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Courtesy : EEP
Generally, power circuits have components that have large thermal capacities, which make it impossible for them to attain very high temperatures quickly except during very large or long disturbances. This requires correspondingly large surge energies. Also, the materials that constitute the insulation of these components can operate at temperatures as high as 200 ºC at least for short periods.
Electronic circuits, on the other hand, use components that operate at very small voltage and power levels. Even small magnitude surge currents or transient voltages are enough to cause high temperatures and voltage breakdowns.
As such, a higher degree of surge protection is called for if these devices have to operate safely in the normal electrical system environment.
Thus comes the concept of surge protection zones (SPZs).
According to this concept, an entire facility can be divided into zones, each with a higher level of protection and nested within one another.
Full Article At: EEP