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

 

[Dcf]

There really are a lot of ******* on this forum,

This like of this post is one of the reasons I've been lurking on this forum recently instead of posting.
son you've had pretty comprehensive and concise explanations of pfc from some of the lads on here. If you don't understand it after that away n stack shelves in tesco.
the trainee only section does indeed sound like a good idea indeed.
there used to be a lot of good lads on here giving good advice and discussions that don't bother coming on anymore.
probably down to juvenile Pish like this.

 

[nicsin02]

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

Would love to work with u m8, never worked with a perfect spark before, I kneel before thee!

 

[nicsin02]

This like of this post is one of the reasons I've been lurking on this forum recently instead of posting.
son you've had pretty comprehensive and concise explanations of pfc from some of the lads on here. If you don't understand it after that away n stack shelves in tesco.
the trainee only section does indeed sound like a good idea indeed.
there used to be a lot of good lads on here giving good advice and discussions that don't bother coming on anymore.
probably down to juvenile Pish like this.

some of the advice was good any I always say thanks to ppl but I don't appreciate advice then a cheeky remark at the same time, and for the record I do understand it, I only have to be to.d something once, getting to the stage were I'm too scared too post anything without some1 quoting apprentice this apprentice that, some ppl are just very sad

 

[thekingiam]

i think the answers on here are justified, we often get alot of questions on here by electricians who should know the answer if they call themselves electricians, this might not be that persons fault as it should of been explained better while undergoing training.
know one should feel ashamed or slated on here for asking questions whatever level of training you are, i am also sorry that you felt it necessary to slate the people that go out of there way to solve other peoples problems.
ps the mods don't apretiate swearing on this forum

 

[trev]

Would love to work with u m8, never worked with a perfect spark before, I kneel before thee!

I'm far from perfect mate, there are lots of things that, because I'm qualified, I should be able to do without any real problem. However, because I have little or no experience of them I will not go near them unless I get someone who knows exactly what they are doing to work with me. Every day is a school day in this trade, that's what keeps it interesting for me
The bit about "well maybe only a little bit" was intended as a little humour

 

[nicsin02]

I'm far from perfect mate, there are lots of things that, because I'm qualified, I should be able to do without any real problem. However, because I have little or no experience of them I will not go near them unless I get someone who knows exactly what they are doing to work with me. Every day is a school day in this trade, that's what keeps it interesting for me
The bit about "well maybe only a little bit" was intended as a little humour

U stated earlier u knew I was an apprentice wen I clearly ain't , just think there's a lot of sad old men on here with 40+ years experience and the just love answering questions to less experienced sparks in a real condasending manner, by no mean calling you old before u jump on that, I can inspect and test quite comfortably, I'm only 22 maybe wen I get to your age I will know as much as you

 

[Marnik]

Would love to work with u m8, never worked with a perfect spark before, I kneel before thee!

I've been in the trade for 40 years, and the day i stop learning is the day they carry me away in a wooden box!!!
You learn something new every day, and if you don't , then you are either fooling yourself or are extremely big-headed and IGNORANT

 

[trev]

U stated earlier u knew I was an apprentice wen I clearly ain't , just think there's a lot of sad old men on here with 40+ years experience and the just love answering questions to less experienced sparks in a real condasending manner, by no mean calling you old before u jump on that, I can inspect and test quite comfortably, I'm only 22 maybe wen I get to your age I will know as much as you

I thought I remembered you saying a few weeks back that you were an apprentice, obviously I'm mistaken. Although it is not clear that you ain't.
I freely admit that I'm no spring chicken so the possible lumping me in amongst the old men doesn't offend me. I tried in my explanation of PFC not to put it over in a condescending manner but to put it over as simply as I could. If that came over as condescending well perhaps I need to consider my approach a little more carefully but as it has served me well for many years I don't see that I have a problem.

 

[Octopus]

There really are a lot of ----ers on this forum,

Keep up this attitude, and you'll soon find yourself an Ex member of this forum

Rather than the crass response, why couldn't you use the "Thanks" button to show some appreciation of the time people have spent extending your knowledge and experience??

 

[nicsin02]

I always say thanks

 

[Dipzy]

22 year old.... says it all

 

[Marnik]

I always say thanks

Just not in a polite manner.
Don't the teach to have respect for your older and experienced people these days?
At 22 , i would have been clipped over the earhole for comments like that.

 

[morph]

Just not in a polite manner.
Don't the teach to have respect for your older and experienced people these days?
At 22 , i would have been clipped over the earhole for comments like that.

and now you would get arrested for it, its all wrong these days ...you cant beat the youngens anymore

 

[Dipzy]

i have a couple around that age, just better not to say anything anymore.... as they clearly think they know better and we should be grateful of being in their company.

