CAPACITOR FAILURES CAN CAUSE
FIRES AND EXPLOSIONS
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.
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.
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.
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.