What is continuity testing at commissioning?
Before a new series loop is ever switched on, a de-energised test proves it is one unbroken conductive path, section by section and end to end, and records the value that every later test is judged against.
A series lighting loop only works if it is one unbroken conductive path: the same current must flow through the cable and through every isolation transformer primary. A single break, a loose connector or a missing transformer stops the whole loop. Continuity testing proves the path is complete, and it is the first electrical test carried out on a new installation.
It is done de-energised, with the CCR off, locked out and disconnected from the loop, using a low-resistance ohmmeter. The cable is tested section by section as it is laid and jointed, then the whole loop is tested end to end. The reading is compared with a value calculated from the design, and the result is recorded as the baseline for all later testing. Never connect a test set to a live or energised loop. Continuity shows the conductors are joined; it says nothing about insulation, so it is followed by insulation testing and only then by energisation.
- Prepare. Take the loop drawings, cable lengths and transformer count, and work out the expected resistance: cable conductor plus every transformer primary.
- Isolate. CCR off, locked out and tagged. Disconnect the loop from the CCR output and prove the circuit dead, including a check for induced voltage, before touching it.
- Test each section. As each cable length is laid and jointed, test it end to end before the joint is closed and the trench or pit is backfilled.
- Test the whole loop. With all transformer primaries connected, put the ohmmeter across the two loop ends at the CCR and let the reading settle.
- Compare. Check the reading against the calculated value, allowing for cable temperature and instrument accuracy. If it is open, split the loop at the pits and test each half.
- Record the baseline. Log the loop, value, date, instrument and tester. Restore the connections, then move on to insulation testing.
What the readings suggest
Close to the calculated value
The path is complete and the conductors, joints and primaries are connected as designed. Record it as the baseline.
Over-range / no reading
There is a break: a cable fault, an open joint or connector, or a loose termination. Split the loop at the pits and test each half.
Higher than calculated
Suspect a poor joint or connector, a loose termination, or more cable than the drawings show.
Lower than calculated
Suspect a transformer missing from the loop, a bypassed section, a short between conductors, or wrong cable lengths in the records.
Reading drifts or wanders
The loop is inductive, so allow the reading to settle. If it still wanders, look for an intermittent joint or a poor test connection.
Always record
Log the loop, section, value, date, instrument and tester. This is the figure that maintenance tests will be compared with.
Frequently Asked Questions
Q1What is continuity testing in AGL commissioning?
It is a de-energised test that proves a new series lighting loop is one unbroken conductive path, through the cable and every isolation transformer primary. It is the first electrical test on a new installation and sets the baseline for later testing.
Q2Why is continuity tested before the loop is energised?
A series loop carries one current through every device, so any break, loose joint or missing transformer stops the whole loop. Finding that with the circuit dead is safe and cheap. Finding it for the first time on energisation means a fault on a live circuit and wasted time.
Q3What instrument is used, and how is the expected value worked out?
A low-resistance ohmmeter in calibration. The expected value is built from the design: the conductor resistance of the cable (length times resistance per unit length, corrected for temperature) plus the primary winding resistance of every transformer in the loop. The measured value is compared with that figure, allowing for instrument accuracy.
Q4Why test cable sections during installation as well as the whole loop?
A fault in one section is far easier to fix before the joint is closed and the trench or pit is backfilled. Testing each section as it is laid and jointed catches faults early, and the final whole-loop test then confirms the sections and the transformer connections work together.
Q5What does a good continuity result not prove?
It proves the conductors are joined, nothing more. It does not show the insulation is sound, that there is no earth fault, that the lamps work or that the secondary connections are good. Insulation testing and functional testing cover those, and energisation comes only after the earlier tests pass.
Q6How is an open circuit found?
Under isolation, split the loop at a convenient pit or connector and test each half. The half that reads open contains the break. Repeat on that half, narrowing it down until the faulty cable, joint or connector is found, then repair it and re-test the whole loop.
Q7How is this different from continuity testing in maintenance?
The method is the same, but the purpose differs. At commissioning the test proves a new loop is complete and records the baseline value. In maintenance the loop is tested again and the result is compared with that baseline, so any change points to a developing fault such as a corroded joint or a damaged cable.
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