Muhammad Faheem Meer, Business Management Consultant
Brugg Kabel AG, Switzerland & Worldwide Cables (Pvt.) Ltd., Pakistan
Power cables are designed to carry their rated current continuously under normal operating conditions. However, electrical systems occasionally experience faults such as:
- Phase-to-phase faults
- Phase-to-earth faults
- Three-phase short circuits
- Equipment failures
- Cable insulation breakdown
During these conditions, fault current may rise to 10–50 times the normal load current, generating enormous heat within a fraction of a second. Although circuit breakers, relays and fuses disconnect the fault rapidly, the cable must withstand this thermal stress until the fault is cleared.
This capability is known as the Short Circuit Rating (SCR), defined as the maximum fault current a cable can safely withstand for a specified duration without suffering permanent damage. It is typically expressed in kA for 1 second or 3 seconds. For example, a cable rated 31.5 kA for 1 second can safely carry a fault current of 31,500 A for one second.
Why is Short Circuit Rating Important?
During a short circuit, almost all electrical energy is converted into heat. Excessive temperature rise can cause:
- Conductor annealing
- Insulation melting or degradation
- PVC softening
- XLPE insulation deterioration
- Failure of metallic screens and sheaths
- Permanent cable damage
Therefore, cable selection should consider not only continuous current-carrying capacity (ampacity) but also short-circuit withstand capability, which is often overlooked during system design.
The Adiabatic Heating Principle
Short-circuit rating is based on the adiabatic heating principle, which assumes that the fault duration is so short that virtually no heat is dissipated to the surroundings. Consequently, the generated heat remains within the conductor, causing a rapid increase in temperature.
Factors Affecting Short Circuit Rating
Several factors determine the SCR of a cable:
Conductor Material: Copper offers higher short-circuit ratings than Aluminium because of its lower electrical resistance, higher conductivity and greater thermal capacity.
Conductor Size: Larger conductors have lower resistance and greater heat-absorbing capacity. Consequently, a 630 mm² conductor has a much higher SCR than a 95 mm² conductor.
Insulation Type: The permissible conductor temperature depends on the insulation material. PVC cables typically operate at 70°C with a maximum short-circuit temperature of 160°C, whereas XLPE cables operate at 90°C and can withstand up to 250°C during short circuits. The higher allowable temperature rise enables XLPE cables to achieve higher SCR values.
Fault Duration: Short-circuit current is inversely proportional to the square root of the fault duration. Therefore, the longer the fault persists, the lower the permissible fault current. For example, a cable rated 30 kA for 1 second can withstand only 15 kA for 4 seconds.
Standard Formula for Cable Short Circuit Rating Calculation
The thermally permissible short-circuit current is calculated using the IEC adiabatic equation:
Isc = kS / √t
Where:
- Isc = Short-circuit current (A)
- S = Conductor cross-sectional area (mm²)
- t = Fault duration (s)
- k = Material constant depending on conductor and insulation
Typical values of k are:
| Conductor | Insulation | k |
| Copper | PVC | 115 |
| Copper | XLPE | 143 |
| Aluminium | PVC | 76 |
| Aluminium | XLPE | 94 |
These values explain why copper conductors and XLPE-insulated cables generally provide higher short-circuit ratings.
Calculation Example
For a 240 mm² Cu/XLPE cable with a fault duration of 1 second:
Using k = 143, the calculated short-circuit rating is approximately 34,300 A, meaning the cable can safely withstand a fault current of about 34 kA for one second.
Design Considerations
When selecting power cables, engineers should ensure that:
- The cable short-circuit rating exceeds the maximum prospective fault current.
- Protective devices clear the fault within the assumed duration.
- Metallic screens, sheaths, armour and earth conductors also satisfy short-circuit requirements.
- Cable cleats and supports can withstand the mechanical forces produced during faults.
- Both thermal and mechanical withstand limits are verified.
Short Circuit Rating is a critical parameter in power cable engineering because it determines a cable’s ability to survive fault conditions without permanent damage. While ampacity governs normal operation, SCR ensures safe performance during abnormal conditions until protective devices isolate the fault. Unfortunately, SCR is not given due importance at cable design stage that may jeopardize complete electrical system and result in suspended operations of worthy installations and plants. A sound understanding of conductor heating, insulation limits, fault duration and applicable IEC standards is therefore essential for reliable and safe cable system design.
