How Are Dielectric Breakdown Voltage Tests Standardized?

This is Part 1 of a seven-part series on dielectric breakage voltage testing in insulating transformer oils. Here we will compare the three standard testing methods. In Part 2, we will examine how each method responds to specific contaminant types and what drives variability in ASTM D1816 test results.

Why Standardized Test Methods Matter

Dielectric breakdown voltage testing is used to detect contamination in insulating oils — including moisture, particulate matter, and oxidation byproducts. The sensitivity of any given method to these contaminants, along with its known interferences and limitations, must be considered when selecting a method and interpreting results. Three standard methods are in common use across the industry, each governed by a recognized standards body and optimized for specific conditions.

The Three Standard Test Methods

ASTM D877: Disk Electrode Method

ASTM D877 is the oldest of the three methods. It uses flat, coin-shaped disk electrodes spaced 0.1 inches (2.54 mm) apart and applies AC voltage at a rise rate of 3,000 volts per second.¹

Pre-test electrode preparation is critical for valid results. Before each test, electrodes must be inspected for contamination, pits, and corrosion, then cleaned and polished per the standard as needed. Edge condition must be verified with a gauge — rounded edges are a disqualifying defect — and gap spacing must be confirmed using round or flat gauges per the standard method.

ASTM D1816: VDE Electrode Method

ASTM D1816 uses electrodes conforming to VDE (Verband Deutscher Electrotechniker) Specification 0370 — the standard from the German Association for Electrical, Electronic, and Information Technologies, which functions similarly to IEEE in the United States.² These electrodes are spherical-segment shaped, each being a section of a 25 mm radius sphere with edges rounded to a 4 mm radius.

D1816 offers two gap settings: 1 mm (0.039 inches) or 2 mm (0.079 inches). The AC voltage rise rate is 500 volts per second — significantly slower than D877. A two-bladed mechanical impeller mounted below the electrodes runs at 200–300 rpm, directing oil flow toward the bottom of the test cell. This agitation is designed to gently disturb the sample and drive any particles present into the electrode gap, increasing method sensitivity to particulate contamination.

As with D877, electrodes must be inspected, cleaned, polished if necessary, and gap-confirmed before the test cell is filled.

IEC 60156: International Standard Method

IEC Standard 60156 uses electrodes that are geometrically similar to the VDE electrodes specified in D1816, with a gap spacing of 2.5 mm and a voltage rise rate of 2,000 volts per second.³

Agitation under IEC 60156 is optional. When used, the impeller operates at 250–300 rpm in the same configuration described for D1816. The method also permits a magnetic stirrer operating 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.

How the Test Is Performed

Across all three methods, the test procedure follows the same general sequence. Electrodes are mounted in a test cell, which is filled with the insulating liquid under evaluation so that the electrodes are fully submerged. The cell is installed in the dielectric breakdown voltage apparatus, and AC voltage is applied and increased at the method-specified rate until breakdown occurs. Breakdown — a disruptive discharge between the electrodes — triggers automatic circuit interruption equipment built into the apparatus.

Comparing Key Method Parameters

The three methods differ in several measurable ways that affect both sensitivity and applicability:

• Electrode geometry: Flat disk (D877) vs. spherical-segment VDE-style (D1816, IEC 60156)
• Gap spacing: 2.54 mm (D877) / 1 mm or 2 mm (D1816) / 2.5 mm (IEC 60156)
• Voltage rise rate: 3,000 V/s (D877) / 500 V/s (D1816) / 2,000 V/s (IEC 60156)
• Agitation: Not used (D877) / Required impeller (D1816) / Optional impeller or magnetic stirrer (IEC 60156)

These differences are not incidental — they directly influence how each method responds to specific classes of contamination and under what conditions each method is most appropriately applied.

Summary:

• Three standard methods are used for dielectric breakdown voltage testing: ASTM D877, ASTM D1816, and IEC 60156 — each differing in electrode geometry, gap spacing, and voltage rise rate.
• Electrode design matters: D877 uses flat disk electrodes; D1816 and IEC 60156 use spherical-segment (VDE-style) electrodes with edges rounded to a 4 mm radius.
• Agitation is a key differentiator: D1816 requires a mechanical impeller; IEC 60156 makes it optional and also permits magnetic stirring under certain conditions.
• Test sensitivity varies by method — each responds differently to moisture, particulate contamination, and oxidation byproducts, which affects method selection and result interpretation.
• Pre-test electrode inspection is mandatory across all three methods — cleanliness, gap confirmation, and surface condition directly affect result validity.

References:

¹ Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes, ASTM D877/D877M-19, ASTM International, West Conshohocken, PA, 2019.

² Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes, ASTM D1816-12(2019), ASTM International, West Conshohocken, PA, 2019.

³ Insulating Liquids – Determination of the Breakdown Voltage at Power Frequency – Test Method, IEC 60156:2025, International Electrotechnical Commission, Geneva, Switzerland, 2025.

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?