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In an electrolytic capacitor, the insulating layer consists of an electrochemically formed film of aluminium oxide with a thickness of only a matter of millionths of a millimetre. This can deteriorate after a long period of disuse (many years) or a shorter period close to or beyond its maximum voltage and/or temperature rating. Also, the liquid used to form the insulating layer can dry out. Poor quality electrolytic capacitors are quite often seen which have failed within their ratings.