IEC 60156:2025

Overview

IEC 60156:2025 - Insulating liquids - Determination of the breakdown voltage at power frequency - Test method - specifies a standardized AC test method to determine the dielectric breakdown voltage (power-frequency) of insulating liquids. The procedure defines required apparatus, sample handling, test-cell geometry and electrodes, and test control so results are comparable worldwide. It applies to insulating liquids with nominal viscosity up to 350 mm2/s at 40 °C and is suitable for both acceptance testing of new liquids and condition assessment during equipment monitoring and maintenance.

Key topics and technical requirements

• Scope: Method for measuring breakdown voltage at power frequency (48 Hz to 62 Hz sinusoidal supply).
• Apparatus components: Voltage regulator, step-up (or resonant) transformer, switching system, current‑limiting resistors and measuring system.
• Voltage control: Automatic, uniform rate of rise; voltage increments shall be smooth and without overshoot. Incremental steps should not exceed 2% of the expected breakdown voltage.
• Electrical integrity: Transformer secondary centre-point earthed; AC source free from significant harmonics (