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Voltmeter measurements for foil insulation

Find out more about testing foil insulation for possible touch voltages.

Where de-energised testing has proven an electrical installation is electrically safe, but further testing with a high impedance voltmeter indicates the presence of a possible touch voltage, further testing needs to be undertaken to determine the nature of this voltage.

A characteristic of a high impedance meter is that electrical measurements can indicate a voltage that may in fact be due to a touch voltage other than directly from the mains supply or electricity generation systems. These voltages have a high impendence source and can occur as a result of capacitive coupling between energised circuits and non-energised, non-electrically connected adjacent wiring or conductive parts.

Due to the coupling effect under normal test conditions, using a high impedance multimeter (most digital volt meters) it is not always possible to determine if the circuit or conductive part under test is energised or de-energised. This may create confusion when performing the test and can lead to unnecessary work practices such as disconnections and isolations.

To determine whether the touch voltage is "sustained" or otherwise, the voltage measurements can be made with a low impedance load connected in parallel with the high impedance voltmeter measuring the circuit. Generally, for a high impedance digital voltmeter, this is achieved with a suitable plug-in resistor of appropriate wattage connected across the terminals of the voltmeter.

Lowering the impedance of the voltmeter will establish whether the voltage is from a sustained or a relatively low impedance source. If the voltage originates from a sustained source the reading will remain relatively constant. Conversely, if the measured voltage is from an induced voltage from within the installation the voltmeter will read very close to zero voltage, indicating the circuit, connection, or conductive part is not energised.

Where the measurements do not exceed the values in clause 3.9.2 of AS/NZS 3017 Electrical Installations - Verification Guidelines then generally this does not present an electrical risk.

When selecting instruments and test devices, the device's function, range and class of accuracy should be appropriate to both work and conditions. Guidance on an instrument's characteristic, including attachments to measure sustained voltages, can be obtained from the manufacturer or their representative.

Further details about the selection, testing and maintenance of instruments can be found in the Electrical Safety Code of Practice for Managing Electrical Risks in the Workplace (PDF, 0.38 MB) while information about measuring touch voltage can be sourced from AS/NZS 3017 (refer clause 3.9).

Testing is electrical work and, where applicable, the requirements of the Electrical Safety Regulation 2013 section 9 (Electrical work on energised electrical equipment) apply to this electrical work.

Further details are also available from the Electrical Safety Code of Practice for Managing Electrical Risks in the Workplace (PDF, 0.38 MB)