Power distribution and other types of transformers can have their internal windings and core submersed in oil. The oil is passed through a radiator mounted to the outside of the transformer ultimately cooling the windings. The oil is also used to insulate the core and windings from the grounded metal tank that they are mounted in. The oil is often used as a diagnostic tool as well. Taking a sample of the oil and analyzing it can be a way to determine the health of the transformer. Although not much oil needs to be extracted from the transformer at one time if the oil is sampled year after year, a good amount of the oil may have been removed. If enough oil is removed so that the windings and core are no longer submersed excess heat can cause a failure to occur.
See the picture below for the recommended and minimum transformer oil level that Cooper Power identifies for Bay-O-Net fuse assembly installation (manual MN132003EN).
Low oil could result in severely damaging the transformer and in some cases the metal tank may rupture allowing all of the transformers oil to leak out.
Preventing a Failure
If you have oil filled transformers and are sampling the oil for diagnostic purposes be sure to include checking the oil level as part of the work procedure or instructions. It may be a good idea to periodically check oil levels as part of preventative maintenance programs. Be sure to check the equipment manufacturers instructions for the recommended oil level. Just because the oil gauge does not indicate a low level some internal parts of the transformer may be exposed. A simple glance at your oil level gauge and adding a little oil may prevent a catastrophic failure. If you do not have an oil level gauge then some simple math can help you decide next steps. The math would involve the quantity of samples you have performed and the volume lost each time. Next steps could result in deciding whether or not to perform proper LOTO and removal of the transformer cap to perform visual inspection of oil level.
Infrared scanning of your equipment might also provide evidence of low oil or impending failure. The picture below shows cool fins but you can see the main tank of the transformer is hot, this transformer's oil was low enough that there was no oil circulation through it's cooling fins.
The image below has proper oil levels and circulation through it's cooling fins.
Thanks to Dennis Hill (and Paul Sullivan) for the excellent presentation at the 2022 IEEE Electrical Safety Workshop which is the source for this article. [Hill, Sullivan (2022) Transformer Failure Due to Oil Sampling. ESW 2022-26, IEEE ESW, Jacksonville FL March 2022]
References to standards for Oil Testing:
NFPA 70B, Recommended Practice for Electrical Equipment Maintenance
ASTM D3612, Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography.
ANSI/IEEE C57.104, Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers.)
ANSI/IEEE C57.106-2002, Guide for Acceptance and Maintenance of Insulating Oil in Equipment
Here is a great article explaining some of the techniques and precautions of transformer oil sampling: https://electricaltesttech.com/transformers/transformer-oil-samples/