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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.

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    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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  • 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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