What Is an Insulation Resistance Test and What Result Should It Give in Sydney?

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What is an insulation resistance test and what result should it give in Sydney? An insulation resistance test, sometimes called a megger test, checks that the insulation around your cables is sound enough to stop electricity leaking to earth, by applying a test voltage and measuring the resistance between live conductors and earth. Under AS/NZS 3000 Clause 8.3.6, the result has to be at least 1 megohm, and every new or altered circuit must pass this test before it is allowed to be energised.

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1. What an insulation resistance test is actually checking

Every cable relies on its insulation to keep electricity travelling along the intended conductor rather than leaking out to earth or to an adjacent conductor. An insulation resistance test is a direct way of measuring how well that insulation is still doing its job, rather than assuming it is fine simply because the cable looks intact from the outside. Under AS/NZS 3000 Clause 8.3.6.1, the test exists specifically to prevent electric shock from inadvertent contact, fire from short circuits, and damage to connected equipment, and it is required on every new installation and most alterations before anything is energised.

2. How the test is carried out

The integrity of the insulation is stressed by applying a direct current test voltage of 500 volts for standard low voltage circuits. Where equipment such as electromagnetic compatibility filters, surge protective devices connected to earth, or sensitive electronic equipment could be damaged by the test, that equipment can be disconnected or switched off first, or the test voltage for that particular circuit can be reduced to 250 volts direct current instead. The test instrument itself has to be capable of holding its terminal voltage within a defined tolerance while measuring typical resistance values, which is why a proper calibrated insulation resistance tester, not a standard multimeter, is used for this job.

3. The 1 megohm minimum and what counts as a healthy reading

AS/NZS 3000 Clause 8.3.6.3 sets the minimum acceptable insulation resistance at 1 megohm, measured both between the conductors of consumer mains and submains, and between live and earthed parts of the installation generally. In practice, a healthy new installation reads well above this floor. For shorter cable runs the standard notes the resistance should be significantly greater than 1 megohm, for example polymeric cables up to 50 metres would typically be expected to read in excess of 50 megohms when new, while PVC insulated cable of the same length might read around 20 megohms at 20 degrees but only 6 megohms at 30 degrees, since insulation resistance decreases as temperature rises and as cable length increases. A small number of specific exceptions apply, including a lower 0.01 megohm minimum for sheathed heating elements in appliances and 0.05 megohm for the functional earth connection of a residual current device, reflecting how those particular components behave differently to standard cable insulation.

4. Where this test fits in the overall testing sequence

Insulation resistance testing is not done in isolation. AS/NZS 3000 Figure 8.1 sets a required testing sequence, starting with earth continuity of the main earthing conductor and bonded parts, then insulation resistance, followed by polarity checks on consumer mains, submains and final subcircuits, correct circuit connections, earth fault loop impedance where applicable, and finally verification that safety switches operate correctly. Our guide to electrical testing and commissioning walks through that full sequence, while this guide focuses specifically on the insulation resistance step within it, and our guide to polarity testing covers the very next step in that same sequence.

5. Why a failed insulation resistance test matters

If a circuit fails this test, it cannot be energised until the fault is found and corrected, and the test then has to be repeated, along with any earlier test in the sequence that the same fault could have affected. A low reading most commonly points to insulation that has been physically damaged during installation, insulation that has degraded with age, or moisture that has found its way into a cable or enclosure, any of which represents a genuine shock, fire or equipment damage risk if left unaddressed. This is also why insulation resistance testing is a standard part of a thorough electrical safety inspection on an older property, since it can reveal insulation problems well before they progress to a visible fault.

Why it's Level 2 ASP work

Insulation resistance testing of the internal wiring within a property is standard electrical contracting work carried out by any licensed electrician. AS/NZS 3000 Clause 8.3.6.1 specifically calls out an additional insulation resistance test between the conductors of consumer mains and submains, which is necessary to minimise the risk of injury or property damage from insulation breakdown on the service connection itself, and testing of that connection falls within Level 2 ASP accreditation.

How High Demand Electrical helps

We are a licensed local team, not a call centre, and we run a proper insulation resistance test, with the correct test voltage and a calibrated instrument, on every new circuit and every consumer mains connection we install, rather than treating it as a box ticking exercise. Servicing homes and businesses across Sydney's West, South West and Sutherland Shire, we also use insulation resistance readings as a genuine diagnostic tool on older properties, where a reading that has crept down over the years can flag a developing problem well before it causes a nuisance trip or a fault. HD Power Pole's guide to the private power pole inspection checklist covers the related testing and inspection steps that apply when a pole and its service cable are checked as part of the same job.

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