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.