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Power Factor correction capacitors rated at 400V on a 415V system - is this an issue

Dave wood

We have a PFC system that has been working happily for 10+ years. The first stage has now blown. The maintenance supplier has stated that as the capacitors are rated at 400V and the system is 415 that is the issue and we should replace all of the capacitors across site. Does this sound correct? if this was an issue surly they would have gone earier. I tend to think the issue is the first stage has been in constant use and has worked past its life expectancy. - any thoughts.

  • 0

    I would expect that 400 volt capacitors should be suited for use on 400 volt NOMINAL supplies. Including almost all public supplies in the UK. Even if the actual measured voltage is at the upper limit of reasonably expected variation.

    I would expect long term use at up to 440 volts to be fine, since the actual voltage of a public supply can vary between 94% and 110% of the nominal voltage.

    A private supply might operate at some higher voltage, but this is very rare, most private supplies are similar to public supplies so as to utilise standard equipment.

  • 0

    Capacitors designed for mains use are really rated to withstand many times that,  for example these  mains rated filter caps are tested to 3000VDC.

    The problem is there can be  kV surges on the mains that can literally blow holes in the insulation. 'Self healing' capacitors will burn back the foil 'plates' around such a pinhole, and carry on, with slightly reduced effective capacitance.

    In the past I have forensically looked at a failed polypropylene capacitor in a location with very nasty mains spikes, and the 'swiss roll' foils inside bore more resemblance to a paper doily or perhaps more a wide lace ribbon when unwound being full of holes when held to the light.

    The clue to such damage before total failure is a slowly reducing capacitance as the active area gets blown away in a series of internal flash-over events.

    I suspect the 400 versus 415 thing is not true, but it may be that the caps have lost value, and are nearing end of life. Measurement will reveal.

    Mike.

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