How is a series circuit tested before it is energised?
A series circuit is proved as a whole loop, after its cables, connectors and transformers have been tested one by one. With the CCR isolated, the loop is checked for resistance and for insulation to earth, then brought up step by step while the current is read at each step.
Cable, connector and transformer tests each prove one part. Series-circuit testing proves the whole loop: every cable, joint, connector and isolation transformer wired in series, out from the CCR and back again. In a series circuit the same current flows through every fixture, so a single break, poor contact or earth fault anywhere affects the entire loop. Series-circuit testing finds it before handover.
The CCR is first isolated, locked off and proved dead, and the loop is disconnected from its output terminals. A low-resistance ohmmeter across the two loop ends gives the loop resistance, which is compared with the value calculated from the cable lengths and the transformer primaries. An insulation-resistance tester then measures the whole loop to earth. Only when both are satisfactory is the loop reconnected and energised, starting on the lowest brightness step, with the current read at each step. Never open a live series loop; an open on an energised loop can put a dangerous voltage across the break.
- Isolate. Switch off the CCR, lock it off under the site permit and prove the loop dead. Disconnect the loop from the CCR output terminals so the CCR is not part of the measurement.
- Measure loop resistance. Connect a low-resistance ohmmeter across the two loop ends. Compare the reading with the value calculated for the cable length and the number of transformers.
- Test insulation. Measure insulation resistance from the loop conductor to earth at the specified test voltage. The value should be high and steady for the whole loop. Discharge the loop afterwards, because the cable holds a charge.
- Reconnect and energise. Reconnect the loop, close the permit and bring the CCR up on the lowest brightness step first. Watch for any earth-fault or open-circuit alarm.
- Read the current at each step. Measure the loop current with a true-RMS clamp meter closed round one conductor, or the CCR meter, at every brightness step and compare it with the nominal step current. Confirm every fixture lights.
- Record. Log the loop resistance, insulation resistance and step currents with the circuit ID, date and instruments. This is the baseline for later tests.
What the readings suggest
Matches the design
Loop resistance is close to the calculated value, insulation resistance is high and steady, and each step current is on nominal. Record it and carry on.
Open loop
No continuity means a break: an unmated connector, a loose terminal or a broken conductor. Split the loop into sections to find it.
Resistance above design
Suspect a poor connector or terminal, undersized cable, or more transformers than designed. Compare with the as-built count.
Resistance below design
Possible short across part of the loop or a missing transformer. Check against the as-built drawing before energising.
Low insulation resistance
Moisture or damage has probably created an earth fault. Section the loop at the transformer pits to find the length at fault.
Step current off nominal
Check the CCR setting and meter, and look for leakage or a faulty transformer. Investigate before handover.
Frequently Asked Questions
Q1What is series-circuit testing in AGL commissioning?
It is the set of tests that prove a complete series loop as a whole. With the CCR isolated, the loop resistance and the insulation resistance to earth are measured; the loop is then energised on the lowest brightness step and the current is checked at every step. Results are recorded by circuit ID.
Q2Why test the whole loop when the parts are already tested?
In a series circuit the same current passes through every fixture, so one fault anywhere affects the entire loop. Cable, connector and transformer tests prove the parts; series-circuit testing proves they work together and match the design. CCR testing proves the regulator, while this test proves the loop it feeds.
Q3Why is the CCR disconnected for the resistance and insulation tests?
So the CCR is not part of the measurement and is not exposed to the test voltage. The CCR is switched off, locked off and proved dead, and the loop is disconnected from its output terminals before any instrument is connected.
Q4What does the loop resistance tell you?
It is compared with the value calculated from the cable lengths and the transformer primary resistances. A close match shows the loop is continuous and built as designed. No continuity means a break, a reading well above the design suggests a poor connection, and a reading below it points to a short or a missing transformer.
Q5Why start on the lowest brightness step?
The lowest step puts the least current and stress on a loop that has only just been connected, so any remaining fault shows up gently. Each higher step is then selected in turn while the loop current is read and compared with the nominal step current.
Q6Can a series loop be opened while it is energised?
No. An open on an energised series loop can put a dangerous voltage across the break. The CCR is always switched off, locked off and the loop proved dead before any connector or cable is opened.
Q7What resistance, insulation and current values are acceptable?
They depend on the circuit design, the CCR class, the cable and transformer data and the project specification, so this page gives no fixed figures. Use the values in the specification and the calculated loop resistance, and compare with earlier readings on the same circuit.
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