Oil immersion (sometimes known as liquid immersion) involves completely immersing the equipment in a suitable oil, in order to exclude the explosive atmosphere. It is typically best suited for bulky, non-rotating equipment (such as high-power transformers) that would be difficult to protect by any other method. In some cases the oil might provide cooling as well as insulation.

Aside from transformers, there are a few rare kinds of equipment that are intended to be fluid-filled for other reasons, but not even all of these are necessarily suited to liquid immersion. This is because there are specific requirements as to the minimum oil depth, pressure relief, level indication, and other items that are not typically installed by default.

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