Capacitors

Large Capacitor Hazards

Capacitors may store hazardous energy even after the equipment has been de-energized, and may build up a dangerous residual charge without an external source. "Grounding" capacitors in series, for example, may transfer (rather than discharge) the stored energy. Another hazard exists when a capacitor is subjected to high currents that may cause heating and explosion. Capacitors may be used to store large amounts of energy. An internal failure of one capacitor in a bank frequently results in an explosion when all other capacitors in the bank discharge into the fault.

Note: High voltage cables should be treated as capacitors because they have capacitance and thus can store energy.

The liquid dielectric in many capacitors, or its combustion products, may be toxic.

Automatic Discharge

Use permanently connected bleeder resistors when practical. Capacitors in series should have separate bleeders. Automatic-shorting devices that operate when the equipment is de-energized, or the enclosure is opened, must be used. The time required for a capacitor to discharge to a safe voltage (50 volts or less) must not be greater than the time needed for personnel to gain access to voltage terminals. In no case must it be longer than 5 minutes.

In the case of equipment with stored energy in excess of 5 J, an automatic, mechanical-discharging device must be provided that functions when normal access ports are opened. This device must be contained locally within a protective barrier to ensure wiring integrity and should be in plain view of the person entering the protective barrier so that the individual can verify its proper functioning. Protection also must be provided against the hazard of the discharge itself.

Safety Grounding

Provide fully visible, manual-grounding devices to render the capacitors safe while they are being worked on. Clearly mark grounding points and use caution to prevent transferring charges to other capacitors.

All grounding hooks must:

• Have crimped and soldered conductors.
• Be connected such that impedance is less than 0.1 (omega) to the ground.
• Have the cable conductor clearly visible through its insulation.
• Have a cable conductor size of at least #2 extra flexible, or in special conditions, a conductor capable of carrying any potential current.
• Be insufficient number to conveniently, and adequately, ground all designated points.
• Be grounded and stored in the immediate area of the equipment in a manner that ensures they are used.

In equipment with stored energy in excess of 5 J, a discharge point with an impedance capable of limiting the current to 500 A or less should be provided. This discharge point must be identified with a yellow circular marker with a red slash and must be labeled "HI Z PT" in large, readable letters. A properly installed grounding hook must first be connected to the current-limiting discharge point and then to a low-impedance discharge point (less than 0.1 (omega)) that is identified by a yellow circular marker. the grounding hooks must be left on all of these low-impedance points during the time of safe access. The low-impedance points must be provided, whether or not the HI Z current-limiting points are needed. Voltage indicators that are visible from all normal entry points should also be provided.

Fusing

Capacitors used in parallel should be individually fused when possible to prevent the stored energy from dumping into a faulted capacitor. Care must be taken in the placement of automatic-discharge safety devices with respect to fuses. If the discharge will flow through the fuses, a prominent warning sign must be placed at each entry indicating that each capacitor must be manually grounded before work can begin. Special knowledge is required for high-voltage and high-energy fusing.

Unused Terminal Shorting

Terminals of all unused capacitors representing a hazard, or capable of storing 5 J or more, must be visibly shorted.