Calculating Earth Fault Loop Impedance (Zs)

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Earth Fault Loop Paths

The diagrams below show the path that fault currents take for three different earthing systems. The purpose of the circuit breaker or fuse is to disconnect the supply within a certain time should a fault current exist.

Equivalent Circuits and Zs
Calculating Earth Fault Loop Impedance (Zs)

The circuit above shows the equivalent earth fault current path when there is a short between line and earth in a TN-S installation. The resistors indicate the different resistance and impedances that exist in the installation and external supply. All cables have resistance and this depends on the CSA of the line and CPC conductors and their lengths.


Zs = The earth fault loop impedance.


Ze = The external loop impedance. i.e. the impedance of the path back to the supply transformer.


R1 = The resistance of the line conductor to the load.

R2 = The resistance of the CPC.

Typical Ze values are

TN-S        0.8 Ohms

TN-C-S    0.35 Ohms

TT            22 Ohms

1. Calculate the earth fault loop impedance and fault current when

A domestic installation is connected to a TN-S supply with a Ze of 0.8Ω. The installation includes a circuit supplying a ring-final circuit with an R1 value of 0.3Ω and an R2 value of 0.7Ω.

Zs = Ze + (R1 + R2)

Zs = 0.8 + (0.3 +0.7)

Zs = 1.8Ω

I =            =                = 127.8 A                 

Uo

Zs

230

1.8

  1. 1.A ring-final circuit in a domestic installation has an R1 value of 0.34Ω, and R2 value of 0.56Ω. The installation is supplied via a TT connected supply with a Ze of 22Ω. Calculate the Zs and prospective fault current of the circuit.


  1. 2.A radial circuit in a domestic installation has an R1 value of 0.5Ω and a R2 value of 0.9Ω. The installation is supplied via a TN-C-S connected supply with a Ze of 0.82Ω. Calculate the Zs and prospective fault current of the circuit.


  1. 3.A lighting circuit in a domestic installation has an R1 value of 0.6Ω and a R2 value of 0.6Ω. The installation is supplied via a TN-C-S connected supply with a Ze of 0.3Ω. Calculate the Zs and prospective fault current of the circuit.


  1. 4.A radial circuit in a domestic installation has an R1 value of 2Ω and a R2 value of 3.34Ω. The installation is supplied via a TN-C-S connected supply with a Ze of 0.85Ω. Calculate the Zs and prospective fault current of the circuit.