 

[thekingiam]

i wonder why you have never asked your fellow sparks you work with that question?
would it be that you would expect a good tounge lashing for not knowing a simplistic question?
i think the patience and politeness some of the members have given you on here is admirable to them,i hope you stay on the forum as it worries me what else you have not been trained in and been afraid to ask. i also hope that you could learn some restraint in thinking we are all on here to ridicule and offend you.people only asked if you where an electrician or apprentice and you over reacted,get over it move on and you will get more from this forum.to say the people on here are old and just like showing off there knowledge is wrong.

 

[Carnegstar]

I think its sometimes difficult to recall experience from training when you need it most.
Like the lad said his lecturer didn`t dive into it the subject well and he wants to understand it more, thats why I joined this forum.
So when im asked a question by my apprentice i can atleast have some confidence about what i am explaining is correct.

 

[shagbite]

jeez, fellas; the lad only asked for a bit of clarification on pfc. i'm sure he could have referenced one of his old college text books for the answer, but people join these forums for a bit of social interaction and to pick the brains of knowledgeable folk like yourselves - sometimes it's better to get an answer in layman's terms. you can argue that he should know the answer, being a practising electrician, but if we all knew it all, then the iet wouldn't make any money from flogging the on-site-guide, which i'm pretty sure we've all used in the past. it's a long while since i was 22, but i'm pretty sure that when i was, i'd have come out swinging as well, if i felt i was being ridiculed. having said that, i also think it's difficult to read people when you're having a conversation through a keyboard (emoticons aside), and it's easy to take offence when non was intended.

 

[Octopus]

jeez, fellas; the lad only asked for a bit of clarification on pfc. i'm sure he could have referenced one of his old college text books for the answer, but people join these forums for a bit of social interaction and to pick the brains of knowledgeable folk like yourselves - sometimes it's better to get an answer in layman's terms. you can argue that he should know the answer, being a practising electrician, but if we all knew it all, then the iet wouldn't make any money from flogging the on-site-guide, which i'm pretty sure we've all used in the past. it's a long while since i was 22, but i'm pretty sure that when i was, i'd have come out swinging as well, if i felt i was being ridiculed. having said that, i also think it's difficult to read people when you're having a conversation through a keyboard (emoticons aside), and it's easy to take offence when non was intended.

It was going OK till he used the txxxxxx word and declined to use the "thanks" button.

What ever happened to "pleases" and "thank yous" - this may be a forum but theres no excuse for being rude.

 

[shagbite]

It was going OK till he used the txxxxxx word and declined to use the "thanks" button.

What ever happened to "pleases" and "thankyous" - this may be a forum but theres no excuse for being rude.

i take your point, but i feel the thread was getting a bit tetchy before the lad started swearing. also, i believe he thanked everyone in advance in his original question.

 

[Octopus]

i take your point, but i feel the thread was getting a bit tetchy before the lad started swearing. also, i believe he thanked everyone in advance in his original question.

He put "cheers guys in advance" in the OP .... which is quite different to using the "thanks" button at the bottom left of each response that you like and or appreciate!!

 

[shagbite]

He put "cheers guys in advance" in the OP .... which is quite different to using the "thanks" button at the bottom left of each response that you like and or appreciate!!

maybe he didn't "like and or appreciate" any of the posts. having looked at the all the people who've posted on this thread, non of them have a very good 'thanks to thanked' ratio.

 

[Octopus]

maybe he didn't "like and or appreciate" any of the posts. having looked at the all the people who've posted on this thread, non of them have a very good 'thanks to thanked' ratio.

So that means they aren't worth thanking??

Pleases and thank yous cost nothing.

 

[shagbite]

So that means they aren't worth thanking??

Pleases and thank yous cost nothing.

that's true. but if the lad thought he was being picked on, i'm sure thanking them was the last thing on his mind.

 

[Chr!s]

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

Well what you seem to be asking is more to do with Discrimination and Back up/ Cascading protection.

You need to be careful in both instances, best to follow type tested where possible.

If you have Discrimination between protective devices then the upstream device will see the full PFC, the device itself may limit the let through dependent upon fault level.

So lets assume we have Discrimination between the 400 kA and the 25 kA.
You measure 14 Ka downstream of the 10 kA MCB, so we either fit a MCB with a Breaking capacity >14 or we look to see if we can rely upon the 25 kA MCCB to provide back up protection.

HTH

 

[Octopus]

that's true. but if the lad thought he was being picked on, i'm sure thanking them was the last thing on his mind.

Posts 4, 5, 6 & 7 are, in my humble opinion, worthy of "thanks"