OVERLOAD CAPABILITY OF THE CONTACTORS

The contactors can withstand 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 higher than the making capacity of the contactor as it is considered that 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 acceptable 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)

Contactor Type N	85	125	190	270	350	550	650	800	1000	1250	1600	2000	3000
Id (Peak val) [KA]	3.5	4.5	5.5	9	11	13	15	20	29	40	40	40	55
Tc [sec.]	2150	1200	1500	2150	1300	2000	1500	1500	1400	1500	1800	2250	2150

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