What is insulation testing?
A de-energised check of how well the cable and transformer insulation holds against earth, measured with a DC test voltage, long before a weak spot becomes a fault.
An insulation resistance (IR) test applies a DC voltage between the circuit conductor and earth and reads the resistance of the insulation in between. The CCR is switched off, locked out and disconnected first, so the reading reflects only the cable and the transformers, not the regulator.
A healthy circuit gives a high, steady reading. A low reading, or one that falls from test to test, points to moisture, damage or ageing in the cable, a joint or a transformer. Insulation testing says nothing about whether the loop is complete, so it is carried out alongside continuity testing.
- Isolate. CCR off, locked out and tagged; confirm the circuit is dead.
- Disconnect. Open the loop at the CCR terminals.
- Discharge & connect. Discharge the cable, then connect the tester between conductor and earth.
- Apply. Apply the specified DC test voltage for the specified time.
- Record. Log the reading, conditions and circuit, and compare with the baseline.
- Discharge & restore. Discharge again, reconnect, remove locks.
Frequently Asked Questions
Q1What is insulation testing in airfield lighting?
A test that applies a DC voltage between the conductor of a series circuit and earth, and reads the resistance of the insulation, to show whether the cable and transformers are still sound.
Q2Why is the CCR disconnected first?
The regulator’s own components would distort the reading, and connecting a tester to a live circuit is unsafe. The circuit is isolated, locked out and disconnected from the CCR before the tester is connected.
Q3What reading shows good insulation?
A high, steady value that agrees with the baseline for that circuit. The acceptable figure depends on the cable rating and the specification, so use the value your project or maintenance manual sets.
Q4What does a falling trend mean?
Insulation that is slowly deteriorating, often from moisture ingress or ageing. Catching it in routine tests lets the faulty length be found and repaired before it fails.
Q5Why record the weather and temperature?
Moisture and temperature change insulation readings. Recording the conditions makes tests comparable, so a change in reading can be told apart from a change in weather.
Q6Does insulation testing replace continuity testing?
No. Continuity shows the loop is complete; insulation shows it is sound. A cable can pass one and fail the other, so both are carried out.
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