SPD Guide: Surge Protection for Electrical Panels

What is surge overvoltage

Short-duration (µs-ms) voltage spike caused by:

• Lightning (direct strike or induced) — up to hundreds of kV
• Switching (motor/transformer disconnection) — 2-10 kV

Three SPD classes

Class I (Type 1 / Class B)

For direct lightning strike (10/350 µs waveform). At the building incomer. Iimp ≥ 12.5 kA/pole, Up ≤ 2.5 kV. Spark gap technology.

Class II (Type 2 / Class C)

For residual and switching surges (8/20 µs). Floor/distribution panels. Imax ≥ 40 kA total, Up ≤ 1.5 kV. Varistor.

Class III (Type 3 / Class D)

Fine protection for end equipment. At the outlet. In ≥ 3 kA, Up ≤ 0.8 kV. Combined varistor + GDT.

Cascade coordination

Class I (main panel) → Class II (sub-panel) → Class III (outlet).

Minimum 10 m cable distance between Class I and II. Otherwise — decoupling inductors needed.

Selection parameters

• Uc ≥ 275 V (L-N), Uc ≥ 350 V (L-PE) for 230/400 V network
• Up: lower is better. Class II Up must be ≤ equipment withstand category
• Network type matters: TN-C, TN-S, TN-C-S, TT — different connection schemes

Do I need SPD in my apartment?

• City apartment >5 floors: built-in appliance protection usually enough
• Private house with overhead line: Class I + II mandatory
• Private house with underground feed: Class II sufficient
• Frequent thunderstorms (mountains, south): Class I + II mandatory

Cost: Class I from $50, Class II from $25. Total home protection: $75-150. Compare to burnt appliances and rewiring.