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Oil Filled


Oil-filled Capacitors


          Oil-filled capacitors have been used for many years in a variety of high-current and/or high-voltage applications, like motor-run, motor-start, and power factor correction.  The term "oil" is used here to mean a wide variety of liquids, many of which you might not think of as being "oils".  For that reason, some people prefer the term "liquid filled" instead of oil filled.  The term "mineral oil" is loosely used to mean just about any non-chlorinated liquid, including refined petroleum oils, silicones, and synthetic esters. 

         Very early high-voltage capacitors used wax and petroleum jelly.  It was almost impossible to achieve void-free construction however (always a big issue in high-voltage capacitors), so as voltage requirements increased, these dielectrics disappeared.  Capacitors made with mineral oil-impregnated paper as the dielectric, and aluminum foil electrodes were first used in the 1920s.  The paper still used is a strong, long-fiber, all wood-pulp type called "kraft" paper (kraft means "strong" in German).  Cotton-pulp paper was found to have a shorter life. 

          In the 1950s, a new class of "oils", the polychlorinated biphenyls (PCB), was introduced as dielectric fluids.  PCBs are a group of 209 compounds in which biphenyl has been modified with chlorine.  About half have been used commercially at one time or another.   PCBs are normally used in mixtures, either with just each other, or with other chlorinated compounds.  Pure PCBs are typically heavy liquids, or even solids at room temperature, but the mixtures can have more fine-tuned physical properties.  The various PCB mixtures were known by the generic name of  "askarels" (actually a trade name).  PCBs were sold under many other trade names, like Aroclor, Pyroclor, Pyranol, Noflamol, Inerteen, and Hyvol.  I have seen about 40 names and there may be many others.  PCBs were first commercialized by Monsanto in this country around 1930.  Monsanto has been called the "sole North American manufacturer", but Im not sure this is true. They were used in a wide variety of industrial processes.  An important one being dielectric fluids.  They were also used as plasticizes, coolants, and as additives in lubricants, inks and adhesives, to name a very few applications.  Capacitors were never the major use for PCBs, they may have accounted for less than 5% of the total.  Big transformers were much bigger users. 

          PCBs were a big hit in the electrical industry; they had reasonably good electrical properties, very good high-temperature stability, high flash points, and low moisture absorption (moisture drives down the breakdown voltage).  Their use in big transformers was actually required by building codes in some areas.  One shortcoming was cost.  The higher cost of PCBs over mineral oils forced changes in capacitor construction to reduce the amount of oil used for a given kVA rating.  A more fundamental problem was the very concept of the classic paper-oil-foil design.  Oils have much higher dissipation factors than more modern dielectrics, and the paper acts as insulation to help hold in the heat.  These problems limited capacitors to about 100 kVA. Note that big power distribution capacitors were not the only ones to use PCBs.  Many smaller capacitors were used in all kinds of electrical equipment, including home appliances.

          In the 1970s, PCBs became associated with a variety of health risks, and by the 1980s, their use and manufacture was either banned or severely restricted in the major industrialized countries.  However, although the U.S. and Canada stopped production in 1976, world PCB production is still considerable if falling.  The Soviet Union was still making PCBs as late as 1990.  In 1988, total production to that time was estimated at 1.2 million tons.  About one third was already in the environment, two thirds in storage or in use, and only a few percent having been destroyed.

          PCBs are believed to cause liver and nerve damage, are believed to cause cancer, and to be teratogens (cause reproductive damage).  Furthermore, their good chemical stability means that they biodegrade extremely slowly.   PCBs are fat soluble rather than water soluble and are not quickly eliminated by the body.  This allows disproportionately high levels of accumulation in body fat from low level exposure (called bioaccumulation).  They are also concentrated by the food chain, sometimes many thousands of times (called biomagnification). 

          PCBs were not the only chlorinated compounds to come under attack.  PCBs are only a part of the much wider problem of the "persistent organochlorines".  Others include chlorinated phthalates, paraffins, terphenyls, naphthalenes, plus a variety of herbicides, insecticides, nematicides, and fungicides, to name but a few.  What makes these compounds so dangerous (and so useful in many cases) is that they are often very similar to compounds found in living organisms, and therefore, can easily interact with these organisms. 

