Test insulating oil – avoid downtimes

Recommendations for the lifeblood of transformers

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Insulating oils ensure the safe operation of power transformers. The ageing condition of these oils changes due to oxidation, thermal stress and contact with the other materials and significantly determines the service life of the transformer.

If the inspection of these insulating oils is neglected, this leads to failures. You can see the serious consequences of these failures in the video. To avoid such accidents, there are recommendations as to when the insulating oils should be checked:

Transformers with a rated voltage between 170 kV and 400 kV

Transformers with a rated voltage between 72.5 kV and 170 kV

Transformers for medium and low-voltage applications up to 72.5 kV

Routine testing

1-2 years

1-3 years

1-4 years

2-6 years

More detailed information can be found in the IEC 60422 standard.

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Taking oil samples made easy:

In order to obtain meaningful results from insulating oil testing, oil samples must be properly taken by instructed personnel. Here are a few tips to help you take samples without any issues:

Use a special sample container: for example, a clean aluminium bottle with a capacity of approx. 1 litre.

Store the sealing ring: When opening the sampling nozzle, the sealing ring may get lost when the safety cap is unscrewed. It must remain on the nozzle.

Avoid air bubbles: Removal should be take place using a device that features an inspection window. This ensures any air bubbles are detected. Air bubbles falsify the oil sample. This device is first screwed onto the nozzle.

Pay attention to your surroundings: When taking samples outdoors or in a dirty environment, make sure that no foreign bodies or drops are able to enter the bottle.

Close: After filling the bottle, make sure it is sealed airtight using the screw cap. The bottle needs to be filled to the brim with oil so that no air remains in the bottle neck. After sealing, the bottle can be cleaned, packed and the excess oil can be disposed of.

Flushing: A hose is attached and the oil is removed until no more air bubbles appear. Flushing the hose with one to two litres of oil is usually sufficient.

Flushing the sample bottle: The sample bottle (already cleaned and dried) is then filled with oil, then placed on its side and swivelled or rotated so that the oil dampens the entirety of the wall surface and absorbs any remaining dirt particles. The oil used for flushing the bottle must then be poured out and disposed of properly at a later point.

Taking samples: For the actual sampling, the hose connected to the sampling nozzle should reach to the bottom of the sample container. The bottle is slowly filled to avoid bubbles until it overflows. Up to 50 % of the bottle's contents may overflow. This produces a homogeneous sample without gas pockets. The overflowing oil needs to be collected, e.g. in a bucket, and disposed of properly.

How often should insulating oils be tested? How often should insulating oils be tested?

Application video

Video tutorial: sampling liquids | BAUR GmbH

Video tutorial on sampling insulating liquids (EN)

This video tutorial demonstrates how to take a sample of insulating liquid according to IEC 60475.

Further topics relating to insulating oil testing:

Download brochure (*.pdf) Download brochure (*.pdf)

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