Part 10 – Short-Circuit Withstand of Cables
Introduction
Under normal operating conditions, the inverter-to-MV panel cable carries only the inverter's rated output current. The conductor temperature remains within its permissible operating limit, and the cable functions safely throughout its service life.
However, electrical systems are not designed only for normal operating conditions.
Every utility-scale solar power plant must also be designed to withstand abnormal operating conditions such as:
- Three-phase faults
- Line-to-line faults
- Single line-to-ground faults
- Double line-to-ground faults
- Equipment insulation failure
- Busbar faults
- Cable termination failures
- Internal equipment faults
During these conditions, the current flowing through the cable may become many times greater than the normal operating current.
Although protective devices disconnect the fault quickly, the cable must remain mechanically and thermally intact until the fault is cleared.
For this reason, professional cable sizing always includes Short-Circuit Withstand Verification.
This verification ensures that the selected conductor has sufficient cross-sectional area to survive the thermal energy generated during the fault without permanent damage.
The OneCalcApp.com AC Cable Sizing Calculator includes short-circuit verification as part of its engineering workflow, allowing engineers to compare cable sizes while still understanding the design methodology explained in this guide.
Why Short-Circuit Verification Is Necessary
Under normal conditions, conductor heating occurs gradually and remains within the allowable operating temperature.
During a short circuit, the situation changes dramatically.
Current may increase several times above the normal operating current.
As fault current increases:
- Heat generation rises rapidly.
- Conductor temperature increases sharply.
- Mechanical stresses increase.
- Insulation experiences severe thermal loading.
- Cable survival depends upon both conductor size and protection operating time.
If the cable cannot withstand this thermal energy before the protective device disconnects the fault, permanent damage may occur.
What Happens During a Short Circuit?
Consider a normal operating condition.
Electrical Current
↓
Conductor
↓
Normal Heating
↓
Stable Operating Temperature
Now consider a fault.
Fault
↓
Very High Current
↓
Extremely Rapid Heating
↓
High Thermal Stress
↓
Protection Operates
↓
Fault Cleared
The cable must survive this entire process.
Understanding Thermal Energy During Faults
The thermal energy produced inside a conductor during a short circuit depends primarily on:
- Fault Current
- Duration of the Fault
- Conductor Material
- Conductor Cross-Sectional Area
- Insulation Characteristics
Higher fault current produces more heat.
Longer fault duration also produces more heat.
Therefore, both fault magnitude and protection clearing time are equally important.
The Adiabatic Assumption
The most commonly used engineering approach for short-duration faults is the adiabatic assumption.
Under this assumption:
The fault occurs for such a short period that very little heat escapes from the conductor during the event.
Therefore:
All the generated heat is assumed to remain inside the conductor.
This assumption provides a conservative and widely accepted method for verifying conductor survival during short circuits.
The Standard Adiabatic Equation
The conductor cross-sectional area required to withstand a fault is expressed by:
Where:
S
Minimum conductor cross-sectional area
I
Prospective short-circuit current
t
Protection clearing time
K
Material constant
This equation forms the basis of thermal short-circuit verification.
Rearranged Form
The same equation may also be expressed as:
This form is useful when the conductor size is already known and engineers wish to determine the maximum short-circuit current the cable can safely withstand.
Understanding Every Variable
Conductor Area (S)
The conductor cross-sectional area determines how much thermal energy the conductor can absorb.
Larger conductor
↓
Higher Thermal Capacity
↓
Better Fault Survival
This is one reason why larger cables generally have greater short-circuit withstand capability.
Fault Current (I)
Fault current is the maximum current expected during a short-circuit condition.
It depends on:
- Utility fault level
- Transformer impedance
- Cable impedance
- Generator contribution
- Inverter contribution
- Network configuration
Professional engineers normally determine this value through a dedicated short-circuit study.
PROTECTION CLEARING TIME (t)
Protection clearing time represents the total time required for the protection system to remove the fault.
It includes:
- Relay operating time
- Circuit breaker opening time
- Arc extinction time
The longer the clearing time:
The greater the thermal energy absorbed by the conductor.
Fast protection therefore improves cable survival.
Material Constant (K)
The constant K represents the thermal characteristics of the conductor and insulation combination.
