Overvoltage category

The second main feature of increased safety - e - is the increase of clearances compared with those of overvoltage category III. To give a rough estimation the clearances of increased safety are comparable with those of overvoltage category IV (or even higher). 

Line- neutral voltage V System voltage V Impulse voltage kV Overvoltage category III IV Minimum clearances mm Pollution degree 3 4 Minimum Voltage clearance V mm ... 

Impulse — Traditionally used to indicate a short duration overvoltage event with certain rise and fall characteristics. Standards have moved toward including the term impulse in the category of transients. 

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