Electrical eqipment and appliances are divided into four groups considering impulse voltage. For all these overvoltage categories a tolerable impulse voltage is given (as front shape value of 1,2/50μs), which the eqipment or apliance has to overcome. Nowadays, in the era of computers and other sensitive devices it has become necessary to expand technical possibilities for successful or complete protection of vitally important services. If overvoltage risik as estimated to be greater than the costs of building the eqipment into electrical instalations, then protection design is done. Overvoltage design has to take into account some facts such as used system of electrical distribution protection eqipment characteristics, needs for not decreasing efficiency of short voltage and overload protection as well as of current circuit breaker and demands of electric distribution company. Good design and realization of overvoltage protection increases the availability of electrical installations.
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