Abstract: A capacitor is composed of an external casing with an internal polyurethane encapsulation containing a polypropylene layered metallized film type capacitor element. Inside the casing is a component in the form of a frangible substrate having at least one conductive track thereon. The track establishes connection between a terminal on the casing and the capacitor element. The substrate is either mounted directly to the inside of the casing or is contained in a cartridge. The substrate is of brittle nature and distorts should pressure increase in the casing to shatter to break the track and isolate the terminal from the capacitor element.