Common diagnostics combine electrical, chemical, and contamination checks. Core methods include BDV breakdown voltage, moisture by Karl Fischer (ppm), acidity neutralization number, dielectric dissipation factor (tan delta or power factor), resistivity, and interfacial tension for oxidation byproducts. Dissolved Gas Analysis (DGA) identifies fault gases for thermal or electrical defects, and furan analysis estimates paper insulation aging. Particle count, color, inhibitor content, and corrosive sulfur tests are added when risk is higher. Trending results over time is often more valuable than single measurements.
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