The IEC 60156:2025 standard outlines the procedure for measuring the dielectric breakdown voltage of insulating liquids at power frequency. This assessment is performed with a specific testing apparatus in which an oil sample is subjected to an increasing alternating current electric field until breakdown occurs. This method applies to all types of insulating liquids that have a nominal viscosity of up to 350 mm²/s at 40 °C.
It is designed for both the acceptance testing of new liquids at the time of delivery and for evaluating the condition of samples collected during equipment monitoring and maintenance activities.