Technical Insight: Critical Role of Copper Tape Screen in 33 kV Cable Fault Performance ⚡🔷

In 33 kV medium-voltage underground cable systems, the copper tape metallic screen is far more than a shielding accessory — it is a precision-engineered component fundamental to:

• Electrical stress management

• Thermal fault withstand capability

• Protection system effectiveness

• Operational safety integrity

Ignoring its design significance can compromise both reliability and fault performance.

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🟦 Core Engineering Functions of Copper Tape Screen

🔹 Electric Field Control
Ensures uniform radial stress distribution across XLPE insulation, preventing localized dielectric overstress and premature breakdown.

🔸 Insulation Reliability Enhancement
Suppresses partial discharge activity and stabilizes long-term dielectric performance, directly increasing service life.

🟢 Charging Current Return Path
Provides a defined low-impedance path for capacitive charging currents under normal operation.

🟣 Electromagnetic Shielding
Limits electromagnetic interference (EMI) impact on adjacent control, protection, and communication circuits — particularly critical in substations and dense cable corridors.

⚫ Touch & Step Safety
Maintains the outer sheath close to earth potential, minimizing induced voltages and enhancing personnel safety.

🔴 Earth Fault Current Conduction
During single-line-to-ground faults, the copper screen — together with metallic armour — forms a controlled low-impedance return path, safely carrying fault current until protection relays isolate the fault.

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📊 Short Circuit Current Rating (SCCR) – Engineering Perspective

The short-circuit withstand capability of the copper tape screen is evaluated as per IEC methodology considering:

• Conductor material resistivity

• Initial and final permissible temperatures

• Fault duration (typically 1 sec basis)

• Thermal constants of copper

• Cross-sectional area of metallic screen

However, a critical engineering refinement often overlooked is:

🧠 Non-Adiabatic Behaviour of Cable Screens

Cable screens are not perfectly adiabatic systems.

During fault conditions:

• Heat does not remain confined to the copper tape.

• Thermal energy dissipates into insulation, sheath, and surrounding materials.

• Temperature rise is therefore lower than purely adiabatic assumptions.

To address this, a non-adiabatic correction factor is applied — enabling more realistic and technically accurate estimation of permissible fault current withstand.

This correction prevents:

• Over-conservative oversizing

• Under-designed fault return paths

• Misinterpretation of IEC short-circuit calculations

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🎯 Key Technical Takeaway

The copper tape screen in a 33 kV cable is:

Not just shielding —
It is a fault current conductor, a stress regulator, a thermal element, and a safety component.

Proper SCCR evaluation — including non-adiabatic correction — is essential for:

• Protection coordination

• Earthing system design

• Cable longevity

• Grid compliance

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