When selecting a cable, its performance under fault conditions is an important design consideration. The cable must be able to withstand the thermal effects of any prospective fault or short circuit until the protective device disconnects the supply.
The main concern during a fault is heat generated by the high current and the effect of that temperature rise on the cable insulation, conductor, joints and terminations.
Principle of thermal withstand
Fault-rating calculations are based on the principle that the protective device will isolate the fault quickly enough that the permitted cable temperature rise is not exceeded.
In practice, this means comparing the prospective fault current and disconnection time with the thermal withstand capability of the cable. The method used depends on the standard, cable construction, conductor material, insulation material and whether adiabatic or non-adiabatic effects need to be considered.
Other fault-condition considerations
- Electro-mechanical stress, where fault levels are high enough to cause cable movement or physical damage.
- Performance of joints and terminations under fault conditions.
- Coordination between cable withstand capability and the protective device operating time.
- Energy-limiting behaviour of protective devices, where relevant.
Related thermal withstand topics
The original knowledge base includes several more detailed thermal-withstand topics. These should be migrated as linked technical notes, with this article acting as the overview.
- The adiabatic equation and k factor
- I2t thermal withstand and energy-limiting devices
- Non-adiabatic effects
For the fault-current inputs used in withstand checks, see Fault Current Calculations for Cable Sizing.
For protective conductor arrangements used in earth-fault paths, see Circuit Protective Conductor (CPC).