Earth fault loop impedance

[mark3092]

Hi all, I am currently studying the 2382 course and am having difficulty understanding Earth fault loop impedance

any help of advice will be much appreciated

Mark

[spadxiii1918]

ELI takes into account the total resistance of the circuit starting from the test point, down the earth path and back to the centre tap point of the suppliers transformer and back along the supply phase conductor through the consumer unit and back along the phase conductor to the original test point.

Zs (ELI) can be broken down into two parts. R1+R2 the internal resistance of the phase and earth circuit from the consumer unit to the end of the final circuit.

Ze is the external ELI from the main earth (at the board) back to the centre tap of the transformer and back along the phase to the main switch.

Zs = Ze + (R1+R2)

Zs = Total earth loop impedance. Ze is the external resistance of the circuit.
R1+R2 is the phase conductor and the earth cpc. Circuit protective conductor

Hope this helps.

[Knipex]

.... and I would also like to provide some explanation as to why EFLI is important.

Regulations require fuses/overcurrent protective devices to operate within a designated time period.

The time period that the device operates within is determined by the amount of Amps that flow during a fault.

The amount of Amps that flow during a fault are determined by the voltage and impedance of the circuit (Ohms law).

So in the case of a type B 32A breaker, it requires 5 X 32A to operate within 0.4S. This means the current required during a fault must be greater than 160A.

If you apply Ohms law then the maximum impedance for a circuit protected by a B32 breaker is 230/160= 1.44 Ohms.

Hope that helps