With lightning season approaching, have you properly selected your cabinet SPD? Improper SPD specification is worse than no installation at all. Save these four core selection rules.

1. Match wiring earthing system first

   4P SPD (full protection for L1/L2/L3/N) for TN‑S system; 3+1 configuration for TT system with independent MOV for neutral conductor. Wrong system matching renders SPD installation ineffective.

2. Avoid undersized Uc continuous operating voltage

   Uc (maximum continuous operating voltage) ≥ 1.1* nominal system voltage. For 220V grid, Uc ≥242V; 275V or 385V rated Uc is recommended for safety. Low Uc causes continuous SPD degradation under normal grid voltage, leading to thermal breakdown and fire hazards.

3. Class rating based on installation location
   • Main incoming cabinet: Class T1(Type1), Imax≥100kA for direct lightning strike protection
   • Floor distribution cabinet: Class T2(Type2), Imax≥40kA against induced surge
   • Terminal control cabinet: Class T3(Type3), Imax≥10kA for fine terminal protection

   Three‑stage coordinated protection is mandatory; standalone terminal SPD lacks front surge interception.

4. Degradation indication & remote alarm are compulsory

   SPD is a consumable degraded after repeated lightning surges. Products without visual status window (green→red) and remote alarm contact cannot alert users to component failure; include these two items in acceptance checklist.

• Earthing conductor cross‑section ≥4mm²
• Lead wire length within 0.5m for optimal surge discharge path
• Maintain proper coordination distance between upstream MCB and SPD

Proper specification brings real surge protection; blind installation provides false safety.