Metal oxide surge arresters (MOSAs) typically have very high reliability and can continue to function as intended almost indefinitely if they have been well designed, properly manufactured, and operated within their specified range of applied voltage, temporary overvoltage, surge magnitude and surge energy, and have not been physically damaged by some external force. However, history informs us that arresters do occasionally fail in service, implying that one or more of the aforementioned factors has been violated. By "fail", we usually mean that the arrester has suffered an internal short circuit, placing a fault (most often a line-to-ground fault) on the system. Almost invariably, this fault results in an operation of a circuit interrupting device (circuit breaker, recloser, fuse, etc.), causing disruption of electrical service. If the arrester is equipped with a disconnector (as is the case for most distribution arresters installed in the United States), the disconnector should operate when the fault current flows following an arrester short circuit, isolating the arrester either from the ground or from the line (depending on where the disconnector is installed); then, upon re-energization, the failed arrester is no longer in the circuit electrically and service can be continued (albeit with some reduced level of surge protection). In the case of a station arrester, a failure will typically cause a system lockout until either that portion of the substation can be by-passed, or until the arrester is physically removed from service. While it is desirable to avoid arrester failures at all times, the consequence of a station arrester failure is typically much more severe than that of a distribution arrester. Often, a considerably greater disruption of service would accompany a station arrester failure, and the cost of replacing a station arrester is typically orders of magnitude greater than that of replacing a distribution arrester.
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