Long duration impulse withstand capability of SPD

In low voltage system, there are varity impulse happened with different peak value and time duration due to the lightning and equipment operation or faults. SPD is the main protective device withstanding those impulse, it is widely used in the low power system. However SPD\´s fault usually lead to big loss in using. Thence SPD\´s reliability is the focus element concerned by users. To improve the reliability of SPD in using is a very important subject in the field of manufacture and application. The common causes of the field failures of SPD may be classified into three categories: (1) The SPD was subjected to one or a few sever impulses that well beyond the maximum ratings of the SPD; (2) A great portion of "Ampere-second" resource of the SPD has been consumed by previous switching over-voltages that occurred frequently in the system (most of them were impulses with low crest but long duration), hence such an SPD may be damaged by an impressed impulse much less sever than SPD ratings; (3) The field failures resulted from interior defects of the SPD. The occurrence probability of the field failures caused by the (1) was very low, and the (3) as well if the SPD products had gone through reliability screening processes prior to delivery, hence the cause (2) was the major cause of the field failures of SPD. Therefore, it is of great importance to improve and guarantee the long duration impulse withstand capability, however it was ignored in the current specification IEC 61643-1. There is a view considering that there is equivalent relationship between the capabilities of withstanding short duration impulse and withstanding long duration impulse. Thence the operating duty test of 8/20us ( which is a typical wave form of short duration impulse)in IEC 61643-1 can substitute to test the SPD\´s capability withstanding long duration impulse. This view is not suitable for the actual situation of MOV type SPD which is widely used in field, according to the following testing da- - ta and theory analysis which prove that there is no any equivalent relation between the capabilities of withstanding short duration impulse and long duration impulse. Thence, to stipulate the requirement and the test method of SPD withstanding long duration impulse is essential. The SPD\´s non-linear components in low voltage distribution system mostly are metal oxide varistor (MOV). A popular viewpoint is that if the surge current MOV withstands is not bigger than Imax, MOV will get damaged, but the field experience has denied this viewpoint. There have been examples that MOV SPD got damaged at a impulse current less than its Imax. This article will, through laboratory result as well as theoretical analysis, address to this issue that the main reason resulting MOV SPD failure at a small surge impulse is its inadequate LLI withstand ability, so it is necessary to add the LLI test to the SPD technical standard. This article will tell the related testing method.