Short-circuit rating
Wire screen short-circuit rating (IEC 60949 adiabatic)
- where
- screen cross-sectional area (mm²)
- number of screen wires
- wire diameter (mm)
- adiabatic screen short-circuit current (kA)
- material constant (A·s½/mm²)
- final screen temperature (°C)
- reciprocal of temperature coefficient (K)
- initial screen temperature (°C)
- earth-fault duration (s)
Explain this equation
Works out the area of a copper wire screen and the earth-fault current it can carry for time t without overheating, using the same adiabatic method as for conductors.
n wires of diameter d_w give the screen area S. θᵢ is the screen's starting temperature, a little below the conductor maximum, and θ_f the limit set by the materials in contact with the screen. K and β are the screen metal's constants.
As for conductors, the heat from I²R is assumed to stay in the metal, and resistance rising with temperature gives the log term. The 10⁻³ gives the answer in kA.
A 16 mm² copper screen from 80 °C to 250 °C carries about 2.4 kA for 1 s or 1.4 kA for 3 s. Halving the duration raises the permitted current by √2.
Screens are thin and lose heat to neighbouring layers, so the true capability for longer faults is higher than this adiabatic figure. Use the earth-fault current, not the three-phase fault current.
Earth-fault current a copper (or other metal) wire screen can carry for time t, by the same adiabatic equation as for conductors.
Enter your values to see the result.
Good to know
The initial screen temperature is usually taken a few degrees below the conductor maximum (e.g. 80 °C for a 90 °C XLPE cable, or a value from the rating calculation). The final temperature is set by the materials touching the screen: often 250 °C next to XLPE semicon, but lower for PE or PVC oversheaths and bedding tapes (manufacturers commonly use limits in the 200–250 °C range). For tape screens or lead sheaths enter n = 1 and choose dw so the area matches, or use the area from the data sheet. Screens are thin, so heat lost to adjacent layers is significant for longer faults.
Common mistake
Using the three-phase fault current: the screen carries earth-fault current, which is limited by the system earthing (e.g. resistance-earthed 11 kV systems).
Limits
Adiabatic method only. IEC 60949 gives a non-adiabatic factor ε for screens and sheaths that can raise the permissible current noticeably for faults over about 0.5 s; this is not calculated here, so results are conservative. Assumes current shares equally between wires.
Sources
- IEC 60949:1988+AMD1:2008 Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects
- IEC 60502-2:2014 Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) – Part 2: Cables for rated voltages from 6 kV up to 30 kV
- Moore, G.F. (ed.) Electric Cables Handbook, 3rd edn, Blackwell Science / Wiley
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.