Earth Fault Impedance

[m00se75]

Can some one help sharing an example on how Earth Fault impedance can be calculated for an underfloor busbar.

After testing the earth fault impedance for the circuit it is showing higher impedance than the max value.

Thanks

 

[ezzzekiel]

ze+R1+R2 IF YOU CAN GET R VALUES

 

[IQ Electrical]

Can some one help sharing an example on how Earth Fault impedance can be calculated for an underfloor busbar.

After testing the earth fault impedance for the circuit it is showing higher impedance than the max value.

Thanks

Don't forget that if the usual methods (supplementary bonding, rcd protection etc.) aren't practical, you can use manufacturers maximum Zs values which usually have a better max value X80% for rule of thumb.
Just make a note on your EIC.

(R1+R2) values on busbar systems are normally comparably small due to the CSA of the conductors-are you starting off with a high Zs before the busbar or using type 'D' MCB's?

 

[m00se75]

Thank you for the response...breaker (MCB) is Type "C", length of run is 28.8m, earth size cable is 10mm2, circuit breaker is 32A, and on the test meter I am getting 0.86 ohm of Zs which is greater than the max impedance..

 

[Deleted member 9648]

Can you change to a type B?.....max 1.15(off top of head).....or if RCD'd 1667 becomes the max.......based on the info given though, as Mr IQ states the Zs would perhaps be a little high at the origin of the circuit.

 

[ridemx27]

From the size of conductors and length you have stated i would of said that should be well within max zs, i think u may have issues with main earth have u taken a ze reding yet, or ra if its tt ? also on ur meter are you testing in high current mode or non trip ? if you have that facillity anyway ?

 

[IQ Electrical]

From the size of conductors and length you have stated i would of said that should be well within max zs, i think u may have issues with main earth have u taken a ze reding yet, or ra if its tt ? also on ur meter are you testing in high current mode or non trip ? if you have that facillity anyway ?

I doubt main earth issues, this could be a sub-circuit from a distribution circuit, from a section board on a TN-S system with a max Ze of 0.8 Ohms, we're only talking 0.86 Ohms at the end of all that!

 

[ridemx27]

could well be but without more info were all left guessing i suppose

 

[m00se75]

Thanks for all replies...

This is one of 4 circuits, others are measuring around 0.93, can someone post a sample of calculation on how it should be measured.

 

[BlueToBits]

Your question is not making sense to me. You don't need to calculate it if you are measuring it. If your measurement it is too high to give a satisfactory disconnection time against the tabulated rating for your protective device, then you should change either the bonding arrangements or use a different device. The only calculating you may need to do is adding or subtracting the loop tests at different points along the circuit to establish if and where best to introduce additional bonding if that is your chosen method of addressing the problem. You may wish or need to use R1+R2 for some of these measurements, but, at the end of the day, it is the Zs at the furthest point of utilisation that needs to be satisfactory.
If it isn't making sense you could call in an electrician for an hour or so and watch, ask and learn for less than £40.