The contactors can withsand for short time duration, current much higher than the rated; in this instance two different phenomena must be considered: the thermal and the electrodynamic. Dynamically the contactors can withstand current peaks to a limit where repulsion of the contacts may occur. The current which causes the repulsion of the contacts could be higer than the making capacity of the contactor as it is considered thet this phenomenon arises when the contactor is already closed and operating. Therefore, the electrodynamic stress in this case is not superimposed to the mechanical bounce effect that can arise during the closing operation which is the principal cause of the contacts melting. Of course, the intensity of this dynamic current must also be thermally tolerable for the contactor and therefore, in the table below, the values "Id" of the maximum accettable dynamic currents are given providing that their duration is no more than 100ms. From the thermal point of view the intensity of the tolerable overload is inversely proportional to the duration and depends essentially upon the time constant Tc of the warming-up curve of the contactor when its rated current is applied. The diagram below gives the ratio t/Tc (where t is the duration of the overload) and the factor K which, when applied to the nominal current of the contactor, determines the intensity of the tolerable overload starting from the cold status.

(Table n. 3)

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