We do not supply these. This page is here because the question comes up and the answer is hard to find written plainly. What we do supply is three phase and single phase pad mounted distribution transformers from 25 to 2500 kVA.
Mineral oil is the default
Most liquid filled distribution transformers are filled with mineral oil. It is cheap, well understood, insulates well and moves heat efficiently. Its weakness is flammability, with a fire point around 160 degrees Celsius, which is what limits where a mineral oil unit can be installed without additional fire protection.
FR3 and ester fluids
Natural and synthetic ester fluids, FR3 being the best known brand, have a fire point above 300 degrees Celsius and are classified as less flammable. That can remove the need for a fire wall or additional separation, and they are biodegradable, which matters near water. They cost more, they are more viscous, and they absorb moisture differently, so they are specified when the installation demands it rather than by default.
What oil testing tells you
A dissolved gas analysis reads the gases in the oil and infers what is happening inside a sealed tank. Acetylene points to arcing, ethylene to overheating, hydrogen to partial discharge. Moisture content and dielectric breakdown voltage tell you whether the oil is still doing its insulating job. On a transformer you cannot open, the oil is the only diagnostic you have.
PCBs on older units
Askarel fluids containing polychlorinated biphenyls were used until the late 1970s. Any transformer of that vintage has to be treated as potentially contaminated until tested, and disposal is regulated. Nothing built today contains PCBs, but this comes up constantly on used and surplus equipment, which is a reason to be careful about provenance.
Common questions
What oil is used in pad mounted transformers
Most commonly mineral oil, which insulates and cools at low cost. Ester fluids such as FR3 are specified where a higher fire point or biodegradability is needed, for example indoors, near water or where fire separation would otherwise be required.
What is the difference between mineral oil and FR3
FR3 is a natural ester with a fire point above 300 degrees Celsius against roughly 160 for mineral oil, so it is classified as less flammable and is biodegradable. It costs more and is more viscous, so it is chosen when the installation requires it.
Why is transformer oil tested
Because it is the only window into a sealed tank. Dissolved gas analysis identifies arcing, overheating and partial discharge from the gases present, while moisture and dielectric breakdown tests show whether the oil still insulates.
Do old transformers contain PCBs
Units built before the late 1970s may contain askarel fluids with PCBs and must be treated as potentially contaminated until tested. Disposal is regulated. Nothing manufactured today contains them, but it remains a real issue with used and surplus equipment.
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