9.3 Insulation Resistance Testing (500 V DC, 1 MΩ)

Classic Values Candidates Must Own

For typical low-voltage installations (nominal circuits in the common 230/400 V range), AS/NZS 3000 Section 8 verification teaches two numbers that appear constantly in EWRB preparation material, exam-meta key stats, and practice banks:

Confirm exact application ranges, any exceptions, and wording in the permitted edition of AS/NZS 3000 for your Aspeq sitting. Higher test voltages (e.g. 1000 V DC) belong to higher circuit voltage categories—not the default domestic/commercial LV story. Lower values such as 0.5 MΩ are not the standard minimum taught for this pathway.

These figures sit in the dead-test block after earth continuity (8.3.6 region) and before polarity/correct-connection checks, still before energised EFLI and RCD tests.

What the Test Measures

An insulation resistance tester (megohmmeter) applies a controlled DC voltage and measures resistance of the insulation system. Desired results are high (megohms), the opposite of continuity testing’s low-ohm goal.

Typical test combinations

• Between live conductors and earth — actives (and neutral where the method requires) referenced to the protective earthing system
• Between actives (and other live–live combinations) as applicable to the circuit arrangement and the method in AS/NZS 3000 / 3017

The goal is to show insulation is intact enough that leakage under test voltage is acceptably low before the installation is put into service or re-energised after PEW.

Care with electronic equipment

Disconnect or isolate sensitive gear before applying 500 V DC:

• Electronic ballasts, dimmers, surge protective devices, control electronics, and many modern appliances can be damaged by IR test voltage
• Connected loads and electronics can also produce falsely low readings that look like insulation failure when the wiring is fine

Open links, remove loads, or isolate equipment so the reading reflects installation wiring insulation, not the internal paths of connected devices—unless the procedure deliberately includes them (exam stems almost always want isolation of sensitive equipment).

Interpreting Low Readings

A reading below 1 MΩ (or trending unexpectedly low) is a stop-and-investigate result, not a “nearly pass.” Common causes:

Do not energise and hope. Dry, repair, disconnect, or re-terminate as required; then retest until the installation meets the minimum and makes engineering sense.

Safety: Prove Dead and Discharge

Insulation testing is performed on a de-energised installation section.

1. Prove dead with a proven testing process before connecting the IR tester—never apply a megohmmeter to a live circuit “to save time.”
2. Apply the specified DC test voltage using a suitable instrument.
3. After the test, discharge capacitance in cables and equipment. Long cable runs store charge after high-voltage DC testing; leave residual charge and the next person to touch the conductors may receive a shock or damage other equipment.

AS/NZS 3017 guidance emphasises correct instrument use and safe method; treat discharge as part of professional IR practice, not optional etiquette.

Exam Trap: IR MΩ vs RCD mA

Candidates routinely mix units and purposes:

Wrong polarity of thinking: “RCD is 1 mA” or “IR must be 30 MΩ because of 30 mA RCDs.” Keep megohms for insulation and milliamps for residual current in separate mental drawers.

Sequence reminder

Visual → continuity → IR → polarity → correct connections → (later) EFLI → RCD. IR is still a dead test. Passing IR does not replace continuity, polarity, or live functional tests.

When IR fails, do not “skip to live tests”

A sub-minimum IR result means the installation is not ready for energisation for EFLI or RCD operation tests. Live testing on wiring with known insulation failure risks further damage, nuisance trips that hide the real fault, and shock/fire hazards. The competent path is: isolate, diagnose, repair or dry out, retest IR to ≥ 1 MΩ, then continue the sequence. Certification evidence should show that insulation was proven before service connection decisions that depend on verification.

AS/NZS 3017 remains the practical methods companion—probe technique, instrument selection, and safe application of DC test voltage—while AS/NZS 3000 Section 8 states the verification requirement and acceptance outcome. On open-book stems, open Section 8 for “what must be achieved,” and use 3017 thinking for “how the test is performed safely.”

Study note

Memorise for speed: 500 V DC, ≥ 1 MΩ, disconnect electronics, prove dead, discharge after. Open Section 8 IR clauses in your permitted book once so clause geography matches the numbers you already know. Drill the unit trap until automatic: MΩ = insulation; mA = RCD.