calculating pscc

[submariner111]

i'm currently studying for the 2391 exam and have been asked on a mock exam paper to calculate the pfc and pscc of an installation. Its a 3phase system supplying circuits of 400/230v 50hz with a Ze value of 0.2ohms.
I know the calculation for pfc = V/Ze but how do you work out the pscc. Its my understanding that you cant. Am i really wrong on this??????

 

[xraytek]

PSCC is measured between L+N whereas PFC is measured between L+E.
For 3 phase systems you dont measure between phases, you just double the single phase figure.
You then make sure the breaking capacity of MCB's is greater than that figure.

 

[submariner111]

cheers for the reply,
im still a bit unsure about how to calculate it without direct measurement. going off what the question said i take it the pfc=400/0.2=2000A (2KA) but what would the pscc be???
your help is appreciated.

 

[xraytek]

cheers for the reply,
im still a bit unsure about how to calculate it without direct measurement. going off what the question said i take it the pfc=400/0.2=2000A (2KA) but what would the pscc be???
your help is appreciated.

There is no way to calculate it with the infomation you have provided, unless you make assuptions.

If you assume TN-C-S then you could use the 0.2 Ohms figure to calculate.
Besides you dont calculate using 400V, you calculate at 230 then double it for 3 phase= 230/0.2*2= 2.3KA

(That what GN3 says if I remeber correctly)

Remember that you quote a higher Ze as possible to be safe, and a higher KA figure as possible too.
If calcs work out ok with the high Ze figure, then a lower reading will make the installation even safer.
Same with KA, if the beaking capacity of MCB is still higher than you calculate, then a lower reading is safer still.

Hope that helps

 

[submariner111]

yeah that has made it a lot clearer. Thanks a lot for your help

 

[zupos40]

PSCC is measured between L+N whereas PFC is measured between L+E.
For 3 phase systems you dont measure between phases, you just double the single phase figure.
You then make sure the breaking capacity of MCB's is greater than that figure.

Hope you do not mind but not quite correct.

PFC = prospective fault current and is the highest of either the PSCC (prospective short circuit current measure betwwen L & N) or Pefc (prospective earth fault current measured L to E at supply with all bonding in place and main earth reconnected)

I suspect your exam question assumes TNC-S then

Pefc = Uo/ze = 230/0.2 = 1150A single phase or multiply by 2 to aproximate 3 phase = 1150 x 2 = 23000A or 2.3 KA

and as TNCS Pscc = Pefc = PFC = 2300 or 2.3kA use you could be more accurate and use 1150 x 1.732 (but not worth it as meter are +/- 20% accuracy when measuring Ze or Pefc or Pscc so multiplying single phase result by 2 statisfactorary.

If it was TN-S not possible to calculate without knowing L - N impedence, really only way to find is by measurement.

 

[xraytek]

Hope you do not mind but not quite correct.

PFC = prospective fault current and is the highest of either the PSCC (prospective short circuit current measure betwwen L & N) or Pefc (prospective earth fault current measured L to E at supply with all bonding in place and main earth reconnected)

Ah yes I forgot that PFC and PEFC are different.
Thanks

 

[johnelectrical]

Hope you do not mind but also not quite correct.

The 1.73 x single phase PSCC, (not pfc) does as you say give the maximum short circuit current if the fault was accross two phases, but the reason GN3 says double it, is because if you had a fault of negligable resistance between all three phases and the neutral it could be approximately double the single phase PSCC

also I think even though you are correct about the PFC and PeFC, I have never come accross a tester which has the choice of PEFC or PSCC. it may be clearer for the younger electricians to make the difference but it is common practice to refer PeFC as PFC

John