pfc testing

[bobby101]

can any body give me the reason why when testing pfc and pefc at a three phas DB why do you have to test all phases to neutral then all phases to earth why not just one phase surely there will be very similar results if not the same just thought iwould ask cause it was buggin me..

 

[Risteard]

can any body give me the reason why when testing pfc and pefc at a three phas DB why do you have to test all phases to neutral then all phases to earth why not just one phase surely there will be very similar results if not the same just thought iwould ask cause it was buggin me..

I don't usually find that they are identical.

Therefore you need to know which one is greatest.

There's not really any need to measure PEFC on a three phase DB. Either double highest PSCC of each phase to neutral or if your meter allows it then test directly between phases.

 

[pushrod]

There's not really any need to measure PEFC on a three phase DB. Either double highest PSCC of each phase to neutral or if your meter allows it then test directly between phases.

can you explain that please, surely a pefc has a completely different route to a pscc and so could be completely different?

 

[tigerpaul]

They may not all be the same.

A loose connection further upstream would cause a higher resistance, therefore a lower prospective fault current. You have to check all three phases to find the highest.

 

[wallyanker]

There's not really any need to measure PEFC on a three phase DB. Either double highest PSCC of each phase to neutral or if your meter allows it then test directly between phases.

I was taught that you must carry out pefc and pscc and the highest reading becomes your pfc. On a three phase board, do a pefc and pscc on each phase, the highest one double and that is your pfc.

Can someone confirm I was taught correctly?
Thanks

 

[WayneL]

can you explain that please, surely a pefc has a completely different route to a pscc and so could be completely different?

The reason is that there isn't a cat-in-hells chance of the PEFC being bigger than the PSCC on a three phase board.

PSCC on three phase is at 400v.

As its the bigger figure that you are after, there's no need to measure PEFC.

 

[brucelee]

hi wayne
so for example if explaining test procedure for 3phase would you not have to include testing pefc as well as pscc

and for practical exam would i just measure the pscc between all lines and neutral multiply the highest line to neutral by 2

 

[pushrod]

The reason is that there isn't a cat-in-hells chance of the PEFC being bigger than the PSCC on a three phase board.

PSCC on three phase is at 400v.

As its the bigger figure that you are after, there's no need to measure PEFC.

cheers - that makes perfect sense

 

[WayneL]

hi wayne
so for example if explaining test procedure for 3phase would you not have to include testing pefc as well as pscc

and for practical exam would i just measure the pscc between all lines and neutral multiply the highest line to neutral by 2

To be honest, mate, we were told to measure PEFC between all Lines and Earth and PSCC between all Lines and neutral and then double the highest reading to give your PFC.

If you are doing the 'doubling' method I suppose this makes sense.

If you are measuring the actual PSCC (between Lines), then measure L1-L2, L1-L3, L2-L3 - the highest reading being your PFC.

Some meters can do this, some can't - so please check
In this case there's no need to measure your PEFC as I said earlier, it's impossible for it to be larger, as the voltage to earth is only 230V.

 

[brucelee]

thanks mate
reason i am asking got my 2391 practical on tuesday and an ma little unsure if i need to test the three phase board for pefc and pscc i know the highest is between lines and to find it you double the highest line to neutral
just done written exam

what a joke that is most of the questions we have not covered at college
i have been studying everyday all day for 4 months and half of the questions they ask are not in GN3 ,bs7671 or any of the many books i have so were i an supposed to get the answers from i have no idea