series circuit is one loop driven by a CCR, so the same current flows through every part of it and the whole circuit is live, not just the equipment near the CCR. A live loop is never opened: the CCR is switched off and the circuit isolated first.
An isolation transformer separates the fixture from the series circuit, but the primary cable and connectors stay in the loop, so they are live while the CCR runs. A dark fixture does not mean a dead circuit. Before anyone touches a cable or connector, the circuit is isolated, locked, proved dead and earthed as the procedure requires.
Frequently Asked Questions
Q1Why is an AGL series circuit hazardous?
The CCR drives the same current through every part of the loop, so the whole circuit is live, not just the equipment nearest the CCR. The voltage across the circuit depends on its load and cable length and can be high, even when the lights look small.
Q2Why must a live series circuit never be opened?
A CCR works to keep the set current flowing. If the loop is broken while current is flowing, a dangerous voltage can appear at the break. Open-circuit protection in the CCR shuts the output down, but only after the break has happened, so it is never relied on to make opening a live loop safe. The CCR is switched off and the circuit isolated before any connector or cable in the loop is opened.
Q3Does an isolation transformer make the cable side safe?
No. The transformer separates the fixture from the series circuit, but the primary cable and its connectors are part of the loop and stay live while the CCR is running. Only the fixture side is separated, and even that is not safe to open on a live loop, because an open secondary on an energised transformer can develop a hazardous voltage.
Q4Does a dark fixture mean the circuit is dead?
No. A lamp or LED fault can leave a fixture dark on a live circuit, the CCR may be running at a low brightness step, and it can be started or restarted from the control tower or a remote control point. Only isolation and a test prove the circuit dead.
Q5Why isolate the circuit before testing it?
An insulation test applies a high DC test voltage to the cable, so the circuit must be isolated first and the CCR locked off so it cannot be energised during the test. The cable holds a charge afterwards, so it is discharged as the test procedure requires.
Q6What must be done before touching a series-circuit cable or connector?
Isolate the CCR, including any remote or automatic restart from the control system, secure the isolation with lock-out tag-out, prove the circuit dead with an approved tester and earth it as the procedure requires. Only then may the cable or connector be handled.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.