Short-Circuit Thermal Limit (IEC 60949)
Standard: IEC 60949 — Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects
Grøft Design® now includes a permissible short-circuit current calculator for cable conductors and screens, implemented in accordance with IEC 60949. When a fault occurs, short-circuit current flows through the conductor and may return via the cable screen for a brief duration. Despite the short fault duration, the deposited energy can raise the conductor and screen temperatures significantly. If the temperature exceeds the maximum permissible limit for the respective metallic layer or adjacent materials, permanent cable damage may result. This calculator enables engineers to evaluate and document the thermal withstand capability of both the conductor and screen under fault conditions.
The calculator is accessible directly within the Cable Designer interface and is not tied to a specific project.
Method
IEC 60949 provides an analytical solution accounting for non-adiabatic heating effects — that is, a fraction of the heat generated in a metallic layer is allowed to dissipate into surrounding materials during the fault, refining the conservative purely-adiabatic assumption. The permissible short-circuit current is a function of:
- Cross-sectional area and material properties of the conductor or screen
- Initial temperature of the respective layer prior to the fault
- Maximum permissible temperature of the respective layer at end of fault
Inputs
| Parameter | Source |
|---|---|
| Cable geometry, material properties, cross-section | Fetched automatically from cable construction |
| Initial conductor temperature | Default: 90 °C; user-adjustable |
| Initial screen temperature | Default: 80 °C; user-adjustable |
| Fault current level and fault duration | Specified by the user |
The calculator reports the peak temperature reached in each metallic layer during the specified fault event, calculated per the IEC 60949 analytical method. For orientation, the calculator also displays the maximum withstand short-circuit current at a fixed 1-second duration for reference temperature limits of 250 °C (conductor) and 200 °C (screen). These reference values are indicative only — maximum permissible temperatures depend on cable materials and must be confirmed with the cable manufacturer and coordinated against project-specific fault duration.
Multi-Screen Fault Current Distribution
Where a cable has multiple screens, the fault current is automatically distributed between them in proportion to their resistance. The user can adjust the distribution percentage using a slider, allowing the engineer to evaluate scenarios where:
- Fault current is shared between all screens (default, proportional to resistance)
- The full fault current is carried by a single screen only
The user is responsible for selecting the appropriate distribution assumption based on the network configuration and fault type.
Notes
- All cable parameters are resolved automatically from the cable construction; no manual entry of material constants is required.
- Advanced users may modify the built-in material and thermal parameters of the IEC 60949 analytical model by opening the Advanced Module, allowing adaptation to non-standard cable materials or project-specific assumptions.
- The conductor and screen are evaluated independently.
- Step-by-step guidance is provided inline within the calculator interface.