Pfc

[nicsin02]

Hi guys would just like to understand PFC a little better, here goes....... I was at a job 2day and cut off had 400 fuses rated at 80ka, it then fed a section board with mccb rated at 25ka, it then went on to feed a d/b with mcb rated at 10ka, obviously I know that under a short circuit fault the 10ka is designed to blow 1st, so my question is when measuring the PFC at the d/b fed from the 25ka mccb I got 7.2ka x2 which gave me 14.4ka what does the result actually mean? Do I want this to be less than 25ka? but what if its not? If I had measured the PFC at the end of a lighting circuit rated at 10ka what is it I'm actually achieving by doing this test?

hope I understand this a little better soon and i don't sound like to much of an idiot but my lecturer never really dived into details about this

cheers guys in advance

 

[sparkymalarky]

Were you carrying out this inspection and test alone? What do you think this measurement means?

 

[nicsin02]

?.........

 

[ExArmy]

PFC is the potential fault current. you want it to be less than the rating on your MCB's if the PFC is 10kA and your MCB's are 6kA, then in the even of a fault (potentially, if the Zs is low enough) the MCB's will not be able to safely disconnect the line, and it will probably fly out of the board on fire in some sort of explosion (i imagine)
are you an apprentice?

 

[trev]

Yes he's an apprentice Exarmy, I remember him from one of his other posts.
Your PFC is always the higher of Prospective Earth Fault Current and Prospective Short Circuit Current and is the highest current that can flow in either of the two scenarios. We'll leave it as single phase just now to keep it simple. PEFC can be worked out by dividing the voltage with the earth fault loop impedance so if your EFLI is 0.1 the sum would be 230/0.1 = 2300 amps or 2.3 Ka.
Your MCBs need to be able to handle this safely so would need to be rated higher than this.

 

[Billythekid]

Hi guys would just like to understand PFC a little better, here goes....... I was at a job 2day and cut off had 400 fuses rated at 80ka, it then fed a section board with mccb rated at 25ka, it then went on to feed a d/b with mcb rated at 10ka, obviously I know that under a short circuit fault the 10ka is designed to blow 1st, so my question is when measuring the PFC at the d/b fed from the 25ka mccb I got 7.2ka x2 which gave me 14.4ka what does the result actually mean? Do I want this to be less than 25ka? but what if its not? If I had measured the PFC at the end of a lighting circuit rated at 10ka what is it I'm actually achieving by doing this test?

hope I understand this a little better soon and i don't sound like to much of an idiot but my lecturer never really dived into details about this

cheers guys in advance

The aim isn't to "blow" anything, the protective device should be installed to achieve a number of goals . . . . . Protection of life, property & cable . . . discriminate with both upstream and downstream devices . . . Whilst being suitable to disconnect the circuit should a fault occur. If a fault occurs it needs to also be capable of withstanding the fault current that'll be passed through it.

I don't mean to sound rude here, are you an electrician?

 

[Deleted member 26818]

You should also be aware that even though individual MCBs may only be rated at 3kA, 6kA, 10kA or even 15kA, with CUs type testing rates the whole assembly at 16kA if protected by a BS1361 or similar fuse.

 

[nicsin02]

Lol....

 

[trev]

Why Lol?

 

[plugsandsparks]

Where i find this pops up the most is when on a large industrial install, say with transformers on site, there will be the normal main dis board, but sometimes a local board is fitted to supply lights, sockets etc and some local machine needs in the sub itself
Now the local board is sometimes just a regular TP board with 10K breakers but because this is a local board to the MDB it can have very high PFC, meaning quite simply the breakers in the board could not reliably disconnect the worst case fault, i.e. it could not do its job .
Although its a simple test, the implications of very high PFCs can be quite costly to fix.

 

[nicsin02]

Why Lol?

U know

 

[ExArmy]

U know

something to do with people assuming your an electrician and don't know how to do your job?
this trainee forum is starting to look like a good idea!

 

[nicsin02]

Don't see the point in a forum wen u just get slated all the time

 

[kingeri]

Be a man about it....prove them wrong!

 

[trev]

Get on to Dan and grab a spot in the new trainees section, you'll be able to ask anything you like in there with a guarantee of no slating....well maybe just a little bit