Transformer Oil - Polychlorinated Biphenyls (PCBs)

Polychlorinated Biphenyls (PCBs)

Well into the 1970s, polychlorinated biphenyls (PCBs) were often used as a dielectric fluid since they are not flammable. PCBs do not break down when released into the environment and accumulate in the tissues of plants and animals, where they can have hormone-like effects. When burned, PCBs can form highly toxic products, such as chlorinated dioxins and chlorinated dibenzofurans. Starting in the early 1970s, production and new uses of PCBs have been banned due to concerns about the accumulation of PCBs and toxicity of their byproducts. In many countries significant programs are in place to reclaim and safely destroy PCB contaminated equipment.

Polychlorinated biphenyls were banned in 1979 in the US. Since PCB and transformer oil are miscible in all proportions, and since sometimes the same equipment (drums, pumps, hoses, and so on) was used for either type of liquid, contamination of oil-filled transformers is possible. Under present regulations, concentrations of PCBs exceeding 5 parts per million can cause an oil to be classified as hazardous waste in California (California Code of Regulations, Title 22, section 66261). Throughout the US, PCBs are regulated under the Toxic Substances Control Act. As a consequence, field and laboratory testing for PCB contamination is a common practice. Common brand names for PCB liquids include "Askarel", "Inerteen", "Aroclor" and many others.

Today, non-toxic, stable silicon-based or fluorinated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and a higher flash points than mineral oil.