Calculating Earth Loop Impedance

I believe this has been covered on CR4 before, but the easiest way is to google "calculating earth loop impedance" for the answer.

Hi jack, thanks for your response,

i have some sample calculation.

first, What is Earth Fault Loop Impedance?

when there is an electrical earth fault on a circuit current flows from the Line (phase) conductor through the fault, down the CPC (earth wire) along the earthing conductor provided by the electricity company for TNS and TNCS or through the earth electrode and planet earth for a TT system into the neutral point of the supply company transformer and through the transformer winding

The circuit (or loop) then is the Transformer winding (impedance) the supply company phase cable the phase cable in the installation and the return earth conductor (resistance)

This complete circuit is called the earth fault loop and the Earth fault loop impedance is the value in ohms measured at the furthest point in a circuit and depends on the length and size of cables and the impednace of the supply transformer. It is measured using an earth fault loop impedance tester. Once a measurement is taken its value can be compared with a maximum value for a given protective device (fuse/circuit breaker) in IEE wiring regs tables. If value is too high then the circuit is not safe.

Calculation Method

This would normally be carried out by the design engineer at design stages of the installation.

Formula

Z_s = Z_e + R₁ + R₂

Z_e = External Loop Impedance Reading

R₁ = Phase Conductor Resistance Reading

R₂ = Circuit Protective Conductor Resistance Reading.

Example :A shower circuit 30 m in length is wired in 6.00 mm²/2.5 mm². The circuit is to be protected by means of a BS1361 30A fuse. The external earth loop impedance reading is given as 0.15 W.

Z_s = Z_e + (R₁ + R₂)

6.00 mm² = 3.08 mW/m

2.5 mm² = 7.41 mW/m

Circuit length = 30 m

*Multiplying factor : 1.2 (Conductor temperature factor)

Z_e = 0.15 W

*Z_s = 0.15 + (10.49 x 30 x 1.2)

1000

Z_s = 0.15 + 0.38

Z_s = 0.52 W

* Max Z_s for BS1361 30A fuse = 1.92 W

if any other type of calculations would be appreciated.

thanks

kumaraguru

As impedance is a value measured in Ohms, the use of Watts to determine fuse size is quite abstruse.

Hi PW Slack,

the impedence value should be Ohms only not as watts.

thanks

kumaraguru

Hello kumaraguru,

Try these sites, they may be useful.

http://www.google.com/search?hl=en&client=safari&rls=en-us&ei=ImIlSv2WFt6TjAfI4szoBw&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=earth+loop+impedance&spell=1

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