Earthing Sizing calculation

 Brief Calculation for earthing :

Protective Conductor Sizes.

Regulation 543.1.1: This can be achieved by either calculation (543.1.3) or selected (543.1.4)

from Table 54.7.

The formula used in 543.1.4 for calculating the size of a protective conductor is covered in great detail in a session inside our 17th edition BS7671.

Selecting from Table 54.7

The protective conductor size is relative to the size of the line conductor which is represented by the letter 'S.

Also, the far right column can usually be ignored as this is only used when the protective conductor and line conductor are of different materials.

Let's Assume, A  transformer of 100 kVA, 11/0.415kV, with an Impedance of 4%

In this case, Transformer's secondary current will be Power / 1.73* Voltage

    =100 kVA / 1.73 * 415 = 139.12 A

    Therefore, Short circuit current, Isc = (100 / 4 ) * 139.12 A = 3478.11 A

So for Earthing Sizing, we need fault current value and operating time.

By Using Data, Short circuit current = 3.47kA, Time = 1 Second, K = 143 for copper, Safety Margin = 1.5, and 1% per year margin for degradation.

S = 24.32

1% per year margin for degradation, i.e for 20 years and a safety margin of 1.5

S =  (24.32+(24.32*20%)) * 1.5 = 43.78, Next standard size = 50 mm2

"Keep in mind the Terminal must be RCD protected in TT earthing System."