AS - 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 the active conductor (Ohms/km)

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

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

Xe is the reactance of the 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 ANZ standard impedance tables.

For Rc, Re the data required is:

Single/Multi

Material

Core Size

R or X = R

Conductor Shape – Default is Circular.

For Xc, Xe the data required is:

Single/Multi

Insulation Type

Core Size

R or X = X

Conductor Shape

Touching Type – Only valid if Single/Multi = Single-Core

These values are then used in the equation in Step I.

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 the protection device

Vp is the earth voltage phase of the protection device

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

Zs is the maximum earth fault loop impedance

Results of this calculation are displayed in Sizing Result tab as shown below.