Is there any way of getting the PEFC if you only have the Ze?

[jaresquire]

PEFC should be measured as it includes the multiple earth paths via the main bonding etc in addition to the main earth. Thus the PEFC will likely be higher than that calculated using Ze

 

[Risteard]

Strictly speaking the PEFC at the origin will be Uo/Zs rather than Uo/Ze as parallel paths are likely to increase the prospective earth fault current.

 

[reefagrim]

just to add the answers given above are correct , the main reason for you to calculate the pfc ( v=ir) is to then show you within what range your measured test should read . in a ideal world you perform the pfc just after the ze test. I believe that you might have misunderstood what the pfc /pefc/ pscc means but its more to do with the time /current characteristics of the protective device . it doesnt matter how many amps your allowing ( decided by the supplier) in to the installation as long as they are higher than the load and the protective device has a breaking capacity that will perform in time ! thus the pfc you calculating is showing you its under the 16k etc breaking capacity needed to blow the protective device.
page 71 in the on site guide will give you the capacity of the device if the device doesnt have the pfc/pscc/pefc or whatever they call it now written on it

 

[trev]

KISS should be the motto in 2391.

I disagree, 2391 is supposed to be tough, it's supposed to make you think. If it was easy it wouldn't be worth having

 

[Ashley2]

I disagree, 2391 is supposed to be tough, it's supposed to make you think. If it was easy it wouldn't be worth having

I don't mean that, KISS, is basically to ensure that you focus on the issue, & in this instance the issue was Ohms law. But in an examination environment your are always looking for that one issue that 'they' might be trying to trip you on. In relation to KISS, I believe that it is relevant in all areas, & not just within this context. KISS focuses on that which is directly relevant, & in this case, Ohms law was directly relevant.

 

[trev]

I actually think it's good that they put in information that is irrelevant to the question. It means you have to read and think "What is it they're wanting here, what information is important and what is not" Just like you would in the real world.

 

[MarkieSparkie]

measurement is better than calculation lol

Not always!
Consider the situation where supply transformer is close to the domestic installation intake and Ze is likely very low. The best BS EN 61557 EFLI testers are typically +-5% accurate +-3 least significant digits (lsd), so when Ze (or Zs) is less than say 0.1Ω the least significant digits become the increasingly significant error term as Z reduces.
Few, if any, EFLI testers actually measure the fault current, they in fact use the ‘measured impedance' as the divisor in a fault current calculation in which the measured voltage (or 230V nominal voltage, depending on the design of the tester) is the dividend and then it just displays the result in A or kA as appropriate. So it is easy to see that if the divisor is a very small number with a significant error, this can result in a very large current with a significant error.
To illustrate the point:
So if the actual Ze =0.05Ω,TN-C-S, then the PFC =230V/0.05Ω =4.6kA
However, our ELFI tester with a resolution of 0.01Ω, may be indicating a wildly different reading…
At increasing low ‘impedance’ values the +-5% tolerance becomes less important to the point where it becomes a second order term and we can effectively ignore it, however the +-3 lsd becomes increasingly dominant.
So, if the EFLI tester reads -3 lsd, Ze becomes approximately 0.05 – 0.03 Ω =0.02Ω, therefore PFC would read 230V/0.02Ω =11.5kA
So, if the EFLI tester reads +3 lsd, Ze becomes approximately 0.05 + 0.03 Ω =0.08Ω, therefore PFC would read 230V/0.08Ω =2.88kA
… do you still have blind faith in your beguiling digital EFFI tester, I hope not.
This is before we further complicate the issue with large CSA line conductors and their significant reactive components and the very low impedances we might expect in some commercial and most industrial installations, where the testing methods often have to be significantly different and more complex.

 

[Guitarist]

Ashley, do you not believe what my learned friends are telling you? I am shocked that so many people have had to say the same thing....

 

[rich.250]

I actually think it's good that they put in information that is irrelevant to the question. It means you have to read and think "What is it they're wanting here, what information is important and what is not" Just like you would in the real world.

Agree ^^

 

[Ashley2]

do you not believe what my learned friends are telling you? I am shocked that so many people have had to say the same thing....

