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Yesterday I saw an industrial capacitor datasheet which stated that discharging the capacitors with resistors improves the life cycle of the capacitors, since it prevents electrical impulse discharges.

Discharging
Capacitors must be discharged to a maximum of 10% of rated voltage before they are switched in again. This prevents an electric impulse discharge in the application, influences the capacitor’s service life and protects against electric shock. The capacitor must be discharged to 75 V or less within 3 minutes. There must be not any switch, fuse or any other disconnecting device in the circuit between the power capacitor and the discharging device. PhaseCap-capacitors either have a premounted ceramic discharge module inserted from above into the middle section of the terminal, or for high voltages/high loads a plastic discharge module mounted at one side of the terminal; alternatively discharge reactors are available from EPCOS. Discharge and short circuit capacitor before handling!"

(Source: TDK - PhaseCap Premium Capacitors Series/Type: MKK800-D-20-01 datasheet)

I wonder how this can be, since if you are working with mains of 230 Vrms, the initial voltage applied to the capacitors depends on the instant you switch on the contactors, but it can vary from 325 V to -325 V. So, since the current through a capacitor is:

Ic = C.(dv/dt)

and arcs are produced from the current flowing, shouldn´t the production of arcs be dependent if there is a potential voltage respective from the mains to the capacitors when they are turned on?

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1 Answer 1

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Yes, the mains phase can be at +325V or -325V when it connects to the capacitor.

Discharging the cap to 0V at least makes sure the change is at most 325V.

If you don't discharge the capacitor, the capacitor may have been disconnected when there is +325V on capacitors, and reconnected when mains phase is at -325V, so there is a 650V difference.

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