Understanding Dielectric Breakdown and Breakdown Voltage Testing in Electrical Oil Sampling

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A new oil sampling customer of our lab asked me the other day do we provide any training materials on different oil analysis tests. I was so pleased to show him the whole LearnOilAnalysis.com portal of articles, books and videos – all free to read and watch. This particular customer had a key interest in electrical oil sampling and although I have a few materials on dissolved gas analysis, I decided to add a specific one on breakdown voltage testing in response to his query. Hence enjoy the article below.

Welcome to another exciting article from LearnOilAnalysis.com, where we explore the fascinating world of oil analysis and provide you with valuable insights and resources to make your life easier. Today, we will dive into the electrifying topic of dielectric breakdown and breakdown voltage testing in electrical oil sampling. So, grab your LOA branded lab coat, put on your safety goggles, and let’s get started!

Dielectric Breakdown: Shockingly Important

Dielectric breakdown is a phenomenon that occurs when an insulating material, such as electrical oil, fails to resist the flow of electric current under high voltage. This can lead to electrical discharge and potentially catastrophic failure of the equipment, such as transformers, that relies on the insulating properties of the oil. Trust me, you don’t want your transformer to suddenly turn into an expensive, sparking paperweight!

The good news is that we can measure the breakdown voltage of insulating fluids to ensure their effectiveness and monitor their condition. This is where dielectric breakdown voltage testing comes into play.

Breakdown Voltage Testing: ASTM and IEC Methods

There are a few methods for measuring breakdown voltage but the two main internationally used methods for measuring the breakdown voltage of insulating fluids are: the ASTM (American Society for Testing and Materials) method and the IEC (International Electrotechnical Commission) methods. While both methods are designed to zap your curiosity, there are slight differences in their approach. For those not interested in the method variation skip to ‘Breakdown Voltage in Video’.

IEC60156 vs ASTM D1816

There are a few methods for measuring dielectric breakdown, but the two most commonly used methods IEC60156 and ASTM D1816. They are very similar and often give very similar results regardless of methods used but for consistency it’s best to keep the method used each time on your samples the same. Being a lab in the UK the most widely used method over here is the IEC one and it is the one we use in our lab, but I will discuss both for our international audience.

Electrode material

Most are brass or bronze, but some can be stainless steel versions too in the ASTM version.

Electrode distance

The ASTM version allows variations on distance of 1 or 2mm. However the IEC is fixed on 2.5mm.

Electrode shape

IEC uses ball shaped electrodes but ASTM uses VDE (Verband Deutscher Elektrotechniker) electrodes which are like two rounds cut in half.

4 Understanding Dielectric Breakdown and Breakdown Voltage Testing in Electrical Oil Sampling
Comparison of common electrode types. Size and gap between electrodes have been adjusted for illustrative purposes to emphasise shape differences.

Test conditions

ASTM uses a much slower rate of voltage increase of 0.5 kV/s compared to IEC 2 kV/s and more replicates of 5 up to 10 compared to 6 of IEC. Hence you tend to get more precision with the ASTM parameters, but I personally find repeating the IEC test 2 or 3 times gives you 12 or even 18 replicates for the test and more than compensates for this. We as a minimum use the average of 12 in our lab.

Breakdown voltage in video

The above description may seem a bit detailed to explain what’s happening, but in simplicity you increase the voltage across an electrode pair at a fixed gap emerged in the oil until electricity jumps across. You can see this happening below.

1 0 gif 2 Understanding Dielectric Breakdown and Breakdown Voltage Testing in Electrical Oil Sampling
As can be seen when the breakdown voltage has been reached a small bolt of electricity passes through the oil and jumps the gap overcoming the insulating properties of fluid. You will also notice some small bubbles forming. These are fault gases. You can find out more about these here.

What’s Normal and Abnormal, Based on Operating Voltage

The acceptable breakdown voltage varies based on the operating voltage of the transformer. Here’s a rough guideline (there are different methods of classifying and setting limits, but these should apply fairly close to whatever each readers local variations are):

  1. Transformers with an operating voltage of 69 kV or below: A breakdown voltage of 30 kV or above is considered acceptable.
  2. Transformers with an operating voltage between 69 kV and 230 kV: A breakdown voltage of 40 kV or above is deemed suitable.
  3. Transformers with an operating voltage above 230 kV: A breakdown voltage of 50 kV or above is required.

If the breakdown voltage falls below these thresholds, it may be an indication that the insulating fluid is contaminated or degrading, and appropriate corrective actions should be taken.

A Jolt of Humor

Why did the electrical oil sample refuse to participate in the breakdown voltage test? It had a lot of resistance.


Understanding dielectric breakdown and the importance of breakdown voltage testing is crucial for maintaining the reliability and longevity of your electrical equipment. By monitoring the condition of your insulating fluids and taking corrective actions when needed, you can ensure a smooth and uninterrupted flow of electricity. Stay tuned for more exciting articles and resources from LearnOilAnalysis.com!

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