Polychlorinated biphenyl's (PCB) is a synthetic transformer insulating fluid, that has found its way into mineral insulating oil via cross contamination.

Polychlorinated biphenyl: Non-specific methods that determines chlorine in oil, as all PCBs contain some amount of chlorine. This test is susceptible to false positive results, for instance, the test indicates the presence of PCB when actually there is none present.

Polychlorinated biphenyl: specific method (ASTM D4059 - Gas chromatography/Electron Capture) that differentiates between PCBs and a related compound e.g. trichlorobenzene.

All commercially produced PCB are complex mixtures of many different congeners (A congener is a PCB molecule containing a specific number of chlorine molecules at specific sites)

Analysing for PCB, therefore, is not a case of simply finding an easily quantifiable compound, but of quantifyg a complex mixture of compounds.

The main reasons for stopping further use are the environmental risks (It is also harmful to human health.). PCB is very stable and its degradation process is slow, it is also biological accumulative in the food chain.

PCB liquid is not more toxic than many other common fluids. The lower the figure, the higher the toxicity
Chemical LD50g/Kg
PCB 8.7
Trichloroethylene 5.2
Acetone 9.8
Methyl alcohol 12.9
Polychlorinated dibenzofuranes less than 0.001

Far more serious are the risks of a fire or an explosion. At temperature around 500 degrees Celsius extremely toxic compounds polychlorinated dibenzfuranes are formed. Small amounts of these compounds have been found at accident where transformers and capacitors have been exposed to fire or have exploded. Even if the amounts have been extremely small and have caused no personal injuries, it has been necessary to perform very extensive and costly decontamination work.