What is circuit isolation?
Making a series lighting circuit safe to work on — supply locked out, loop disconnected at the CCR, then proved dead and earthed.
Circuit isolation is separating a series lighting circuit from every source of energy so it can be worked on safely. The circuit is switched off at the CCR, its supply is opened and locked out, and the loop is disconnected at the CCR output. A series loop is one closed path, so the whole circuit is isolated, never just the part being worked on.
Isolation is only complete once the cable is proved dead at the work point and earthed on each side of it. A live series circuit must never be broken: the CCR drives current round the loop, and an opened loop can put dangerous voltage across the break. Switching the lights off at the control desk is not isolation, because the supply is still live and the circuit can be restarted.
- Permit. Get the permit to work and agree the isolation points.
- Switch off. CCR off, with control on local so it cannot be started remotely.
- Isolate and lock out. Open the supply isolator, then lock and tag it.
- Disconnect. Open the loop at the CCR output terminals or output isolator.
- Prove dead. Test at the work point with an approved tester, proved before and after.
- Earth. Short and earth the cable on each side of the work point.
Frequently Asked Questions
Q1What is circuit isolation in an airfield lighting circuit?
Separating a series lighting circuit from every source of energy so it is safe to work on: CCR off, supply isolator open and locked out, loop disconnected at the CCR output, then proved dead and earthed at the work point.
Q2Why isolate the whole circuit and not just the part being worked on?
A series loop is one closed path carrying the same current through every transformer, so any point on it can be live while the circuit is live. The whole circuit is isolated first; the loop is opened at the work point only once it is dead and earthed.
Q3Why is switching the lights off not enough?
Switching off at the control desk leaves the supply to the CCR live, and the circuit can be restarted locally or remotely. Isolation needs the supply opened and locked out by the person doing the work.
Q4What happens if a live series circuit is opened?
The CCR keeps trying to drive its set current round the loop, so voltage can build up across the break, giving a shock and arc hazard. CCR open-circuit protection may trip, but it must never be relied on as a safety measure.
Q5Why prove dead and earth after isolating?
Opening a switch does not prove the cable is dead. The wrong circuit may have been isolated, and long cable runs can hold stored or induced voltage. Testing at the work point with an approved tester proves it, and earthing each side guards against accidental re-energisation.
Q6How is circuit isolation different from an isolation transformer?
Circuit isolation is a safety procedure that makes a circuit dead before work. An isolation transformer is permanent equipment at each light that separates the lamp from the high-voltage series loop, so a lamp fault does not by itself open the loop. It does not make the circuit safe to work on.
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.