# Prospective fault current 3 phase

Discussion in 'Electricians' Talk' started by madmaz, Mar 19, 2006.

Anyone know the formula to calculate the PSCC at the origin of a three phase supply ??

2. ### sinewaveScrewfix Select

It'll be the highest figure you can measure, which will usually be between two of the phases.

3. ### steve CFNew Member

if your meter is only single phase meter then test between

phase and neutral

phase and earth

which ever is the highest times it by 2 to give the total

never connect your single phase meter across two phase or you will damage it

i understand that i want to calculate it with numbers and not measure it directly i know for single phase it is supply voltage divided by the phase - neutral loop impedance, so what is it for three phase don't think is is 415/Zpn is it must be a correction factor ??

5. ### sinewaveScrewfix Select

Sorry steve didn't see your post so once i calculate it i multiply by 2 for three phase, thanks.

7. ### steve CFNew Member

can you not use 230/ loop impedance between phase and neutral X 2

yes that is what i will do just wanted to know the formula i know where the x2 comes from now it is the approximate sq root of 3 thanks for that mate.

9. ### Lokkars DaisyNew Member

Ah, forgot to mention that bit Steve! :O

I'm so use to 3 phase testing kit that you forget
that there is still kit out there that'll go bang if
you connect it to 400 volts! :O

Never mind
you could always get it fixed here!

:^O :-|

11. ### state-itNew Member

You'd think it was the square root of 3, but it is x2. Forgotten exact reason. The question comes up on 2391 paper every now & then. Can't remember exact reason, but it is 2x. Have to dust the old notes down. Or even try to remeber where they might be.

12. ### eawr89New Member

Assuming that the phase and neutral conductors are of the same impedance (Z), the approximate single phase fault current is given by 240/2Z. The approximate three phase fault current is given 240/Z. Therefore to determine the three phase fault current from a single phase measurement we take the ratio of these two i.e. 240/Z divided by 240/2Z which equals 2. Apologies for using the &#147;old&#148; voltage levels!

13. ### LectricianScrewfix Select

Where does the 240v / 2Z come from??

For single phase, this doesn't fit??? Or is it me???

14. ### eawr89New Member

Hello
Phase conductor impedance Z plus neutral conductor impedance Z equals a loop of 2Z.
The reasoning behind using just conductor impedance is to eliminate the complication of what to do with the &#147;source&#148; impedance. You can therefore assume this to be equal to zero or that it is included within the impedance of the conductors. Either way it is the ratio that we are after. As a further example, if we know the phase to phase impedance then again the three phase impedance can be determined through ratios but this time 240/Z divided by 415/2Z which is approximately 1.154 (2 divided by the square root of 3).

15. ### LectricianScrewfix Select

Ahh - OK. When you were saying Z......I was assuming P N type loop.