pscc pfc values

[Farmelectrics]

just done eicr on a 3 phase db LI= 1.33KA L2=1.44 L3=1.48 THESE ARE THE PSCC VALUES SO ADDED TOGETHER THEY GIVE ME 4.25 KA THE LAST TEST DONE 5 YEARS AGO WAS 5.5KA NO WHAT WOULD CAUSE THIS TO BE LOWER NOTHING HAS CHANGED IN DB AND I NO THE READINGS ARE ACCEPTABLE YOUR THOUGHTS PLEASE

 

[timbobelfast]

Are you using the same test kit as last time?

Would PFC not fluctuate at different times of the day with network load demand?

 

[Farmelectrics]

no tim diferent tester ok take your point that could have an affect thanks

 

[JUD]

Aren't you just supposed to double (or more accurately x 1.73) the highest of the readings to get PSCC on 3 phase.

So in your case PSCC would be 2.96kA (double your L3 value). I don't think you're supposed to just add them all together.

That's what I was taught anyway.

 

[rich.250]

As above^^^^
Double highest L-N reading.

Or if your tester is capable you can test between phases.

 

[SirKit Breaker]

Jud has got it, and it also looks like the bloke who did the last test did it wrong as well.

Cheers...........Howard

 

[Farmelectrics]

Well I thought you double it my self had a quick look in gn3 not had time to read properly yet but all I did see was your pscc will always be higher than your pfc on a 3phase system anyone find we're it's says it's doubled let me no what page my gn3 is at work cheers

 

[JUD]

Page 52 last paragraph

 

[Farmelectrics]

Thanks wiil look up in morning

 

[Farmelectrics]

Sir kit I've recorded reading as L1=1.33 L2 =1.44 L3 =1.48 I've not added them together on the test sheet but the previous test sheet just has one value of 5.5 ka

 

[Farmelectrics]

Ok will check in morning in gn3 but jud said you double them values so will alter test sheets in morning

 

[Silly Sausage]

........................

Your readings are line to earth measurements.
Line to line short circuit fault current is higher due to the higher voltage difference (as you know!)
V line = root3 x V phase.........hence record the highest reading x root3

 

[Farmelectrics]

So archly maybe what your saying is the previous tester has got the higher reading and times it by 1.73 maybe that's we're he's got his 5.5 ka reading

 

[trebor]

The line to line pscc will be the highest, the rule of thumb as others have said is single phase pscc x 2, this should be recorded as in your example as 1.48 x 2 to avoid any doubt by office persons verifying test sheets

The previous result is pretty close to x2 x2 of your measurements so maybe he was using twu thumbs!

 

[Farmelectrics]

strange on one of the 3 phase boards we tested over the weekend the pfc was highier than the pscc what could cause this

 

[timbobelfast]

You mean PEFC?

PFC is the higher of PSSC and PEFC.

Was it a TN-S?

As TN-C-S should roughly be the same...

 

[trebor]

I have had this result also on a db , 3 from origin, after a bit of head scratching we decided it was parallel earth pats connected to to DB3 which were at a lower impedance at that point than origin due to the length of the sub mains, as we moved back to origin the pscc became the higer value

Also on TN-C-S pscc on line to neutral and pefc are much the same unless you isolate the installation and disconnect the earthing conductor, there is little point in this however as the whole idea is to know the worse case value

 

[telectrix]

sounds as though the previous guy may have read the phase/phase value then doubled it.

 

[Farmelectrics]

tim its a tns system and yes i ment my pefc was higher than pscc on a 3 phase board

 

[ackbarthestar]

tim its a tns system and yes i ment my pefc was higher than pscc on a 3 phase board

That would be Phase to neutral = psc and Phase to earth = pfc ?

If so, then your earthing system from the DB is running parallel back to the main intake. Very low R2 value.

You could try an R2 measurement with a R1 (L1,L2,L3) at the DB then add that to the probable Ze at the main board, if possible. Compare the measured values with the calculated values. Allow 30% for maximum operating uncertainties with the measured values.

Its not so uncommon.