Capacitors are devices which store charge as an electrostatic field. When the supply connected to a capacitor is removed, the capacitor still retains the charge within itself.
Thus, when a capacitor is switched off, it still contains charge. Hence, an engineer working on a capacitor that has not been discharged can get an electric shock. It is, therefore, vital that all capacitors and other energy storage devices be discharged prior to service. Power capacitors usually have a resistor known as a bleeder resistor connected in parallel. The function of this resistor is to discharge the capacitor once the power supply has been removed.
These resistors are usually designed to reduce voltage across the capacitor to less than 50V (the permissible safe voltage for humans) within 5 minutes.Hence, service work in a capacitor should be started only after five minutes.
As a final precaution, the capacitors need to be discharged manually prior to starting the work. (See article on Manual Discharge of capacitors)