Capacitors are generally rated for the system line-to-line voltage (e.g., 240 V, 480 V). However, in a harmonic filter application, they should be selected to withstand overvoltages and overcurrents caused by fundamental and harmonic current flow through the series connected tuning.

IEEE Std 18-2002 requires shunt capacitors, under contingency conditions, to withstand continuous voltages up to 110% of rated rms voltage and continuous currents up to 135% of nominal rms current based on rated kvar and rated voltage.

When applied in harmonic filters, the normal voltage and current may exceed these levels even before system contingencies are considered. Consequently, capacitors selected for use in normal shunt capacitor applications may not be suitable for use in harmonic filters.

Harmonic filter capacitors should be selected based on their expected duty under normal and contingency conditions. The capacitor manufacturer should be consulted when specifying capacitors for harmonic filter applications.

Capacitors should be manufactured in accordance with UL 810-1995. Capacitors should be protected by a UL-listed/recognized protective device. For capacitors that do not contain a UL-listed/recognized protective device, the capacitors should be protected with external current limiting fuses or other external protective devices that are UL-listed.

Capacitor cells connected with a wiring harness may also be protected with UL listed current limiting fuses (even if they have internal pressure sensitive interrupters) to protect in the event of a failure of external resistors, bushings, or connecting wires.

IEEE Std 18-200215 specifies that the terminal-to-case test for the internal insulation of indoor capacitors should be performed at 3 kV rms (capacitors rated 300 V or less) or 5 kV rms (capacitors rated 301 V to 1199 V) for 10 s.

The terminal-to-terminal test should be 10 s at 2 × rated rms voltage (ac test) or 4.3 × rated rms voltage (dc test). Each capacitor rated 600 V or less must be provided with a discharge resistor(s) to reduce the residual voltage from peak of rated to less than 50 V within 1 min of de-energization (5 min for capacitor units rated higher than 600 V) to meet the requirements of IEEE Std 18-2002 and Article 460-6 of NFPA 70-2002 (National Electrical Code).

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