EN
Dielectric Strength Test
Dielectric strength (breakdown) test of insulating oil per IEC 60156: breakdown voltage measurement and acceptance criteria with accredited, assessed results.
Dielectric strength testing measures the resistance of insulation oil against electrical breakdown and is the fastest screening electrical parameter of oil quality. The sample is placed between two electrodes in a standard oil test cell; following IEC 60156 (or its national counterpart TS EN 60156) the voltage is raised by a constant ramp and the instant of arc (breakdown) across the electrodes is recorded. The operation repeats six times in a row; the average (and standard deviation) of the results reports the oil's instantaneous dielectric strength (kV). The test is also interpreted from the darkening-carbonisation trace on the sample after breakdown.
Why a Critical Item?
Basis strength alone does not reveal the chemical health of the oil — breakdown caused by particles and moisture occurs before chemical degradation. A low basis value can be the first signal of water emulsion, cellulosic particle or contamination in the oil, and the next step is dielectric losses evolving into a growing fault (partial discharge, thermal runaway). Results are therefore interpreted together with the other oil analyses; sub-threshold values lead to filtration-dehydration (Fuller's earth-vacuum unit) and retesting.
Procedure Steps and Acceptance Values
- The sample is brought to room temperature, stirred and transferred to the cell; the electrode gap is set to 2.5 mm.
- Voltage rises by a 2 kV/s ramp (per the IEC method); breakdown is noted and the series repeats after a 60-second rest.
- Average kV and % deviation are computed; typical practical acceptance bands: new oil 60 kV+; in service 40-50 kV caution/monitoring; below 30 kV intervention (filtration-drying) class — the limit is finalised by equipment voltage class and oil type per manufacturer and standard.
- High consistency along the series (low deviation) is a sign of sound sampling; a sudden drop raises moisture-particle suspicion.
- Breakdown events after rest intervals are counted, and the electrodes are cleaned and dried with a suitable solvent between the series; contamination accumulation on electrode surfaces also suppresses the reading.
Legislation and Standards
The test is part of the periodic test programme under Law No. 6331 and the Regulation on High Current Electrical Installations with maintenance specifications. Method standard IEC 60156 / TS EN 60156; supervision and limit approaches IEC 60422 and IEC 60599 (gas interpretation). The instrument (breakdown test set) is calibrated; the operator is an authorised electrical engineer; results are reported as laboratory or field measurement records, evaluated by trending with the annual oil panel.
Outcome
At the end of measurement the breakdown voltage average, deviation and assessment note appear in the signed report; for sub-threshold values a retest and conditioning (drying-filtration) proposal is communicated. The test is an item of our Transformer Tests and Analyses group and of our Insulation Oil Test panel; for other parameters see Density and Acidity, or contact us for a programme quotation.
FAQ on Dielectric Strength Test
It measures at how many kV the oil breaks down between two electrodes; it directly shows the electrical insulation margin of the oil. Moisture, gas bubbles, fibre-particle contamination and degradation products lower the breakdown voltage. Therefore this measurement is also used to verify the effectiveness of an applied drying-filtration treatment as much as the particles and moisture inside the oil.
Per the IEC 60156 methodology, with a standard electrode gap (typically 2,5 mm) and a stepped-ramp applied voltage, run with sample-darkness-ambient-temperature-rest preparation. Generally five consecutive breakdowns are measured and the median-mean reported; the distribution, not a single value, carries meaning. With high moisture-particles the result is low-unstable; then it is read together with dissolved water and particle count.
The acceptance threshold varies with oil condition and voltage class: high for new-clean oil (e.g. above 60-70 kV), lower thresholds accepted for operating oil, but a continuous fall or scattered multiple results is a caution sign. Below limit requires drying-degasification-filtration or change. The result is compared as a trend against the same transformer s past measurements; the slope, not a single snapshot, is the basis.
A marked and repeated fall of breakdown voltage, high particles with moisture, or low strength coinciding with gases that indicate internal arcing (C2H2): these say the insulation system is degrading and raise the risk of continuing under load. Then vacuum drying-filtration, fault localisation (DGA-tan delta) and, where needed, a take-out plan are arranged. Pre-post test notes are reported to verify the treatment.