          PCBs and their many cousins present several types of exposure risks.  The first is contact during handling of the oil, manufacturing of the capacitors, and leakage of the capacitors in use.  The second is the risk of fire.  There are a number of cases in which electrical fires resulted in the contamination of entire buildings with PCBs and their combustion products, such as chlorinated dibensofurans and dioxin.  Decontamination after such fires has been extremely expensive, in some cases exceeding the original cost of the building.  These fires are also a terrible risk for firefighters.  A third risk is from PCBs entering the environment and into our food chain, through accidental (in some cases deliberate) spills, or disposal in unsecured landfills.

          Large PCB-filled transformers and capacitors have mostly been destroyed or decontaminated because the risk of fire or leakage is considered to be too great to leave them in service.  However, small PCB capacitors (such as motor-starter size) may still be used in industrial equipment if all the proper paperwork is filed.  Penalties for failure to follow the rules can be severe.  By Federal law, all non-PCB liquid-filled caps must be marked as such.  If a capacitor is not so marked, it must be assumed to contain PCBs.  Some small PCB capacitors were in the distribution pipeline long after manufacture stopped, and it may still be possible to purchase one by accident.  The EPA defines small as less than 3 lbs of PCBs.  There is still a significant number of PCB capacitors in old consumer products.  Submersible pumps for water wells are a special problem.  They have a long life so there are probably lots of them still in service.  Plus, if the capacitor leaks, the PCBs can go directly into drinking water.         

          PCBs can be destroyed by high-temperature incineration; the chlorine is released as hydrogen chloride.  Mineral oils contaminated by PCBs (as from transformers) can be cleaned by chemical means, and a number of companies do this.

          The banning of PCBs started the search for satisfactory substitutes.  Since then, a variety of mineral oils have been used in capacitors and other electrical applications, and research continues today.  The various fluids used include high-temperature hydrocarbons (like diaryl ethane), silicones, fluorocarbons, and synthetic esters.  Polypropylene and polyethylene glycols and related compounds are sometimes used, as are caster oil and rapeseed oil.  Some chlorinated hydrocarbons with good biodegradation and photodegradation properties are used as well.  Not all the replacement fluids are as nontoxic as might be wished, and some of them are now under attack as possible health hazards.  There is fear that some of these compounds may present future disposal problems.  At least they are more biodegradable than PCBs.

          For more information see:

          A very few companies use wax in motor-start and motor-run capacitors.  This eliminates the danger of leakage, some oil-filled caps contain so much oil they slosh if shook.      

          In the 1960s, polypropylene (PP) was introduced as a dielectric, and was used in PP/oil/paper hybrid designs.  PP has a much lower dissipation factor than oil, and a higher voltage breakdown.  PP has a dissipation factor of less than 0.1%, while the various oils are typically 0.5 to 3.5%, depending on type and temperature.  Oil and paper were still used, as a way to eliminate air pockets that would shorten the life of the capacitors.  PP allowed for construction of capacitors with much higher kVA ratings.  This started a "long march" of improvements in high-power capacitors.  There has even been significant improvment in the quality of kraft paper, and its additivies.  The another major step was to eliminate the aluminum foil in favor a vacuum metallized layer directly on the PP film.  Metallized PP film has much better self-healing properties, but also has a higher ESR so it is not used in all applications.  Some manufacturors replaced the aluminum film with zinc, or more recently, with proprietary alloys for better self-healing.  Paper may still be used to hold the oil because oil does not wet the PP film very well.  Roughening the film allows it to hold an even film of oil without the paper.  This is called "hazy film" (Hazy is a G.E. trademark I believe).  Capacitors that use the traditional paper/oil layer are sometimes called "soggy film" capacitors.  All of these construction methods have their strengths and weaknesses, and all are still in use. 

          Some people claim that PCBs dangers are exaggerated and largely unproven, and there has been some call to reintroduce them into certain applications.  Whatever the case, the levels of PCBs in the environment and in ourselves is reported to be on the decline. 

          For more information on PCBs see: includes lists of capacitors that do and do not contain PCBs