2. Surge Phenomena & Overvoltages · Page 3 of 6
Insulation Coordination Basics
How surge levels relate to equipment withstand voltages.
Withstand voltage categories
Equipment is designed for certain impulse withstand levels depending on installation category (overvoltage category I–IV in IEC thinking).
Role of the SPD
An SPD lowers the expected transient voltage at a point in the installation so that it stays below the withstand of connected equipment, including a safety margin.
Cascaded protection
Upstream devices take energy; downstream devices refine the voltage. Coordination ensures the upstream SPD operates first under high-energy events.