Charles C. Roberts, Jr.

Ph.D., P.E.


Oil filled capacitors are electrical components that store electrical energy and are used in a wide variety of electrical applications. Figure 1 is a view of a typical oil filled capacitor. The two terminals on top are for wire connections. The device is often bolted to an electrical chassis via the mounting base. The housing contains the capacitor surfaces and a dielectric oil, which is used to increase the dielectric strength of the gap between the capacitor plates and reduce undesirable electrical discharges (corona discharge). The dielectric oil inside can range from PCB based materials to silicone oils. Typical oils are castor oil, canola oil, mineral oil, ethyl hexyl phthalate, phenyl xylyl ethane, isopropyl biphenyl and polydimethyl siloxane. Failure of oil filled capacitors can occur, resulting in ignition of the dielectric fluid and causing a fire or explosion.

Figure 1




Figure 2


A case study involving an incident where a capacitor failure damaged a small building serves to illustrate the consequences of oil filled capacitor failure. Figures 2 and 3 are views of a small block wall building that was a shelter for a radio transmitter. The transmitter suddenly quit operating and the radio station went off the air. Investigators found the walls of the building pushed out as indicated by the arrows in Figure 2. The damage was consistent with slight internal pressure inside the building, similar to that of a low energy explosion. Figure 3 shows slight overpressure damage near a ventilation duct as indicated by the arrow.

Figure 3


Figure 4

Further investigation revealed thermal damage to the inside of an electronic cabinet depicted in Figure 4. Using thermal damage patterns as a guide, a failed capacitor, which had been badly damaged, was found in the electronic cabinet (Figure 5). The arrow in Figure 5 points to an oil leakage pattern originating from an oil filled capacitor.  There was evidence of oil leakage (arrow) from the capacitor, allowing air to fill the void inside. As the capacitor plates become exposed to the air, the dielectric strength of the plate gap decreases, causing a sudden high energy electrical discharge between plates. This discharge has sufficient energy to ignite oil vapor and cause an explosion, albeit a small one.

Figure 5


Capacitor failure, such as that previously described, can occur for a variety of reasons. They are defective manufacture, defective design, improper installation, shipping damage or an intervening biologic. Defective manufacture includes not enough fluid in the capacitor, insufficient plate gap or improper sealing of the capacitor housing. Defective design includes improper electrical specification (using the unit at an excessive voltage) or insufficient cooling of the electronic equipment. Examples of improper installation are excessive strain on the capacitor housing from mounting or deforming the unit during installation. Damage to the capacitor case can result in plate gap reduction that can cause a discharge and capacitor failure. Finally, an intervening cause such as a rodent (biologic) contacting the two electrodes simultaneously will likely cause a capacitor failure.