Back in the 2391 days

[Hawkmoon]

Short question (2006 paper):


'Small commercial unit owned by a paint company:

Supply characteristics as follows..
The supply forms a 400/230V 50Hz TN-C-S system. The main fuses are 100A BS 88-2 and measured value of PFC is 1600A. The earthing conductor is a 25mm copper conductor and the main equipotential bonding conductors to gas, water and structural steel are 16mm copper.

Determine Ze from the above information'


Remember this causing much confusion...

Answers?

 

[LeeH]

I'll be the first to put myself out on a limb.

The PFC would have been doubled so half that.

Then it's ohms law. Cannot see why the size if the eq bonding conducters are relevant as these are disconnected for Ze.

And the main earth conductor is also irrelevant as that is what it is.

 

[telectrix]

but was it doubled or x 1.732? or even PEFC as opposed to PSCC??

 

[LeeH]

Its a ---- question TBF. I've had a few over the past few weeks where you only get half a story and it's up to you to make assumptions.




Anyway got go to Los Santos.....

 

[Hawkmoon]

I can tell you I was told that approx 50% of students forgot the 3 phase aspect and applied ohms law to the 1600

Approx 20% went with 800


anyone else??

 

[LeeH]

I'll give it to my tutor next week and see what he makes if it. He certainly knows his onions.

 

[Hawkmoon]

Yes but shouldn't we also 'know our onions'

 

[Hawkmoon]

I'll be the first to put myself out on a limb.

The PFC would have been doubled so half that.

Then it's ohms law. Cannot see why the size if the eq bonding conducters are relevant as these are disconnected for Ze.

And the main earth conductor is also irrelevant as that is what it is.

There were other questions relating to it in addition to the Ze issue

 

[Hawkmoon]

but was it doubled or x 1.732? or even PEFC as opposed to PSCC??

From the question the PFC was 'measured'

On 3 phase supply PSCC is normally PFC

 

[Hawkmoon]

I'll be the first to put myself out on a limb.

The PFC would have been doubled so half that.

Then it's ohms law. Cannot see why the size if the eq bonding conducters are relevant as these are disconnected for Ze.

And the main earth conductor is also irrelevant as that is what it is.

Respect to you Lee - 80 odd views and you are the only one man enough to post up close to an answer lol

I got this wrong (not in exam but after in class where it was put up by a lecturer)

 

[Bick]

There should be a new qualification, City & Guilds 2399 'Initial verification and deciphering of vague exam questions'.

I've always thought their questions tend to be ambiguous and open to several interpretations in a large number of questions I have come across.
Not very helpful when you are under pressure in an exam situation!

 

[LeeH]

I have my Design and Verification exam tonight so an answer would be nice!

 

[Guest 004]

I have never used a 3 phase loop meter.
So if the value was measured then no root 3 will apply.

 

[Guest 004]

Having said that you can get 3 phase loop meters. I normally just measure 1 phase then factor in root 3

 

[RichardMoore]

Assuming that a pfc can only be measured off a singles supply one at a time them doubled by calculation. I would work it out as 230v divided by 1600a pfc = 0.14 ze

 

[Guest 004]

The problem is clearly on how the PFC was measured. Which is not clear on the question, as it says 230/400. So you either go with 230 to give 0.14, or what I am leaning with 400 which is 0.25

 

[Hawkmoon]

Assuming that a pfc can only be measured off a singles supply one at a time them doubled by calculation. I would work it out as 230v divided by 1600a pfc = 0.14 ze

Congrats - not for your answer but for having the balls!

 

[Guest 004]

Yep unclear on how the value was achieved, so hopefully this question is no longer in the databank

 

[RichardMoore]

The problem is clearly on how the PFC was measured. Which is not clear on the question, as it says 230/400. So you either go with 230 to give 0.14, or what I am leaning with 400 which is 0.25

why would you go with 0.25 on 400v ? Is it because it feels better but it wrong

 

[Guest 004]

^ Just ohms law isn't it

 

[LeeH]

You cannot use 400V as you would never get this to ground. Only short a short circuit between phases.

^ Just ohms law isn't it

 

[Guest 004]

Oh hum

 

[MDJ]

0.287 ohms

 

[Guest 004]

^ Interesting how did you get to that?

I am going with 230/1600 = 0.14 ohms

 

[Guest 004]

If the PFC given was calculated with root 3. Then 1600 would have to be divided by root 3 giving 923.8. Then it's 230/923.8 = 0.248

Don't remember the exam being that complicated really. So I am staying with 230/1600

Final Answer

You can shoot me down in flames now.

 

[Guest 004]

BTW passed 2391 in 2000

 

[Hawkmoon]

0.287 ohms

This is the answer I came up with (back in the day) - unfortunately not correct

 

[Hawkmoon]

The lecturer who showed my group the question said it was likely it was designed to mislead (or a 'trick question')

 

[LeeH]

The lecturer who showed my group the question said it was likely it was designed to mislead (or a 'trick question')

And the model answer is!!??

 

[Guest 004]

That's what we have already said

 

[Guest 004]

This is the answer I came up with (back in the day) - unfortunately not correct

Respect

 

[spark 68]

I would have went with 800, and came up with 0.29

On my 2391 we were told to double the highest SP PSSC to derive the TP PSSC
As this is a TNC-S supply the SP PFC will be the same as the PEFC (without the bonding)

But the question is not clear, the PSSC measurement issues aside (technically root 3), the PEFC would be taken with all bonding connected too.

