How Do You Diagnose a Failed ASTM D1816 Result?

This is Part 5 of a seven-part series on dielectric breakage voltage testing in insulating transformer oils. Part 4 examined ASTM D1816 sensitivity in detail — how the method responds to specific contaminant types and what drives result variability. Here we will cover the factors that cause D1816 values to vary and what drives result inconsistency across samples and test conditions. Part 6 will examine D1816 applications for new oil, oil being processed for installation, and oil in newly commissioned transformers.

Starting With Cross-Reference Testing

When an ASTM D1816 result falls outside the acceptable range, the first step is cross-referencing against complementary test data — moisture, liquid power factor, liquid screen, and dissolved gas analysis (DGA).¹ This allows the most common causes of depressed D1816 values to be systematically evaluated and, in many cases, eliminated.

Moisture, oil aging, and most contamination types can frequently be ruled out through this cross-reference. When they are, the probable cause narrows to one of two factors: dissolved gas content or an abnormally high suspended particle count.

Evaluating Dissolved Gas as the Cause

Dissolved gas content is the first variable to assess when other causes have been eliminated. A total dissolved gas content of 80,000 ppm or greater increases the likelihood that gassing is the primary driver of the depressed D1816 value.

When Gas Content Is High

When dissolved gas content is at or above the 80,000 ppm threshold, routine follow-up testing is unlikely to identify a meaningful cause for the low dielectric breakdown voltage — and corrective action is generally neither necessary nor effective. The appropriate response in this situation is to defer D1816 retesting until the next routine sampling interval and to include an ASTM D6786 particle count distribution analysis at that time.²

When Gas Content Is Not Elevated

When dissolved gas content does not suggest gassing as the cause, the appropriate response is to repeat the D1816 determination on a fresh sample immediately and to run an ASTM D6786 particle count distribution analysis concurrently. This combination is generally sufficient to establish the cause of the abnormal result.

Gap Setting and Its Effect on Result Sensitivity

D1816 can be run at either the 1 mm or 2 mm gap setting. While both are available and can be run per customer specification, the 2 mm gap is the recommended default for in-service oil evaluation.

The 2 mm gap is slightly less sensitive than the 1 mm setting, which reduces the frequency of dissolved gas interference producing a threshold exceedance in oil that is otherwise acceptable. This makes results easier to interpret with confidence and reduces the rate of inconclusive outcomes driven by gas content rather than genuine oil condition issues.

Summary:

• Cross-reference testing is the first diagnostic step — moisture, liquid power factor, liquid screen, and DGA results should be consulted before drawing conclusions from a failed D1816 result.
• When other causes are eliminated, the probable cause narrows to dissolved gas content or abnormally high suspended particle counts.
• 80,000 ppm total dissolved gas is a key threshold — at or above this level, gassing becomes the primary suspect and routine corrective action is generally neither necessary nor effective.
• Below the gas threshold, immediate retest on a fresh sample combined with ASTM D6786 particle count distribution analysis is typically sufficient to identify the cause.
• The 2 mm gap setting is recommended for in-service oil evaluation — its reduced sensitivity relative to the 1 mm setting lowers the incidence of dissolved gas interference producing inconclusive or misleading results.
• Deferring retest is appropriate when high gas content is the suspected cause — retesting should be scheduled at the next routine sampling interval with D6786 included.

References:

¹ Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes, ASTM D1816-12(2019), ASTM International, West Conshohocken, PA, 2019.

² Standard Test Method for Particle Count in Mineral Insulating Oil Using Automatic Optical Particle Counters, ASTM D6786-15(2023), ASTM International, West Conshohocken, PA, 2023.

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?