What is insulation-resistance testing?
After continuity is proved and before the loop is energised, a DC voltage is applied between the conductor and earth to show the insulation is sound, and the megohm reading becomes the baseline for later tests.
Continuity shows a loop is joined up; insulation resistance shows it is safe to energise. The test applies a DC voltage between the conductor and earth and reads the resistance of the insulation in between, in megohms. Sound cable and sound transformers leak almost nothing, so the reading is high and steady.
It is done de-energised, with the CCR off, locked out and disconnected from the loop, after continuity has passed and before energisation. A damaged cable, a wet joint or a faulty transformer shows up here as a low or falling reading, long before it becomes an earth fault on a live circuit. The result is recorded as the baseline for later tests. The test voltage can be hazardous and the cable holds a charge afterwards, so discharge it before touching anything, and never connect the tester to a live loop.
- Prepare. Take the cable rating and specification to find the test voltage, hold time and minimum reading. Check the tester is in calibration.
- Isolate. CCR off, locked out and tagged. Disconnect the loop from the CCR output and prove it dead.
- Discharge and connect. Make sure the cable is discharged, then connect the tester between the loop conductor and earth.
- Apply. Apply the specified DC voltage for the specified time and let the reading settle.
- Read and compare. Read the megohm value and compare it with the specified minimum. If it is low, split the loop at the pits and test each section.
- Discharge and record. Discharge the cable again, then log the reading, voltage, time, temperature and weather as the baseline. Restore the connections.
What the readings suggest
High and steady
At or above the specified minimum and holding still. The insulation is sound. Record it as the baseline.
Low reading
Suspect moisture, a damaged cable, a wet or poorly made joint or connector, or a faulty transformer. Split the loop at the pits and test each section.
Reading keeps falling
Leakage is growing under the test voltage, which usually means moisture or damage. Do not energise until it is found and cleared.
Near zero
Points to a hard earth fault. Locate it by splitting the loop, then repair and re-test. See Earth Fault.
Varies with conditions
Temperature and damp change the value, so compare like with like and always note the conditions.
Always record
Log the loop, voltage, time, reading, date, conditions and instrument. Later tests are judged against this figure.
Frequently Asked Questions
Q1What is insulation-resistance testing in AGL commissioning?
It is a de-energised test that applies a DC voltage between the loop conductor and earth and reads the resistance of the insulation in megohms. It shows whether the cable, joints and transformers of a new series loop are sound before the loop is energised, and it sets the baseline for later tests.
Q2Why is it done after continuity and before energisation?
Continuity shows the loop is complete; it says nothing about insulation. A loop can pass continuity and still have a wet joint or a damaged cable. Finding that with the circuit dead is safe, whereas finding it on energisation means an earth fault on a live circuit.
Q3Why must the CCR be disconnected for the test?
The regulator has its own components and earth connections, which would distort the reading, and the DC test voltage could damage it. The loop is isolated, locked out and disconnected so that the reading reflects only the cable and the transformers.
Q4What test voltage and minimum value are used?
They come from the cable rating, the transformer data, the applicable standard and the project specification. This page gives no fixed figures, so always use the values in the specification for the job.
Q5Why discharge the cable after the test?
Cable behaves like a capacitor, so it stores charge from the DC test voltage and can give a shock afterwards. The tester discharges it, and the cable is proved discharged before it is touched or reconnected.
Q6How is a low reading traced?
Under isolation, split the loop at a convenient pit or connector and test each part. The part with the low reading holds the fault. Narrow it down, repair or replace the damaged cable, joint or transformer, and re-test the whole loop.
Q7How does this differ from the maintenance and transformer insulation tests?
The method is the same, but the purpose differs. At commissioning the test proves a new loop is sound and records the baseline. In maintenance the same test is compared with that baseline to spot ageing. The transformer test looks at one transformer's windings, whereas this one tests the whole loop of cable and transformers together.
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