IEC 61443: Short-Circuit Temperature Limits of Electric Cables with Rated Voltages Above 30 kV (Um = 36 kV)

IEC 61443 specifies the temperature limits, calculations, and guidelines for electric cables with high-voltage ratings during short-circuit conditions. The standard ensures that cables can safely withstand thermal stresses without damage to their insulation, conductors, or protective layers.

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1. Scope and Purpose

The scope of IEC 61443 includes:

• High-voltage cables rated above 30 kV (Um = 36 kV).
• Thermal limits for conductors, insulation, and screen layers during short circuits.
• Guidelines for designing cables to withstand short-circuit currents.

Purpose:

• Ensure cables remain operational after short-circuit conditions.
• Prevent thermal degradation of insulation and metallic layers.
• Define permissible temperature rises for different cable materials.

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2. Permissible Short-Circuit Temperatures

The standard specifies maximum permissible temperatures for various cable materials during short circuits:

• Copper Conductors: 250°C
  
• Aluminum Conductors: 200°C
  
• XLPE Insulation: 250°C
  
• EPR Insulation: 200°C
  
• Metallic Screens and Sheaths: Depends on the material and configuration, typically between 200°C and 250°C.
  

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3. Key Formulas for Thermal Calculations

(a) Heat Energy Generated in the Conductor:
\[ Q = I^2 \cdot R \cdot t \]

(b) Temperature Rise in the Conductor:
\[ \Delta T = Q / (m \cdot c) \]

(c) Maximum Short-Circuit Current:
\[ I_{\text{max}} = \sqrt{(A \cdot \rho \cdot c \cdot \Delta T) / t} \]

(d) Resistance as a Function of Temperature:
\[ R_T = R_0 \cdot (1 + \alpha \cdot (T - T_0)) \]

(e) Energy in the Metallic Screen or Sheath (if applicable):
\[ Q_{\text{screen}} = I_{\text{screen}}^2 \cdot R_{\text{screen}} \cdot t \]

(f) Thermal Balance for Complex Cable Designs:
\[ \Delta T_{\text{total}} = \Delta T_{\text{conductor}} + \Delta T_{\text{screen}} + \Delta T_{\text{insulation}} \]






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4. Parameters and Variables

The following parameters are used in thermal calculations:

• I: Short-circuit current (Amperes)
  
• t: Duration of the short circuit (Seconds)
  
• R: Resistance of the conductor or screen (Ohms)
  
• A: Cross-sectional area of the conductor (m²)
  
• \(\rho\): Resistivity of the material (Ohm·m)
  
• m: Mass of the conductor or screen (kg)
  
• c: Specific heat capacity of the material (J/kg·K)
  
• \(T, T_0\): Final and initial temperatures (Kelvin)
  
• \(\alpha\): Temperature coefficient of resistance (1/K)
  

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5. Design Considerations

To ensure compliance with IEC 61443, cable designs must consider:

• Adequate cross-sectional area to minimize temperature rise during faults.
• Thermal properties of insulation and metallic layers.
• Proper installation to allow heat dissipation.
• Coordination with protective devices to limit fault durations.

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6. Example Calculation

Consider a copper conductor with the following parameters:

• Cross-sectional area (\(A\)): \(300 \, \text{mm}^2\)
• Short-circuit current (\(I\)): \(25,000 \, \text{A}\)
• Duration (\(t\)): \(0.5 \, \text{seconds}\)
• Initial temperature (\(T_0\)): \(90^\circ \text{C}\)
• Final permissible temperature (\(T\)): \(250^\circ \text{C}\)
• Resistivity of copper (\(\rho\)): \(1.72 \times 10^{-8} \, \Omega \cdot \text{m}\)
• Specific heat capacity (\(c\)): \(385 \, \text{J/kg·K}\)

1. Calculate heat energy generated:
  \[ Q = I^2 \cdot R \cdot t \]

2. Determine the temperature rise:
  \[ \Delta T = Q / (m \cdot c) \]

3. Verify the final temperature does not exceed \(250^\circ \text{C}\).

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7. Practical Applications

IEC 61443 applies to:

• High-voltage power transmission cables.
• Substation and switchgear connections.
• Industrial applications with large fault currents.

Proper compliance ensures:

• Safe operation under fault conditions.
• Longevity of cable systems.
• Prevention of catastrophic failures due to overheating.

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8. Conclusion

IEC 61443 provides essential guidelines for designing and verifying the thermal performance of high-voltage cables under short-circuit conditions. By adhering to these guidelines:

• Cables can withstand fault conditions safely.
• Thermal stress on materials is minimized.
• Electrical systems remain reliable over their operational lifespan.

References:

• IEC 61443: "Short-circuit temperature limits of electric cables with rated voltages above 30 kV."
• Material data sheets for copper and aluminum.
• Industry best practices in high-voltage cable design.