It accounts for:
- Conductor material
- Initial conductor temperature
- Final permissible conductor temperature
- Insulation type
Different conductor materials and insulation systems have different K values.
Therefore, engineers should always use the value specified in the applicable standard or project design basis.
Relationship Between Current and Fault Energy
One important observation is that fault current does not increase conductor heating linearly.
Instead:
Thermal Energy ∝ I²t
This means that both fault current and fault duration have a significant influence on conductor thermal stress.
This explains why high fault levels require careful coordination between:
- Cable size
- Protection settings
- Circuit breakers
- System design
Why Protection Coordination Matters
The cable and the protection system work together.
The protection device is responsible for:
- Detecting the fault
- Disconnecting the faulty circuit
- Limiting fault duration
The cable is responsible for:
- Carrying the fault current safely until protection operates
Neither can be designed independently.
A correctly sized cable with slow protection may still fail.
Similarly, fast protection may allow a smaller conductor to satisfy the thermal withstand requirement.
Short-Circuit Verification Process
Professional engineers generally follow this sequence:
1. Determine the prospective fault current.
2. Identify the protection clearing time.
3. Select the preliminary conductor size.
4. Apply the adiabatic equation.
5. Verify that the conductor withstand capability exceeds the calculated fault level.
6. If the requirement is not satisfied, increase the conductor size or review the protection coordination.
Common Engineering Mistakes
Typical mistakes include:
- Ignoring short-circuit verification completely.
- Using incorrect fault current values.
- Assuming instantaneous protection operation.
- Using incorrect clearing times.
- Mixing values from different standards.
- Forgetting to coordinate the cable with the protective device.
- Selecting conductor size based only on ampacity.
These errors can compromise the safety and reliability of the installation.
Engineering Best Practices
Professional engineers generally recommend:
- Perform a dedicated short-circuit study before final cable selection.
- Obtain fault levels from the approved electrical design.
- Verify protection clearing times from relay coordination studies.
- Use the applicable IEC, NEC or IS methodology consistently.
- Coordinate cable design with protection engineers.
- Document all assumptions and calculation inputs.
Following these practices improves both safety and long-term reliability.
Applicable Standards
Short-circuit verification is commonly based on internationally recognised standards and engineering guidance, including:
IEC Standards
- IEC 60909 – Short-Circuit Current Calculation.
- IEC guidance for thermal withstand verification of power cables.
- IEC 60502 – Power Cables.
NEC
- Article 240 – Overcurrent Protection.
- Article 310 – Conductors for General Wiring.
Indian Standards
- IS 7098 – XLPE Power Cables.
- IS 1255 – Installation and Maintenance of Power Cables.
- Relevant adopted IEC guidance for thermal short-circuit withstand verification.
Engineers should always follow the standards specified in the project documentation and local regulatory requirements.
How Onecalcapp Helps Engineers
Short-circuit calculations involve multiple design parameters, including fault current, conductor size, protection clearing time and thermal withstand verification.
Repeating these checks manually across several cable options can be time-consuming during detailed engineering.
The OneCalcApp.com AC Cable Sizing Calculator assists engineers by organising these inputs and supporting systematic verification of cable selections while still allowing every result to be checked manually using the engineering methodology discussed throughout this guide.
Rather than replacing engineering judgement, OneCalcApp.com helps streamline repetitive calculations and improves design efficiency during solar EPC and industrial electrical projects.
Summary
At this stage, the selected cable has successfully undergone:
- Full Load Current Calculation
- Preliminary Cable Selection
- Cable Derating
- Corrected Ampacity Verification
- Voltage Drop Verification
- Power Loss Evaluation
- Short-Circuit Thermal Withstand Verification
The cable now satisfies both normal operating conditions and fault conditions.
However, one final engineering review remains before the cable can be approved for construction.
Engineering Takeaway
Short-circuit withstand verification is a critical safety check in professional AC cable sizing.
The final cable must be capable of withstanding the expected fault current for the complete protection clearing time without exceeding the permissible conductor temperature.
The final design therefore depends on proper coordination between:
Fault Level
↓
Cable Cross-Section
↓
Conductor Thermal Capacity
↓
Protection Clearing Time
↓
Short-Circuit Withstand Verification
Calculate. Verify. Design with confidence.