BS7671 Help with some PFE questions

[clanky]

Thanks to being quarantine for a week i finally managed to have a go at the the practice exams in the course and i have a few questions for the more knowledgeable of you as im wasnt entirely sure of the correct method to get to the right answer. Thanks for any help in advance:

‘how is the nominal voltage at origin of an installation determined?’
I thought the answer was in the start of paragraph 313.1 and is quoted as ‘..calculation, measurement, enquiry or inspection.’ but the answer in the exam was enquiry only?

‘radial power circuit rated at 16A has been installed on a TT system what is the max. disconnection time for the AC circuit?
The correct answer in the exam is given as 0.2s but (using the table 41.1) I was confused as that assumes the supply is between 120 and 230v. I know its unlikely but for 50 to 120v the time is 0.3s. Which is obviously more. Am I missing something?

What is the condition of external influence in which an installation supplied from a low voltage network incorporating overhead lines would not normally require additional protection against overvoltages of atmospheric origin.
I struggled to find anything in the body of the main text for this one so I just used the table listing AQ1 as ‘normal’ Although I got the question right, I felt like I was just making an educated guess. Is there anything im missing?

Apologies for being a pendant but i have plenty of time before i have an opportunity to sit the real exam so i may as well use it usefully.

 

[clanky]

And a couple of more queries!

What is the use of a single core steel wire armoured cables restricted to?
Looking at section 523.201 there is no mention of what circuits that these are used for but in the exam the answer was DC circuits only. This one stumped me as i just assumed it was ok for both?

Which is the minimum degree of protection against external mechanical impact for electrical vehicle charging equipment in public areas?
The correct answer for this one was IK08 but in my version of 7671, sect. 722.512.2.203 states IK07 so im assuming my copy of the BS standard is out of date? Where can I check for updates?

 

[Lister1987]

And a couple of more queries!

What is the use of a single core steel wire armoured cables restricted to?
Looking at section 523.201 there is no mention of what circuits that these are used for but in the exam the answer was DC circuits only. This one stumped me as i just assumed it was ok for both?

Have a read of 521.5.2

 

[Lister1987]

Which is the minimum degree of protection against external mechanical impact for electrical vehicle charging equipment in public areas?
The correct answer for this one was IK08 but in my version of 7671, sect. 722.512.2.203 states IK07 so im assuming my copy of the BS standard is out of date? Where can I check for updates?

This changed in Amendment 1

722.512.2.203 Impact (AG)
Equipment installed in public areas and car park sites shall be protected against mechanical damage (impact ofm edium high severity AG3). Protection of the equipment shall be afforded by one or more of the following:

• the position or location shall be selected to avoid damage by any reasonably foreseeable impact
• local or general mechanical protection shall be provided
• equipment shall be installed that complies with a minimum degree of protection against external mechanical impact of IK08 in accordance with the requirements of BS EN 62262.

 

[clanky]

Have a read of 521.5.2

Ahhh bingo! cheers.

 

[Lister1987]

What is the condition of external influence in which an installation supplied from a low voltage network incorporating overhead lines would not normally require additional protection against overvoltages of atmospheric origin.
I struggled to find anything in the body of the main text for this one so I just used the table listing AQ1 as ‘normal’ Although I got the question right, I felt like I was just making an educated guess. Is there anything im missing?

External Influences, Appendix 5, pick out key words; external influence, not normally require, atmospheric origin (know that 'atmospheric origin' is science talk for lightning) Head for Appendix 5, look for lightning; AQ, AQ1 Negligible (aka not normally required)

 

[Lister1987]

Thanks to being quarantine for a week i finally managed to have a go at the the practice exams in the course and i have a few questions for the more knowledgeable of you as im wasnt entirely sure of the correct method to get to the right answer. Thanks for any help in advance:

‘how is the nominal voltage at origin of an installation determined?’
I thought the answer was in the start of paragraph 313.1 and is quoted as ‘..calculation, measurement, enquiry or inspection.’ but the answer in the exam was enquiry only?.

Look at the model form in Appendix 6 for EIC or EICR, look for U/Uo and read carefully

 

[Lister1987]

‘radial power circuit rated at 16A has been installed on a TT system what is the max. disconnection time for the AC circuit?
The correct answer in the exam is given as 0.2s but (using the table 41.1) I was confused as that assumes the supply is between 120 and 230v. I know its unlikely but for 50 to 120v the time is 0.3s. Which is obviously more. Am I missing somethin

Read the table heads carefully

Column 2 reads: 120 V ˂= U0 ≤ 230 V

the 'Less than or equal to' symbol in the book often catches people out as they just see the < part of it.

