2395 exam question

[electric-tec]

Hi All

Just thought id post this and see what the verdict is,had a couple of emails from good guys telling me ther thoughts.

Senario.. A small workshop with a canteen and toilets are wired in 70 deg single core thermoplastic cables enclosed in tubing, the light in the toilet are wired from a photcell and the fittings are flourecent, the Ze is 0.3ohms and pfc 16kA.


Q, A radial circuit fedding a heater in th office is fed from a 40A beaker the value of the combined conductors are 0.23ohms, calculate the volts drop for the circuit,

i answered. (ri+rn) x Ib = 0.23 x 40 = 9.2v

then had a brainstorm from doing previous stuff in class of adding a correction factor of 1.2 then with 5 min to go decided to change 1.2 to 1.04 as in GN3 it states 1.04 for cables that are single 70deg not enclosed or bunched or embedded so answer became 9.56v still under then after exam looked at GN3 only to find i used the 1.04 for the mv/m for cable factoring for bonding and not temperature,

So when my papers marked he will see i had 1.2 then changed to 1.04 showed all equations and calculated the percentage.

QUESTION IS do you think i will score anything or nothing was a 15 mark question ?

 

[tony mc]

Have a read of this post

This question has been asked a few times !

Today's 2395

 

[Amp David]

Well i used the 1.2 correction factor as you are calculating the vd for the cable at full operating temp of 70 deg c

 

[tony mc]

I have to disagree re using correction factors as you working out the volt drop under working conditions not fault conditions!

IMO you will not gain any marks if you changed your first answer and workings out!

Correction factors are for use when calculating that the circuit will disconnect and comply under fault conditions.

 

[telectrix]

correct. the mV/A/m figures in the osg tables already assume a temp.of 70deg.

 

[Amp David]

The question that was written advised that the resistance was a measured value at an ambient temp of 20 degs.

There wasn't any reference to any mV/A/m figures from osg on the paper though

 

[Amp David]

Taken from the C and G Examiners report.

Candidates were given information related to a particular circuit and asked to determine i) the
expected voltage drop and ii) whether this value complies with the requirements of BS 7671.
Many candidates failed to make any allowance for the change in conductor temperature when
calculating the expected voltage drop. When determining compliance a large number of
candidates failed to indicate why their conclusion was compliant or non-compliant.


So it looks like the 1.2 temp factor is to be used after all.

 

[telectrix]

stupid question anyway. the value of Ib is not given.so how can you calculate the VD?

 

[electric-tec]

hi guys everyone got mixed opinions on this one,

i must have done something correct as i passed:wink5:

i had 1.2 then changed it to 1.04

but will never know even my lecturer started questioning himself with the osg and gn3 lol

 

[Amp David]

stupid question anyway. the value of Ib is not given.so how can you calculate the VD?

40 amp

 

[telectrix]

40A is In, the MCB rating. Ib < ( or equal) to In, might be anywhere between 30A and 40A.