In the event of overvoltage or thermal overload or ageing at the end of the capacitor‘s useful service life, a non-healing dielectric breakdown may occur which will cause rising pressure inside the capacitor. To prevent it from bursting, the capacitor is fitted with an obligatory overpressure switch (opener). It enables external monitoring and – if necessary – disconnection of the capacitors through external devices. It has to be noted that this safety system can act properly only within the permitted limits of loads and overloads.