PCB's
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PCB's or Polychlorinated Biphenyl is a Hydrocarbon material with strong electrical insulation properties and high temperature flash point which makes it a perfect dielectric for transformers and other liquid filled electrical apperatuses. But, PCBs have been demonstrated to cause cancer, as well as a variety of other adverse health effects on the immune system, reproductive system, nervous system, and endocrine system. So now it is a banned substance by the EPA. From it's invention in 1929 until 1979 when it was banned, a great deal of equipment was manufactured with this substance due to it's superior fire prevention and insulation properties.
Since 1979, the EPA has progressively improved it's policy in regulating this substance to the point where corporations in general have found owning these kinds of transformers and other electrical apperatuses as more of a liablity than a benefit. The expected life cylce of electrical equipment by design is approximately 30 years. Many transformers and other apperatus if maintained correctly can last up to twice as long as this. In 2009 the last PCB filled apperatus's will be over 30 years old. These facts have to be weighed carefully when equipment is found to contain PCB's.
At this point in time, it is very challenging to make a case to spend time and money and accept temporary risks that are involved with retrofiling any PCB electrical apperatus. Especially when the transformers are clearly in habitable spaces that must comply with fire and health and safety codes. The easiest way to mitigate this risk today is by replacement and proper disposal and decontamination.