IEC – Cable Size Under Earth Fault Condition

This section describes how the core size is calculated under when earth fault conditions.

Earth conductors are assumed to be parts of the same cables in this release.

In order to calculate the earth fault current first the earth fault loop impedance needs to be calculated. This means the impedance of the active conductor (conductor core) and earth conductor.

Step – I Calculate Earth Loop Impedance (If Cable Catalogue has valid values)

Zs = ( L/(1000 * P) ) * Ö[(Rc + Re)2 + (Xc + Xe)2]

Where:

L is the length in meters

Rc is the resistance of active conductor (Ohms/km)

Re is the resistance of earth conductor (Ohms/km)

Xc is the reactance of active conductor (Ohms/km)

Xe is the reactance of earth conductor (Ohms/km)

P is the number of cables in parallel

Rc, Re, Xc, Xe are the resistive and reactive components of the conductor from the cable catalogue.

Step – II Calculate Earth Loop Impedance (If No Cable Catalogue values)

If conductor resistive and reactive values are not available in the cable catalogue, then impedance values are taken from the BS standard impedance tables.

Rc, Re = [ R * L * cosø ] / (A * 1000)

Where:

R is the Resistivity of conductors in normal service, taken to be equal to the Resistivity at the normal service temperature, i.e. 1.25 times the Resistivity at 20°C, or 0.0225Wmm2/m for copper and 0.036Wmm2/m for aluminium

L = route length of circuit, in metres

A = cross-sectional area of conductors, in mm2

cosø is the power factor

Xc, Xe = [ X * L * sinø ] / 1000

Where:

X is the reactance per unit length of conductors, which is taken to be 0.00008W/m in the absence of other details

L = route length of circuit, in metres

cosø is the power factor; in the absence of precise details, the power factor is taken as 0.8 (sinø = 0.6)

Zs = (L/(1000 * P)) * Ö[(Rc + Re)2 + (Xc + Xe)2]

Where:

Zs is the maximum earth loop impedance

L is the length in meters

Rc is the resistance of active conductor (Ohms/km)

Re is the resistance of earth conductor (Ohms/km)

Xc is the reactance of active conductor (Ohms/km)

Xe is the reactance of earth conductor (Ohms/km)

P is the number of cables in parallel

The results of this calculation are displayed in Sizing Result tab as shown below:

Step – III Calculate Earth Fault Current

Ief = (c * Vp) / Zs

Where:

Ief is the earth fault current required to trip protection device

Vp is the phase to earth voltage at the protection device

c is a factor to take into account that nominal voltage is higher that the voltage at the protection device. Typical value is 0.8.

Zs is the maximum earth fault loop impedance

The results of this calculation are displayed in Sizing Result tab as shown below: