The capacitor that condenses the spark occurring from the points opening is nothing more than two conductors (aluminum sheets) separated by a dielectric (non conductor) sheet. The type material and thickness of the insulator determines the voltage rating of the capacitor. Put it in a container with two leads, one per conductor, maybe a mounting strap, seal it up, stamp it and there you have it.
The value is in the order of .01 microfarads and that value seldom comes in very low voltages as that's just not a lot of C to have to package. These units are usually referred to as paper/plastic types as the insulation could be either. So to have the unit capable of several hundred volts is probably the norm. Don't remember seeing the rated voltage on the unit, just the paper box they came in if it were there at all and I don't remember that.....just a part number.
I don't know the exact voltage that occurs across the points when they open with the capacitor present, but the purpose of the capacitor is to slow the rate of rise of the voltage spike and limit the amplitude of it to keep these values below arcing values and prevent the points from arcing as arcing will burn them up in short order.
There is a mathematical equation expressing that voltage and shape and it requires knowing the value of primary inductance of the ignition coil, the actual current flowing when the points open and the value of the capacitor.
So, with all that hooplah, I doubt (too) that your previous capacitor failed due to voltage rating. Lots of griping on here about sub standard imported units.
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