How to Select Uc for SPDs (According to IEC 61643)

How to Select Uc for SPDs (According to IEC 61643)

The Maximum Continuous Operating Voltage (Uc) is a critical parameter when selecting a Surge Protective Device (SPD). It ensures the SPD remains stable under normal operating conditions while effectively clamping transient overvoltages.  

 

1. Key Factors Influencing Uc Selection  

According to IEC 61643-11/12, the following must be considered:  

A. System Voltage & Configuration

- Nominal System Voltage (Un):

  - Single-phase: 120V, 230V, etc.  

  - Three-phase: 230V/400V, 400V/690V, etc.  

- System Earthing (Grounding) Type:  

  - TN-S/TN-C-S:Neutral and protective earth combined or separated.  

  - TT: Local earth connection (higher risk of overvoltages).  

  - IT: Isolated or impedance-grounded neutral (higher Uc required).  

B. Temporary Overvoltages (TOV)  

- Voltage spikes due to faults, switching, or neutral disconnection.  

- TN/TT Systems:TOV can reach 1.45×U₀(phase-to-neutral voltage).  

- IT Systems:TOV can reach√3×U₀(due to phase-to-phase faults).  

C. Voltage Tolerance & Safety Margin

- Standard±10% voltage fluctuation in power networks.  

- Uc must be higher than the worst-case voltage to avoid SPD degradation.  

 

2. Uc Calculation Formulas (IEC 61643-12)  

A. For TN/TT Systems (Solidly Grounded Neutral)

Uc≥1.1XUo(Uo=phase-to-neutral voltage)

Example (230V System):

Uo=230V

Uc≥1.1X230V=253V

Standard SPD Uc:275V (next available rating).  

 

Why 275V?  

TOV can reach 1.45×230V = 333.5V (SPD must withstand this without damage).  

- 275V SPDs are designed to handle this (some high-end SPDs go up to 320V).  

 

B. For IT Systems (Ungrounded or High-Impedance Grounded)  

Uc≥U_L-L (line-to-line voltage)

Example (400V IT System):  

U_L-L = 400V

Including±10% fluctuation: 400V×1.1 = 440V  

Standard SPD Uc:440V or 500V (next available rating).  

 

Why Higher Uc?  

- In IT systems, phase-to-ground faults can cause√3×U₀overvoltages.  

- Example: For 400V IT, worst case is 400V×1.73≈ 692V.  

- SPD must not trip under this condition (select Uc≥440V or 500V).  

 

3. Practical Case Study: Selecting Uc for Different Systems  

Case 1: Residential Building (TN-S, 230V/400V)  

- System: TN-S, 230V phase-to-neutral, 400Vphase-to-phase.  

- Calculation:

  Uc≥230V×1.1=253V

- Selected Uc:275V (standard SPD rating).  

 

Case 2: Industrial Plant (IT System, 690V)  

- System: IT, 690V phase-to-phase.  

- Calculation:  

  Uc≥690V×1.1 = 759V

- Selected Uc:760V or 800V (standard rating).  

 

Case 3: Solar PV System (DC, 1000V)  

- System: DC, 1000V nominal.  

- Calculation:  

  Uc≥1000Vdc×1.1 =1100Vdc

- Selected Uc:1200V or 1500V(depends on manufacturer).  

 

4. Common Mistakes in Uc Selection  

❌Choosing Uc too low→SPD may degrade or fail prematurely.  

❌Ignoring TOV→SPD may activate unnecessarily during faults.  

❌Mismatching system type (TN vs. IT)→Wrong Uc selection.  

 

Final Advice:  

✔Always check IEC 61643-12 for detailed SPD selection.  

✔Consider worst-case TOV scenarios (e.g., neutral faults).  

✔Use manufacturer-recommended Uc values for specific applications.  

 

Would you like a specific example for your system? Let me know your voltage and earthing type!