 

[clanky]

External Influences, Appendix 5, pick out key words; external influence, not normally require, atmospheric origin (know that 'atmospheric origin' is science talk for lightning) Head for Appendix 5, look for lightning; AQ, AQ1 Negligible (aka not normally required)

Apologies, i missed out some info on my original listing and how you describe the above is exactly how i answered the question. But i just felt that this wasnt answering the question directly, but more interpreting the book in an incorrect fashion to derive at the correct answer. I could be wrong, but doing the PFE's, the questions went through the standard in a basic numerical order ie from the start to the end, and this question was obviously the exception as it required to access the appendices before finishing the questions on the rest of the chapters. Hence why i wondered if the correct info was listed elsewhere.

Or am i overthinking this?

 

[Lister1987]

Apologies, i missed out some info on my original listing and how you describe the above is exactly how i answered the question. But i just felt that this wasnt answering the question directly, but more interpreting the book in an incorrect fashion to derive at the correct answer. I could be wrong, but doing the PFE's, the questions went through the standard in a basic numerical order ie from the start to the end, and this question was obviously the exception as it required to access the appendices before finishing the questions on the rest of the chapters. Hence why i wondered if the correct info was listed elsewhere.

Or am i overthinking this?

Have a read of page 5, excerpt below, my emphasis;

Chapter 44 Protection against voltage disturbances and electromagnetic disturbances
Section 443, which deals with protection against overvoltages of atmospheric origin or due to switching, has been
redrafted.
The AQ criteria (conditions of external influence for lightning) for determining if protection against transient
overvoltages is needed are no longer included. Instead, protection against transient overvoltages has to be provided
where the consequence caused by overvoltage (see Regulation 443.4):
(i) could result in serious injury to, or loss of, human life, or
(ii) could result in interruption of public services/or damage to and cultural heritage, or
(iii) could result in interruption of commercial or industrial activity, or
(iv) could affect a large number of co-located individuals.
For all other cases, a risk assessment has to be performed in order to determine if protection against transient
overvoltage is required.
There is an exception not to provide protection for single dwelling units in certain situations.

--
While it is true the test will generally walk through the book from part 1 to part 7, isn't not uncommon for an occasional reversion to an earlier part of the book, just think you got unlucky with that one

 

[clanky]

While it is true the test will generally walk through the book from part 1 to part 7, isn't not uncommon for an occasional reversion to an earlier part of the book, just think you got unlucky with that one

Brilliant! thanks for that. I'll take this one as a win but grudgingly!

 

[clanky]

Read the table heads carefully

Column 2 reads: 120 V ˂= U0 ≤ 230 V

the 'Less than or equal to' symbol in the book often catches people out as they just see the < part of it.

Thanks but my problem is that voltage wasnt referenced (only that its AC) in the exam question so it seems to be based on an assumption that its the middle column and not the first which would make the disconnection time 0.3 rather than the shorter 0.2s

 

[Julie.]

Thanks but my problem is that voltage wasnt referenced (only that its AC) in the exam question so it seems to be based on an assumption that its the middle column and not the first which would make the disconnection time 0.3 rather than the shorter 0.2s

I happen to agree with you.

This is one of the big issues with C&G, sometimes you have to read the question very carefully as there is some subtle inference pointing to a particular answer, yet in others there is an assumption that you know what the examiner wants rather than what they ask!

If the question was "a remote farm supplied by the dno...." then there is an inference the voltage would be nominal, and 0.2s would be correct, however the question is "what is the maximum..." and therefore 0.3s should be correct as 0.3 is bigger than 0.2!

Unfortunately there are always dubious questions with C&G and experience with their foibles is all that can help you.

You have to chalk the occasional "wrong" answer to this unfortunately.

 

[clanky]

You have to chalk the occasional "wrong" answer to this unfortunately.

Thanks and i think im starting to realise this. The method of questioning seems to prevalent in C&G courses with the only sole intention (as far as i can see) to avoid anybody achieving the 100% pass rate!

 

[clanky]

Ok, just finished another PFE and theres a few more 'foibles' to contend with...

Q2) ..what is excluded from the scope?
I picked Lightning Protection Systems but it seems the correct answer is Highway Street Furniture which isn’t listed in 110.2 But it even says highway equipment and street furniture as being covered within the scope in 110.1.1!
Anybody?

