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

Discuss Is there any way of getting the PEFC if you only have the Ze? in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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#### Ashley2

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

#### D Skelton

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(Post edited after reading OP's past posts)

Ohms law.

The Ze is the external earth fault loop impedance (resistance for simplicities sake)

Divide the voltage (230V) by the Ze and you will get the potential current flow (prospective earth fault current)

I=V/R

Someone may correct me here but I believe that the PFC should always be measured, not calculated.

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#### Ashley2

I would have thought that the PFC is always measured (Taking the higher of the PEFC/PSCC), & then a calculation to ensure that the protective device is able to handle the PFC.

Some of the information given was as follows:
The Ze given was 0.8Ω
The voltage given was 230 V a.c.
The protective device was 100A.

I am trying to remember the above. I am positive about the voltage & protective device, & about 90% positive about the Ze figure.

I initially thought it was Ohms law, but the figures did not tally with that, i.e. PEFC should be less than the 100A. This is why I asked whether there was a way of calculating the PEFC with the Ze given.

#### telectrix

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it is ohm's law I = V/R, so in the case you mention. I = 230/0.8 = 287.5A. what makes you think it should be <100A. the rating of the fuse has nothing to do with it.

#### scotsparky

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It all depends on the earthing system

In a TN-C-S the PEFC and FSCC current will be the same
In a TN-S and TT system they will be different ats the Neutral and Main earth are taking different routes

Always just measure and I put down the lowest result taken as thats the worst case

#### D Skelton

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I would have thought that the PFC is always measured (Taking the higher of the PEFC/PSCC), & then a calculation to ensure that the protective device is able to handle the PFC. This is correct

Some of the information given was as follows: Information to what?
The Ze given was 0.8Ω
The voltage given was 230 V a.c.
The protective device was 100A.
PEFC should be less than the 100A. This is why I asked whether there was a way of calculating the PEFC with the Ze given.
On a TN system, PEFC will never be lower than 100A. The lowest possible PEFC on a TN system is on a TN-S with a value of 287.5A

On a TT system PEFC will more than likely be lower than about 2A and between 2 and 10A if it's a really good TT system (domestic)

I must admit, I am slightly confused. Are we on the same wavelength?

Edit:
it is ohm's law I = V/R, so in the case you mention. I = 230/0.8 = 287.5A. what makes you think it should be <100A. the rating of the fuse has nothing to do with it.
Beat me to it

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#### Ashley2

The 100A information was already given, for the protective device, as was the Ze for the supply. And the question asked what the PEFC was. Bear in mind that this was a part B scenario for 2391, & the 100A for the protective device was already given, therefore, in the first instance, the PEFC must be less than the protective device. This is why it became questionable, in relation to calculating, using ohms law.

#### D Skelton

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The 100A information was already given, for the protective device, as was the Ze for the supply. And the question asked what the PEFC was. Bear in mind that this was a part B scenario for 2391, & the 100A for the protective device was already given, therefore, in the first instance, the PEFC must be less than the protective device.
If the PEFC was lower than a protective device, it would never operate during an earth fault. Hence the reason why (rightly or wrongly so) RCD's are relied upon to provide earth fault protection in TT systems.

PEFC is calculated using voltage and Ze only, forget the info about the 100A protective device because it seems to be confusing you and throwing you off.

If what you're worried about is thousands of amps flowing through a OPD rated at only 100A, you're concerned about the wrong rating. Assuming the protective device is a 60898, it will be rated at 6kA or above for fault conditions.

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#### Ashley2

I am looking at 435.5.1: which states that the breaking capacity rating of each protective device shall be not less than the prospective fault current at its point of installation". I strongly suspect that it is ohms law, but the 100A protective device was the mind boggler, taking into account 435.5.1.

#### D Skelton

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I am looking at 435.5.1: which states that the breaking capacity rating of each protective device shall be not less than the prospective fault current at its point of installation". I strongly suspect that it is ohms law, but the 100A protective device was the mind boggler, taking into account 435.5.1.
The breaking capacity of a breaker is not the same as its rating. It is rated at 100A but its breaking capacity will more than likely be 6000A or higher if it's a BS60898 or around 2000A if it's a BS3036

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#### Ashley2

Yes, its not the 100A protective device, its the Icn value, "& for the majority of applications the prospective fault current at the terminals should not exceed this value". It is ohms law, KISS should be the motto in 2391.

#### IQ Electrical

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Yes, as D Skelton points out, you're confusing the overcurrent rating with the short circuit capacity of the device.

