Ipsc and Ipfc at sub distribution board

[filo]

Hi,
Hope someone can help.

I need to test the prospective fault current at a garage distribution board (i.e. a sub board) but the no trip function of my tester will not work and trips the RCD which serves the garage circuit in the main board.

As I understand it, the figure to take for prospective fault current is the highest out of Ipfc and Ipsc.

So if the highest at the main board happens to be Ipfc then I can easily calculate Ipfc at the garage board by adding R1+R2 of the distribution circuit to the Z given by the meter when testing Ipfc (not sure what this is called but it's like Ze but with the main bonding conductors connected!) then dividing it into 230.

Here's the question:
If the highest figure at the main board happens to be Ipsc then should I be calculating the Ipsc at the garage board using R1+Rn of the distribution circuit rather than R1+R2?

Any clues would be greatly appreciated.
Thanks

 

[trebor]

measure R1 + R2 at the sub board and calculete in ohms and convert to amps and subtract from pfc at source, as a point pfc is not required at db`s if the protective devices ka are equal or higher than the sorce cb`s or fuses as long as the source cb/ fuses can withstand the fault current

 

[filo]

Surely Ipsc is the maximum current that can flow as the result of a L-N fault so it's the L+ N impedance that's important not the R1+ R2
??

 

[trebor]

sorry I should have R1 -Rn as well. it the highest of the 2 that is important and it is not always beteween lives

 

[filo]

Thanks, I think I know what I'm doing now!

 

[Chr!s]

Hi,
Hope someone can help.

I need to test the prospective fault current at a garage distribution board (i.e. a sub board) but the no trip function of my tester will not work and trips the RCD which serves the garage circuit in the main board.

As I understand it, the figure to take for prospective fault current is the highest out of Ipfc and Ipsc.

So if the highest at the main board happens to be Ipfc then I can easily calculate Ipfc at the garage board by adding R1+R2 of the distribution circuit to the Z given by the meter when testing Ipfc (not sure what this is called but it's like Ze but with the main bonding conductors connected!) then dividing it into 230.

Here's the question:
If the highest figure at the main board happens to be Ipsc then should I be calculating the Ipsc at the garage board using R1+Rn of the distribution circuit rather than R1+R2?

Any clues would be greatly appreciated.
Thanks

PEFC and PSSC may be measured and then calculated in the way you describe, is this a single phase system?

You need to obtain your Ze(PEFC) at the origin and your Zl(PSSC). Then you need to measure the resistance upto the required point, this will include all live conductors and and circuit protective conductors. Then you can add the resistance of the live loop(Zl) for PSSC and the earth loop(Zs) for your PEFC.

Once you have your resistance then you can calculate your fault current.

If the circuits are greater than 100 amp or utilizing steel armour(PEFC) you may need to consider reactance.

Regards Chris

 

[filo]

Yes, this is single phase & <100A.

Thanks very much, this confirms what I thought - I couldn't find a proper explanation in any of my books on testing and inspection.