Q5) Which describes a Neutral Conductor?
Based on the definitions in Part 2, where it describes a line conductor as a conductor of AC OTHER than the neutral and Live Part includes a Neutral Conductor, i chose wrong. The ‘correct’ answer in the exam was ‘a line conductor’ but during the course, according to my scribbled notes, the neutral was defined as Live only.

Q40) ...Standby Generator to provide switched alternative to public supply. Which is NOT a precaution to prevent parallel operation?
From 551.6.1 I selected ‘an arrangement with separate switches for each supply’ which was wrong. The correct answer apparently should have been ‘An automatic changeover switch with interlock’ but, ‘An automatic changeover switching device with a suitable interlock’ is listed as a suitable precaution. Isnt this just semantics?

As before, huge thanks for all the input. As im doing this all online, the help at the college has dried up.

 

[clanky]

‘how is the nominal voltage at origin of an installation determined?’
I thought the answer was in the start of paragraph 313.1 and is quoted as ‘..calculation, measurement, enquiry or inspection.’ but the answer in the exam was enquiry only?

Look at the model form in Appendix 6 for EIC or EICR, look for U/Uo and read carefully

back to this one, im still confused as to why 313.1 isnt correct? ie assessment of general characteristics or is it just because the word 'installation' has been used in the question?

 

[Zerax]

You have to chalk the occasional "wrong" answer to this unfortunately.

That's probably why they won't tell you which ones you got wrong either... would open up a whole can of worms and potential legal action !

 

[timhoward]

It’s also possible your practise software has errors it in. I didn’t have any issues with ambiguous questions in the actual exam. I also know that with the effort you are putting in you will easily pass.
The only issue I had was not finding a couple of things (still got 93% though)

 

[timhoward]

Q2) ..what is excluded from the scope?
I picked Lightning Protection Systems but it seems the correct answer is Highway Street Furniture which isn’t listed in 110.2

If highway street furniture was the correct answer this is wrong. Lightening Protection systems is likely the correct answer. Occasionally you get part answers that are deemed to be incorrect by not having the whole text, so some wally might decide that unless BS EN 62305 is mentioned in the answer it isn't right....but I suspect this is a website / software error.

Q40) ...Standby Generator to provide switched alternative to public supply. Which is NOT a precaution to prevent parallel operation?
From 551.6.1 I selected ‘an arrangement with separate switches for each supply’ which was wrong.

If you read the list in the regulation you mentioned, the key thing is that in all cases you can't possibly have both supplies on at once. A separate switch for each doesn't prevent you turning them both on. The point of the regulation is that you mustn't ever be able to do that.

‘how is the nominal voltage at origin of an installation determined?’
I thought the answer was in the start of paragraph 313.1 and is quoted as ‘..calculation, measurement, enquiry or inspection.’ but the answer in the exam was enquiry only?

This one is rather horrible. I feel the word "origin" is key, if you look that up in part 2 you get "the point electricity is delivered to the installation". In general that bit isn't yours to be messing around with and inspecting/measuring and you have nothing to base calculations on.
It's a poor question in my view, and again, worse than any real-life exam question I remember.

 

[pc1966]

Q5) Which describes a Neutral Conductor?
Based on the definitions in Part 2, where it describes a line conductor as a conductor of AC OTHER than the neutral and Live Part includes a Neutral Conductor, i chose wrong. The ‘correct’ answer in the exam was ‘a line conductor’ but during the course, according to my scribbled notes, the neutral was defined as Live only.

I don't have a copy of the questions, etc, but this is one odd aspect where common usage of 'live' differs from the definitions.

Live = Any current-carrying conductor under normal use (so all but the CPC)
Neutral = A live conductors that somewhere is connected/referenced to Earth.
Line = The live conductor(s) not referenced to Earth

Indeed it really is the referencing of a current-carrying conductor to Earth that defines neutral, and how/where that is done is of course on of the key aspects of the supply.

While an IT supply may not have any reference to Earth, most of any significant power level (e.g. excluding shaver supplies) do by some high impedance so they technically can have a neutral even though a single line-Earth fault has no immediate impact on operations as normally the earthing impedance is quite high and designed to sustain such a single fault. Used for situations where you really don't want an immediate interruption if at all possible (e.g. operating theatre supply, ship electrics, etc) and it is under skilled supervision.