Short-circuit current assessment is one of the fundamental pillars in the design of any electrical circuit. For heavy-duty mobile service cables, subjected to severe operating conditions, this analysis transcends mere regulatory compliance, becoming a critical factor for operational safety, asset integrity, and the prevention of catastrophic failures. The ability of a cable to withstand the thermal stress imposed by a fault current, even for fractions of a second, determines the resilience and reliability of the entire installation.

Regulatory Basis and Thermal Limits

To establish a robust and reliable technical criterion, we resorted to internationally consolidated standards, such as the German one. VDE 0250According to its guidelines, the calculation of the permissible thermal limit for short-circuit current in mobile service cables with elastomeric insulation must consider a scenario of practically adiabatic heating, where the generated heat does not have time to dissipate into the environment.

The stipulated reference parameters are:

• Initial Temperature (θi​): 80 °C – This value is not arbitrary. It represents the most critical operating condition, in which the cable is already at its maximum steady-state temperature, operating at full load. The calculation is therefore based on the worst possible thermal scenario before the fault occurs.
• Final Short-Circuit Temperature (θf​): 200 °C This is the maximum limit that the conductor's insulating compound can reach without suffering irreversible degradation of its dielectric and mechanical properties. Exceeding this threshold, even momentarily, can compromise the cable's lifespan and safety.

The nominal short-circuit currents (Icc​The values ​​provided in Innovcable's technical tables are calculated based on these parameters for a standard duration of 1 second.

Adjusting Short-Circuit Capacity as a Function of Time

In practice, the tripping time of protective devices (circuit breakers and fuses) is rarely exactly 1 second. To adapt the tabulated value to the reality of your project, it is essential to understand the relationship between current and time during a fault.

This formula demonstrates the inverse and quadratic relationship: the faster the protection device, the greater the short-circuit current capacity that the cable can withstand.

Advanced Calculus for Specific Design Conditions

For projects that adopt different assumptions, such as different operating temperatures or the use of cables with superior insulation performance (such as XLPE, which can withstand up to 250 °C in short circuit), the permissible short-circuit current can be determined more accurately using the general formula, derived from Onderdonk's equation:

Implication for Engineering: The correct application of these concepts is vital. The cable's withstand capability must always exceed the calculated short-circuit current at the point of installation, and the operating time of the protection device must be coordinated to ensure that the cable's thermal limit is never exceeded.

We invite you to reflect: In your projects, how do you ensure selectivity and coordination between the thermal withstand curve of the cables and the operating curve of the protection devices to guarantee maximum protection and reliability? Share your experiences in the comments.