Almost all run capacitors use oil and paper as the dielectrics. Because of the hazards involved with oil, a system of protection was devised to prevent fires from occurring within the equipment. The figure on the right shows how a fusing device is incorporated into the capacitor. This safety feature is designed to prevent an explosion or fire that could result from a shorted capacitor, but it can also create a service problem if the capacitor is not discharged correctly.
A capacitor can hold a charge for several days (depending on its size and capacitance value). The old “tried and true” method of discharging a capacitor was to use a screwdriver or jumper wire to short across the capacitor’s terminals. If you were to place a direct short across the terminals, you could possibly destroy a good capacitor. To prevent this, capacitor manufacturers recommend that a bleeder resistor be used as a discharging device. A 15,000-Ω, 2-W (brown, green, orange) resistor with jumper clips is the easiest and safest way to discharge a capacitor. If you are in doubt about whether a capacitor is charged or not, simply bridge the capacitor with the resistor.