In electrical applications, selecting the right SPD (Surge Protective Device) requires careful consideration of multiple factors. These include the system’s grounding method, susceptibility of equipment to lightning strikes, division of lightning protection zones, cable lengths, coordination between protection levels, short-circuit current at protection points, and the number of parallel discharge paths.

Key Steps for SPD Selection:

1. Determine the equipment’s impulse withstand voltage (Uw).

2. Identify the type of AC low-voltage distribution system (e.g., TT, TN-S, IT).

3. Select the maximum continuous operating voltage (Uc) of the SPD.

4. Choose the appropriate test category and define impulse current (Iimp) and nominal discharge current (In).

5. Calculate the voltage protection level (Up).

6. Determine the number of poles based on the grounding system.

7. Verify the effective protection distance; add supplementary SPDs if needed.

8. Ensure energy coordination between SPDs and select dedicated overcurrent protection devices.

SPD Selection Guide by Application:

1.Power Systems

Level 1 (Main Distribution Cabinet): T1 Class (IEC 61643 Type 1), In≥12.5kA, Up≤4kV.

Level 2 (Sub-Distribution Cabinet): T2 Class (Type 2), In≥20kA, Up≤2.5kV.

Level 3 (Equipment Front-End): T3 Class (Type 3), Up≤1.5kV (e.g., socket SPDs).

2.Signal Systems(Network/Telecom): 

Dedicated SPDs with voltage levels matching equipment (e.g., Up≤30V for CCTV).

3.Photovoltaic Systems: 

DC SPDs with Uc≥1.2×system voltage (e.g., 1200V DC for 1000V systems).

4.Special Applications:

Communication towers: Multi-level SPDs (T1+T2+T3), ground resistance ≤4Ω.

Explosive environments: ATEX/IECEx-certified SPDs.

Outdoor use: IP65-rated, UV-resistant.

Installation Best Practices:

· Maintain minimum distances between SPDs (e.g., 10m cable or decouplers).

· Limit wiring length to ≤0.5m using "V" or "star" grounding.

· Ensure ground resistance ≤10Ω (≤4Ω for lightning-prone areas).

· Coordinate Up values across levels (e.g., T1 Up > T2 Up > T3 Up).

Maintenance & Testing:

· Check SPD status indicators (green=OK, red=failed).

· Test with SPD analyzers for leakage current and residual voltage.

· Replace every 5-8 years or after severe lightning strikes.