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#### 1shortcircuit

the PEFC must be less than the protective device
Correct, look at the side of a 60898 and on the side it will say 6000. Meaning that device will take UP TO 6000amps of fault current before being blown into pieces which would obviously create a lethally dangerous situation.

#### Farmelectrics

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measurement is better than calculation lol

#### telectrix

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if the OCPD is a fuse to BS1361 or BS88, or whatever they change the BS number to tomorrow lunch time, the breaking capacity is in the region of 16KA or 33KA.

#### jaresquire

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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

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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.

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#### 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

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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

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#### 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

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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.

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#### 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.

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#### Guitarist

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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

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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 ^^

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#### 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".

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#### Guitarist

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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.

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#### 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

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#### 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.

J

#### 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

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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

G

#### Guest55

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.
Youre coming across as a real jerk pal.
You started a thread and asked a question.
It was answered in the second post and everyone should have gone home happy.
You like to use long-winded lingo in otherwise empty tedious posts.
As internet trolls go youre small change so go jog on.

M

#### mtfpuk2011

This.was in.the.2391 on.the 9th this.month.why warry should have reaulta.by the 12th.October if worse come worse.re sit.ir

Sent from my GT-I9000 using Tapatalk 2

#### ipf

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This.was in.the.2391 on.the 9th this.month.why warry should have reaulta.by the 12th.October if worse come worse.re sit.ir

Sent from my GT-I9000 using Tapatalk 2
Grammer and spelling ? 'Full stop' to that too. Never seen such small sentences.

M

#### mtfpuk2011

Yh well.I.have dixscler so.I sat 2391 and just wate see what happens

Sent from my GT-I9000 using Tapatalk 2

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#### 1shortcircuit

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.
I think you have suffered another confusion here Ashley.

The use of emoticons and symbols such as :thumbsup or Good Luck should not be taken in a negative way. I have based my post on posts that you have made in not only this thread but I went to the effort of browsing previous posts in other threads to confirm my suspicion.

If you feel that my post is irrelevant and the post such as MarkieSparkie's is useful and you understand then that is superb, perhaps I judged you wrong? Perhaps I didn't? Who cares? I was actually trying to offer some critique and remind you that learning to walk before you run is a wise move. If you choose to disregard this that too is completely cool. I am most certainly not going to lose any sleep over it my friend.

I wish you luck with your understanding of the basics of the trade and sincerely wish you all the best on understanding the more difficult areas too. I hope you do well and pass your course:thumbsup

PS I am a big boy and do not need the assistance of/feel the need to attack/defend any member whether they be on my friends list in my profile or not.

F***ing Dime Bar!!!!

R

#### reefagrim

I have to say i have assessed over 1500 electricians in their competence in their ablilty to test and i would say i found some good some shocking and some average. Ofa all of then that are trained electricians it was obvious that so many became lazy in their thinking of the test it be came a chore and after much prompting answers were given which i wanted to hear. the regs and assoicated GN books have the relevent infomation that you need and yes it can be confusing sometimes depends on the teacher or the back ground of the learner actually learning. of what i have read (and i did type a error i missed a o.o oops smack on the wrist) i have not only seen the question answered in many examples but also the " why " this is information that has been given freely and what you do not think is relevent to the question of the 2391 exam actually is ! it the whole point of it. maybe you should ask why have you been given the questions in the first place or a copy of the exam they want you to pass!! but any one person can test i can teach my 6 year old the correct proceedure and method but he will not understand the "why" of what he has done . i would say anyone who has been as fortunate to have such a deeply explained answer given by experienced and competent persons wouldnt be so negative.

#### Guitarist

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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.
I'm really not sure how you can accuse me of being a "cyber-bully" when I merely ask that you show the members on this forum some gratitude, rather than keep arguing with them about basic knowledge. I very much resent that accusation.
It is people like you on this forum that make a lot of us think "why bother?"
As for "ranting"? Hmmm....
All of us on here give our knowledge freely and to help other electricians who genuinely need help (and occasionally DIY'ers too).
Anyway, I wish you all the best and hope that with enough help on the forum you will be able to learn some of the basics of our profession.

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#### Ashley2

If you feel that my post is irrelevant and the post such as MarkieSparkie's is useful and you understand then that is superb, perhaps I judged you wrong?ng Dime Bar!!!!
The input that MarkieSparkie bought to this topic, I really enjoyed, & I am sure that you are as capable as him, in this respect, so please do not perceive my comments in this regard. If my perception, in terms of what I wrote after your post, is wrong, relating to the profile issue, then I apologise.