 

[Guest 004]

With respect your teacher was a c u next Tuesday.

I have many arguments on site today about root 3, and doubling.

All current books will use root 3.

 

[spark 68]

With respect your teacher was a c u next Tuesday.

I have many arguments on site today about root 3, and doubling.

All current books will use root 3.

The 'rule of thumb' given in GN3 uses the double SP PSSC rule, as this errs on the side of caution Pg 55 GN3

 

[Guest 004]

^ rule of thumb is a quick answer and not correct. Not going to say anything more on the matter. Unless you want to start another thread.

 

[Hawkmoon]

I would have went with 800, and came up with 0.29

On my 2391 we were told to double the highest SP PSSC to derive the TP PSSC
As this is a TNC-S supply the SP PFC will be the same as the PEFC (without the bonding)

But the question is not clear, the PSSC measurement issues aside (technically root 3), the PEFC would be taken with all bonding connected too.

This is the area to consider!

 

[spark 68]

Technically you are correct re: root 3, but as the 2391 and 2394/5 use GN3 as their reference this is what is taught on these courses.

 

[Guest 004]

Really? ? I was taught root 3.
I give up. ..

 

[spark 68]

Not really sure why we are splitting hairs whether to double or root 3, as this reading is not very accurate anyway, so much so that on SP installs you are supposed to take three readings and pick the highest, a slight change of loop impedance measurement has a massive effect on the PFC reading, most 'common' instruments that we use do not have the accuracy, hence the double errs on the side of caution rule.

Incidentally I have a 400V capable meter and it gives a lower reading than the calculated.

 

[spark 68]

Here is a past 2391 paper Q & A, sorry about the formatting of the equation in the answer, but you get the gist.

Q20: For a TNC-S system, supplied at 230V, 50 Ha, state
a) why PSCC and PEFC are the same (1 mark)
b) the value of PFC if Ze was 0.01 (1 mark)
c) the approximate value of PFC between phases if the single-phase value
was 10 000A.
(1 mark)

A20: PSCC and PEFC
a. PSCC and PEFC are the same because the neutral and earth conductors are
combined up to the service head where the measurement is taken.
b. If Ze = 0.01, then using the formula
a
oc
s I
U
Z = where Uoc is taken as 240V for a
nominal 230V supply voltage (Uo), then PFC 24kA
0.01
= 240 =
c. If single-phase PFC is 10kA, an approximate value for PFC between phases is
found by doubling this value to 20kA.

 

[Hawkmoon]

For those who have tried this question the answer I was given (by the lecturer) was along the lines of:

You cannot determine Ze from the information given - as the PFC measurement is carried out with bonding connected.


Although most (in the class) agreed they should have thought of this, there was agreement that the question was misleading as it implied that you could.

 

[spark 68]

Bloody Typical obtuse C&G trick questions, lol

 

[Hawkmoon]

Bloody Typical obtuse C&G trick questions, lol

Interesting though that the question you put up has the same issue Qb

 

[MDJ]

Hawkmoon you tease LOL, great Thread though Fella, although I was wrong I still enjoyed it. Well done for a decent thread worth reading for once.

 

[Guest 004]

Must admit did not appreciate the answer.

Concur good thread

 

[Hawkmoon]

Hawkmoon you tease LOL, great Thread though Fella, although I was wrong I still enjoyed it. Well done for a decent thread worth reading for once.

Don't feel bad, nobody in the class (around 20 guys) got it right although there was a few smart asses there lol

As I said half did not even consider the 3 phase element so it was a good learning exercise.

 

[spark 68]

I actually only ever had a quick look at past papers before doing my 2391 (passed 1st time btw), as I found them more of a hindrance than a help.

They would often switch methodology by asking very similar questions on different paper and using a different method giving a different answer.

I am not sure of the date of these past papers, a lot were using 16th ed terminology which were not much use in a 17th ed based exam., some had 240V calcs in, or a hybrid like the question/answer I posted.

They were useful in so far as to see the layout, structure and format of the exam, but that was about it.

 

[MDJ]

Don't feel bad, nobody in the class (around 20 guys) got it right although there was a few smart asses there lol

As I said half did not even consider the 3 phase element so it was a good learning exercise.

Well to be fair it was the early eighties when I did most of my college work, apart from going back to tech for the 16th and then the 17th editions and the odd testing course I am pretty much out of practice in the classroom.

 

[Hawkmoon]

I actually only ever had a quick look at past papers before doing my 2391 (passed 1st time btw), as I found them more of a hindrance than a help.

They would often switch methodology by asking very similar questions on different paper and using a different method giving a different answer.

I am not sure of the date of these past papers, a lot were using 16th ed terminology which were not much use in a 17th ed based exam., some had 240V calcs in, or a hybrid like the question/answer I posted.

They were useful in so far as to see the layout, structure and format of the exam, but that was about it.

Agreed - Useful for judging what you needed to have in memory (closed book), layout etc - but as you say, a bit limited otherwise.

 

[atm84]

I have my Design and Verification exam tonight so an answer would be nice!

I did that course. Complete waste of time considering our tutor left us to do our course work in class. We only went through past papers on the last day if the course and they were provided by another guy who happened to get the papers from one of his mates who did the course a few years previous with a different training provider.

I'm now doing a degree.