AS - Cable Size Under Short Circuit Calculation

This section describes the fault current calculation that needs to be performed after the cable has been sized.

The adiabatic method shown below calculates maximum short circuit current. The larger the cable, the larger the current the cable can withstand without damage.

Step – I Calculate Source Short Circuit Current

Isc =     (Er * 1000) * (100)

                V * (Imp %)

Where:

V = Phase voltage (Vph * Ö3/2) for 1 phase, Line voltage Vl for 2 phase or Line voltage (Vl * Ö3) for 3 phase

Er = Equipment rating with units in kVA

Imp % = Equipment Impedance expressed as a percentage

Step – II Calculate Source Impedance

Zsc = V/ Isc

Where:

Zsc = source impedance in ohms

Isc = phase current in amps

V = voltage in volts

Step - III Calculate Cable Impedance 1 (When cable catalogue has valid values)

If the cable catalogue data includes the resistive and reactive value then use the following equation:

Zc1 = (L/1000) * Ö [(Rc)2 + (Xc)2])

Where:

L is length in meters

Rc is resistance of active conductor (Ohms/km)

Xc is reactance of active conductor (Ohms/km)

Zc1 is cable impedance in Ohms

Step – IV Calculate Cable Impedance 1 (If No cable catalogue values)

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

For R the data required is:

Single/Multi

Material

Core Size

R or X = R

Conductor Shape – Default is Circular.

For X 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

If cables are in parallel the No. of Cables: value (see the screen shot above) will be 2 or more (in future).

The equation to calculate the total impedance is:

1/ ZTOTALC1 = 1/ Zc1A + 1/ Zc1B + 1/ Zc1C …….+ 1/ Zc1n

Where:

ZTOTALc1 = total impedance for cable in ohms

1/ Zc1A = impedance for cable if ‘No. of Cables:’ value = 1, in ohms

1/ Zc1B = impedance for cable if ‘No. of Cables:’ value = 2, in ohms

1/ Zc1C = impedance for cable if ‘No. of Cables:’ value = 3, in ohms

1/ Zc1n = impedance for cable if ‘No. of Cables:’ value = n, in ohms

Since the system only allows 2 cables to be in parallel the equation can be simplified to:

ZTOTALc1 = Zc1 / N

Where:

Zc1 is in Ohms

N = number of cables in parallel, N = 1 when no cable in parallel

Step – V Calculate Cable Impedance 2 (If No cable catalogue values)

In this example there is a cable between switchboard busbar SWB-2 and incomer MCC-1.

Before performing the calculation the system needs to check if it is a power cable.

Then if there is an impedance value for the cable.

Then if it is in parallel.

The calculation is exactly the same as the Cable Impedance 1 calculation.

Step – VI Calculate Total Impedance

Z = Zsc + ZTOTALc1N + ZTOTALc2 + ……. + ZTOTALcn

Where:

Z is total impedance for Isc in ohms

Zsc = source impedance in ohms

ZTOTALc1 = total impedance for cable in ohms

ZTOTALc2 = total impedance for cable in ohms

ZTOTALcn = total impedance for cable in ohms

Step – VII Calculate Short Circuit Current for Sized Cable

Isc = V / Z

Where:

Isc is short circuit current in amps

V is voltage in volts

Z is total impedance for Isc in ohms

Step - VIII Calculate K Factor

The value of K, which is a material factor, is taken from AS Table 52

The K factor is used in equations to calculate the minimum core size under short circuit temperature.

Initial Temperature = Conductor Temp

Final Temperature = Maximum temperature, this limit values are taken from AS Table 53.

To return K values, consider the Material and Initial Temperature

To return the final temperature or maximum temperature limit, consider the insulation type and core size.

Step – IX Calculate Core Size based on Short Circuit Current

A = Ö ((Isc/P)2 * t)/K

Where:

A is the cross-sectional area of the current-carrying component, in square millimetres

Isc is the short-circuit current

t is the short circuit duration, in seconds

K is a constant depending on the material of the current-carrying component, the initial temperature and the final temperature

P is the number of conductors in parallel