Fault Current Calculations for Cable Sizing

Electrical faults can occur because of insulation failure, equipment failure, mechanical damage, incorrect operation or external events such as lightning. During a fault, a large current may flow through the system until a protective device clears the fault.

Fault current calculations estimate the current available at a point in the system. This is needed for protective-device selection, short-circuit ratings, earth-fault protection and cable fault withstand checks.

Basic fault-current equation

At its simplest, fault current can be estimated from Ohm’s law using the voltage driving the fault and the impedance of the source and fault path:

The impedance includes the relevant source, transformer, cable and return-path impedance. For more rigorous network calculations, standards such as IEC 60909 define detailed methods and correction factors.

Three-phase fault

In a balanced three-phase fault, all three phases are shorted together. A simplified calculation is:

Here V_LL is the line-to-line voltage and Z_p is the phase impedance of the system seen from the fault location.

Phase-to-phase fault

For a phase-to-phase fault, two phases are shorted together. A simplified calculation is:

Single-phase-to-ground fault

For a single-phase-to-ground fault, one phase is connected to earth or ground through the fault path. A simplified calculation is:

In this expression, Z_g is the grounding or earth-fault loop impedance of the system.

Use in cable sizing

The maximum fault current must be compared with the short-circuit rating of switchgear and protective devices. For cables, the fault current and disconnection time are used to check conductor and protective-conductor thermal withstand.

The minimum fault current can also be important, particularly for earth faults, because it determines whether the protective device will operate within the required time.

For cable thermal withstand under fault conditions, see Cable Thermal Withstand Under Fault Conditions. For the wider design workflow, see Cable Sizing.

Related fault-calculation topics

The original knowledge base lists the following related subtopics, which are good candidates for separate migration:

• Circuit protective conductor (CPC)
• Earth fault loop impedance
• IEC 60909 fault calculations
• Network fault level
• Transformer secondary fault level