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#### 1shortcircuit

in terms of what I wrote after your post, is wrong, relating to the profile issue, then I apologise.
Absolutely no need to apologise in my opinion. Everyone has an opinion and I am far from always being right.

One thing for sure, I won't be losing sleep over anything posted on a forum

MarkieSparkie's posts are always VERY informative but often the content goes way over my head as I'm more of a small words/pictures man lol

:thumbsup

#### spark 68

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As always there will be differences of opinion.

I was always taught to use the Ze {measured} +(R1+R2) {again measured} and record the calculated results method.

The reason my mentor taught me this method, was if your calculated (two measured results added together) complied, then any parallel paths which lowered the Zs was a bonus, but not guaranteed.
You were actually measuring and calculating the worst case results, which were likely (but not guaranteed) to be better in reality, so if your results complied they could only get better, this he said was proper testing and proving compliance.
Likewise if you just measured your Zs, things could change later leaving you with a non compliant circuit.

On my own jobs I do the two measurements and calculate method and record this result, but still check the measured Zs just to double check that everything is ok.

I have worked on sites where they have wanted the actual measured Zs, but on the other hand I have been boll*cked on yet another site (same firm, different gaffer) where my Zs has been lower than the Ze (due to parallel paths), so it seems you cannot win, as always he who pays the piper calls the tune.

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#### Ashley2

Absolutely no need to apologise in my opinion. Everyone has an opinion.
Thank you for your response. And with reference to my initial query, my conversation with D Skelton was spot on, he put me right & I could have kicked myself for having lost that aspect:

D Skelton: (8). “PEFC is calculated using voltage and Ze only, forget the info about the 100A protective device because it seems to be confusing you and throwing you off”.

My response was:
Ashley2: (11). Yes, its not the 100A protective device, its the Icn value, "& for the majority of applications the prospective fault current at the terminals should not exceed this value". It is ohms law, KISS should be the motto in 2391.

The above remark was in response to my input into the exam question, namely that I should have kept it simple, in terms of answering this part, & I should have kept to Ohms law, instead of gallivanting into other areas. As far as I was concerned the matter had been concluded, thanks to D Skelton & IQelectrical.

#### D Skelton

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As always there will be differences of opinion.

I was always taught to use the Ze {measured} +(R1+R2) {again measured} and record the calculated results method.

The reason my mentor taught me this method, was if your calculated (two measured results added together) complied, then any parallel paths which lowered the Zs was a bonus, but not guaranteed.
You were actually measuring and calculating the worst case results, which were likely (but not guaranteed) to be better in reality, so if your results complied they could only get better, this he said was proper testing and proving compliance.
Likewise if you just measured your Zs, things could change later leaving you with a non compliant circuit.

On my own jobs I do the two measurements and calculate method and record this result, but still check the measured Zs just to double check that everything is ok.

I have worked on sites where they have wanted the actual measured Zs, but on the other hand I have been boll*cked on yet another site (same firm, different gaffer) where my Zs has been lower than the Ze (due to parallel paths), so it seems you cannot win, as always he who pays the piper calls the tune.
I agree 100%. When determining Zs, I believe calculation is always best because like you said, you are working out what the Zs would be in the worst case scenario with no parallel paths.

When working out PEFC however, I was always taught that measurement is best because you want to include all the parallel paths to find out what the worst case scenario is.

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#### spark 68

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When working out PEFC however, I was always taught that measurement is best because you want to include all the parallell paths to find out what the worst case scenario is.
Agreed,
:yes:

#### D Skelton

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my conversation with D Skelton was spot on, he put me right
thanks to D Skelton & IQelectrical.
In future, there is a little 'Thanks' button at the bottom left hand corner of every post. Just tap that if you wanna say thanks to someone for writing a helpful and informative post

In all seriousness though I'm glad you finally grasped the concept, when I first started studying I found things hard to grasp, hell I'd even go as far to say that most of us on here find one or more things hard to understand still. We all have our weaknesses and we're all still learning however one thing I've noticed that seperates the men from the boys in this game is that the men still have a keen interest in learning about our work no matter how experienced they are. The boys however know it all already (or at least think they do)! So my advice is never give up your quest for more knowledge! The hungrier you are for it, the more seriously you will be taken IMHO.

I'd also agree with the KISS acronym, one sure way of getting yourself in a right old mess in this line of work is overthinking things! We've all done it!

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