Pscc / Pfc confusion

[eddieboy]

Hi all. Can someone please explain the difference between the two. Also if I know the Ze on a single phase installation is it a simple Ohm's law calculation to find out the Pscc?

 

[Guest123]

Prospective Fault Current (PFC) is the highest value of the two readings of Prospective Earth Fault Current (PEFC) and Prospective Short Circuit Current (PSCC)

PEFC is measured between L&E, PSCC is measured between L&N. On a 3P install PSCC is doubled (as a rule of thumb) as the highest fault current will always be between the phases, unless you test meter can measure between phases.

 

[BillyBoBob]

PFC is your Prospective Fault Current.
It made up from the higher readings of two tests.
PSCC = Prospective Short Circuit Current. Tested between LIVE conductors. L1,L2,L3 & N
PEFC = Prospective Earth Fault Current. Tested between Line conductors to Earth. L1-E, L2-E, L3-E
On single phase installations PEFC is usually higher.
On three phase installations PSCC is usually higher because it's at 400v rather than 230v.
PFC should be tested because all parallel paths for earth are taken into account. When testing Ze you remove all parallel paths.
Your trying to find the maximum fault current that can flow within you installation.
Ohms Law. Less resistance more current.

 

[eddieboy]

So should I test between phase and neutral, phase and earth then record the higher figure on the test sheet?

 

[Guest123]

So should I test between phase and neutral, phase and earth then record the higher figure on the test sheet?

Yes.

As an extra a TN-C-S system should have identical values for both as it's a PEN conductor.

 

[ezzzekiel]

which means the neutral and earth share a common cable from sub station upto the cutout, then seperates into neutral and earth from there.

 

[BillyBoBob]

As an extra a TN-C-S system should have identical values for both as it's a PEN conductor.

In therory yes, However; with both main and supplementry bonding in place on Water, Gas, Oil, structral steel, etc, there will be additional parallel paths for currents to flow down.
Essentially we create (with bonding) a larger Earth back to the star point of the transformer.

 

[andyb]

PFC is your Prospective Fault Current.
It made up from the higher readings of two tests.
PSCC = Prospective Short Circuit Current. Tested between LIVE conductors. L1,L2,L3 & N
PEFC = Prospective Earth Fault Current. Tested between Line conductors to Earth. L1-E, L2-E, L3-E
On single phase installations PEFC is usually higher.
On three phase installations PSCC is usually higher because it's at 400v rather than 230v.
PFC should be tested because all parallel paths for earth are taken into account. When testing Ze you remove all parallel paths.
Your trying to find the maximum fault current that can flow within you installation.
Ohms Law. Less resistance more current.

Not really, the pscc is usually higher, although as Lenny points out, on a tncs system they should be about the same.

 

[BillyBoBob]

Not really, the pscc is usually higher, although as Lenny points out, on a tncs system they should be about the same.

Can you explain why?

 

[andyb]

With tncs, the earth & the neutral are the same conductors so you would expect the same reading.But as its name sugests, with a tns system the neutral & earth are separate and so would have a diferent resistance.Usually (not allways) the earh has a greater resistance than the neutral and so the current flowing to earth in an earth fault would be less than that flowing through the neutral in an l/n fault.

 

[pushrod]

I would think the comparison between pscc and pefc on a single phase depends very much on the earthing arrangement. For the testing of a pfc bonding should also be in place, unlike when testing Ze.

With a TT the pscc will almost certainly be larger due to the high earth electrode resistance reducing the pefc.

With a tncs as the neutral is effectively the same conductor as the earth you would expect similar results,and because they have the same parallel paths

With a tns the earth to the transformer is separate from the neutral and will probs have a larger resistance than the neutral, so making the pscc more likely to be the larger. However, the parallel earths available to a pefc will complicate matters and so could bring it to a higher value. Making the case to test both a stronger one.

Edit must improve my proof reading !!

 

[Risteard]

Parallel paths will cause the current to increase not decrease as there is a lower impedance.

 

[pushrod]

Parallel paths will cause the current to increase not decrease as there is a lower impedance.

Doh and i read thru that twice - was trying to keep it short rather show the calc - cheers Risteard

 

[Risteard]

No probs. I thought I'd read it wrong at first!

Don't worry about it - we all know what happened to the last person who never made a mistake...

 

[andyb]

With a tncs as the neutral is effectively the same conductor as the earth you would expect similar results, but because there are parallel paths for earth then the pefc will be higher (due to lower impedance) than the pscc (just as a tested Zs is usually lower than a calculated one)

The parallel paths are there for the pscc as well as the pefc, so you are reading the same thing both times.
So the results would be similar.