With all due respects, I have found the responses highly interesting, however, the one you posted, as per above does not do anything for me. In terms of interesting, take for instance the one copied below, from:

MarkieSparkie

"Not always!
Consider the situation where supply transformer is close to the domestic installation intake and Ze is likely very low. The best BS EN 61557 EFLI testers are typically +-5% accurate +-3 least significant digits (lsd), so when Ze (or Zs) is less than say 0.1Ω the least significant digits become the increasingly significant error term as Z reduces.
Few, if any, EFLI testers actually measure the fault current, they in fact use the ‘measured impedance' as the divisor in a fault current calculation in which the measured voltage (or 230V nominal voltage, depending on the design of the tester) is the dividend and then it just displays the result in A or kA as appropriate. So it is easy to see that if the divisor is a very small number with a significant error, this can result in a very large current with a significant error.
To illustrate the point:
So if the actual Ze =0.05Ω,TN-C-S, then the PFC =230V/0.05Ω =4.6kA
However, our ELFI tester with a resolution of 0.01Ω, may be indicating a wildly different reading…
At increasing low ‘impedance’ values the +-5% tolerance becomes less important to the point where it becomes a second order term and we can effectively ignore it, however the +-3 lsd becomes increasingly dominant.
So, if the EFLI tester reads -3 lsd, Ze becomes approximately 0.05 – 0.03 Ω =0.02Ω, therefore PFC would read 230V/0.02Ω =11.5kA
So, if the EFLI tester reads +3 lsd, Ze becomes approximately 0.05 + 0.03 Ω =0.08Ω, therefore PFC would read 230V/0.08Ω =2.88kA
… do you still have blind faith in your beguiling digital EFFI tester, I hope not.
This is before we further complicate the issue with large CSA line conductors and their significant reactive components and the very low impedances we might expect in some commercial and most industrial installations, where the testing methods often have to be significantly different and more complex".

 

[Guitarist]

With all due respects, I have found the responses highly interesting, however, the one you posted, as per above does not do anything for me.

With all due respect to you, I wasn't intending to "do anything for you".
I just think that after all the excellent advice you have been given, a few "thanks" may have been in order to my learned friends rather than disagreement.
I hope that you have now got the hang of the difference between what a fuse can safely carry, and what it will "blow" at.

 

[1shortcircuit]

Ashley, AGAIN, with all due respect. Looking at previous threads you have created and posts proving that you are getting the basic things like Ze and Zs mixed up I don't think posts like MarkieSparkie's would do you any good.

You need to learn to walk before you can run. Master the understanding of the basics and then perhaps move on to more informative posts such as MarkieSparkie's:thumbsup

Good Luck

 

[Ashley2]

Suffice to say that Guitarist has 1 Friend mentioned in his Profile, & I do not have to go a million miles to see why 1shortcircuit came in straight away to criticise me after Guitarist. The concept of cyber bullying is not a new phenomenon.

This is a forum, the purpose of which is to promote critical thinking, meaningful problem solving & knowledge construction, take your cyber bullying elsewhere. I will decide what will do me good, it is not for anyone to make that decision for me.

I have found the responses from other members highly interesting & I found the one posted by Guitarist, did not do anything for me. That response correlated with some of the others that Guitarist has posted in other threads, in that they tend to have a low interest value, & tend to rant, the others did not. There was a world of difference between what MarkieSparkie wrote & the rant that came from Guitarist.

 

[Jurasic Spark]

"This is a forum, the purpose of which is to promote critical thinking, meaningful problem solving & knowledge construction,"

2391 is not supposed to be a give away qualification, but it is not rocket science. Most of it revolves around Ohm's Law and knowing how and when to apply it.

I think you are getting confused with:

In, Icn and Ics.

Maybe a browse through Part 2 of BS7671 and the chapter on PFC in GN3 might clear things up.

​

 

[trev]

Suffice to say that Guitarist has 1 Friend mentioned in his Profile, & I do not have to go a million miles to see why 1shortcircuit came in straight away to criticise me after Guitarist. The concept of cyber bullying is not a new phenomenon.

This is a forum, the purpose of which is to promote critical thinking, meaningful problem solving & knowledge construction, take your cyber bullying elsewhere. I will decide what will do me good, it is not for anyone to make that decision for me.

I have found the responses from other members highly interesting & I found the one posted by Guitarist, did not do anything for me. That response correlated with some of the others that Guitarist has posted in other threads, in that they tend to have a low interest value, & tend to rant, the others did not. There was a world of difference between what MarkieSparkie wrote & the rant that came from Guitarist.

Ashley, Guitarist is among the nicest, well mannered and clued up posters on here. If you think he's bullying you then you have very thin skin, wait till you annoy some of the guys here. Then you'll know mate