How Does IEC 60156 Compare for Dielectric Breakdown Testing?

This is Part 7 of a seven-part series on dielectric breakage voltage testing in insulating transformer oils. Part 6 examined ASTM D1816 applications for new oil, oil being processed for installation, and oil in newly commissioned transformers. Here we will complete the discussion of standard dielectric breakdown voltage methods, examining IEC 60156 and its applications.

How IEC 60156 Is Run

IEC 60156 uses spherical electrodes geometrically similar to the VDE electrodes specified in ASTM D1816, with a gap spacing of 2.5 mm and a voltage rise rate of 2,000 volts per second. Agitation is optional — when used, an impeller operates at 250–300 rpm in the same configuration as D1816. The method also permits a magnetic stirrer at a comparable rate, provided there is no significant risk of removing magnetic particles from the oil. Because magnetic particles affect dielectric breakdown behavior in service, their removal during testing would produce unrepresentative results.

What to Expect From IEC 60156 Values

D1816 values for properly maintained transformers are typically considerably higher than those obtained by either ASTM method — frequently exceeding the maximum reportable value of many dielectric breakdown voltage instruments. Most instruments report a maximum of 60 kV, which is generally sufficient for evaluating new oil from a supplier or in-service oil under routine conditions.

For oil being processed for installation, or when a more precise value is required for in-service oil, an instrument capable of measuring beyond 60 kV is advisable.

Sensitivity Profile and Dissolved Gas Behavior

The contaminants and conditions that affect IEC 60156 results are essentially the same as those that affect D1816 — moisture, oil oxidation, non-dielectric contamination, and suspended particles all influence the result. Dissolved gas content can also depress IEC 60156 values, as it does with D1816, but the magnitude of that depression is less pronounced under the IEC method. This makes IEC 60156 somewhat more robust than D1816 in high dissolved gas environments, though the sensitivity to other contaminant types remains comparable.

Acceptance and Maintenance Values Under IEC Standards

IEC 60156 values are referenced in two IEC standards, each governing a distinct application.

Unused Oil: IEC 60296

IEC 60296, Fluids for Electrotechnical Applications – Unused Mineral Insulating Oils for Transformers and Switchgear, specifies that unused oil received in bulk from a supplier must have a minimum dielectric breakdown voltage of 30 kV by IEC 60156.¹ After vacuum filtration in the laboratory, the minimum rises to 70 kV. Note that IEC 60296 has been revised — the current version is IEC 60296:2020, which should be referenced for current values.

New and In-Service Oil: IEC 60422

IEC 60422, Mineral Insulating Oils in Electrical Equipment – Supervision and Maintenance Guide, addresses dielectric breakdown voltage at two stages:

Stage 1: For new oil in new equipment after filling but prior to energizing, acceptable values are specified by voltage class as indicated in the table below.

Note: The acceptable limits for these oils vary based on the highest operating voltage of the equipment. The water content limit is ≤ 20 mg/kg for equipment < 72.5 kV and < 10 mg/kg for all equipment ≥ 72.5 kV.²

Stage 2: For in-service transformers, IEC 60422 classifies results as Good, Fair, or Poor, with corresponding maintenance recommendations:

• Good: Normal condition. Continue routine monitoring intervals.
• Fair: Deterioration detectable. Increase sampling frequency and cross-check with moisture content, particle count distribution, dissipation factor or resistivity, and acid number.
• Poor: High risk. Schedule immediate maintenance. Recondition the oil to return it to acceptable condition.

Note: IEC 60422 has also been revised — the current version is IEC 60422:2024, which should be referenced for current values.

Temperature and Test Validity

As with all dielectric breakdown voltage methods, IEC 60156 results are sensitive to oil temperature. Testing hot oil — whether drawn directly from an oil processor or from in-service equipment — will produce results that are not reproducible or valid. Oil must be allowed to reach an appropriate temperature before testing is conducted.

Summary:

• IEC 60156 uses VDE-style spherical electrodes with a 2.5 mm gap and a 2,000 V/s voltage rise rate — geometrically similar to D1816 but with optional rather than required agitation.
• IEC 60156 values are typically higher than those from either ASTM method — results for well-maintained transformers frequently exceed the 60 kV maximum of standard dielectric breakdown instruments.
• Dissolved gas depresses IEC 60156 results less severely than it does D1816, making the IEC method somewhat more robust in high dissolved gas environments.
• Two IEC standards govern acceptance criteria: IEC 60296 for unused oil (30 kV as received, 70 kV after vacuum filtration) and IEC 60422 for new and in-service oil evaluation by voltage class and condition category.
• IEC 60422 uses a three-tier condition classification — Good, Fair, and Poor — with specific monitoring and maintenance recommendations for each tier.
• Temperature control is a validity requirement — testing hot oil from processing equipment or in-service transformers produces non-reproducible results and should be avoided.
• Both IEC 60296 and IEC 60422 have been revised — current values should be drawn from IEC 60296:2020 and IEC 60422:2024 respectively.

References:

¹ International Electrotechnical Commission. (2003). Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear (IEC Standard No. 60296:2003).

² International Electrotechnical Commission. (2013). Mineral insulating oils in electrical equipment – Supervision and maintenance guidance (IEC Standard No. 60422:2013).

In This Series:

• Part 1: How Are Dielectric Breakdown Voltage Tests Standardized?
• Part 2: What Does ASTM D877 Actually Detect — and When?
• Part 3: How D1816 Became the Standard for Dielectric Breakdown Testing
• Part 4: How Do You Interpret ASTM D1816 Results Accurately?
• Part 5: How Do You Diagnose a Failed ASTM D1816 Result?
• Part 6: How Does ASTM D1816 Apply to New Insulating Oil?
• Part 7: How Does IEC 60156 Compare for Dielectric Breakdown Testing?