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One very important rating of capacitors is "working voltage". This is the maximum voltage at which the capacitor operates without leaking excessively or arcing through. This working voltage is expressed in terms of DC but the AC equivalent is about only one half of that DC rating. As the AC frequency increases the working voltage rating decreases further still.
Heating of the dielectric also decreases the working voltage.
The "dielectric strength" or number of volts that the dielectric will stand per 0.001" of dielectric thickness, varies considerably with materials. Some approximate examples are:
Air 80V, Glass 200 - 300V, Mica 2,000V and Ceramics 80 - 200V.
The thickness of the dielectric depends on the design working voltage. Some electrolytic capacitors formed to work at say 450V DC but used at 300V DC may re-form to the lower voltage.
Should an electrolytic capacitor be connected across a circuit with the polarity reversed the film will deform, the capacitor becomes a good conductor, very high current flows, heat develops and the electrolyte boils and the capacitor will explode, often with dire results.
Absolute care must be taken in observing correct polarity when installing polarised capacitors in circuits. This means the average electrolytic capacitor can not be used in AC circuits. Special capacitors are manufactured for this purpose.
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