2391 written exam last nite

[bobby101]

did the exam last nite i thought the exam was not to bad still quite tough and the dreaded VD question came up by VD i mean voltage drop i think i managed to work it all out but i made the stupid stupid stupid mistake of saying it was within six percent instead of five dont know how heavily i will get penilised for that but was quite impressed with myself for being one of the very few to work it out dont get me wrong i did make a few more schoolboy errors on the exam too but fingers crossed got my practical on mon at 8am aarrgghh had a few beers last nite though how did the rest of you find it and what questions did you think were nuts cause i think there were a few

 

[SafetyFirst]

There seems to be a unanimous conclusion that these 2391 exams aren't worded very well. How many folks took this exam on here? I'm thinkin we should start a revoloution or something, at the least get a petition sent out to ****** and no thrills telling them that whoever writes out this exam wants sacking!!!

 

[brucelee]

agree mate but they a law unto themselves

 

[mag8485]

I doubled the length of the conductors, but now I'm confused:

Table 4D1B in Appendix 4 states that 10mm sq cable in conduit drops 4.4mV/A/m at 70 °C
The title at the top states: 'Single-core ... cables', so I'd assume the values are for one conductor only.

The table in the question paper is different. 'Figure 1 below shows information on the resistance of conductors in mΩ/A/m.'
The bottom row says nothing more than: 1m of a 10mm² conductor has a resistance of 1.83mΩ at 20°C.
At 1amp, this gives a voltage drop of 1.83mV.
The scenario also mentions single cables in trunking and conduit.

Sadly, when I compare the figures with BS7671, logic and common-sense cease to apply.
The values on the exam paper should be the same as the values listed in BS7671, table 4D1B, but reduced a certain amount because they're at 20°C instead of 70.

BS7671 says a 1m length of 10mm ² cable drops 4.4V per amp,
C&G say that the same cable drops 1.83mV.

Temperature correction is 4% increase in resistance per 10°Cincrease in temperature, i.e. 20% for a 50°C difference.


The difference in these two figures is 58%.

What's going on?

The 4.4 mV/A/m is for 2 cables (L+N) single phase 10mm copper conductor at 70 degrees.
if you divide that by 2 you get 2.2 mV/A/m for one 10mm conductor at 70 degrees

The mV/A/m value is doubled to add the neutral conductor so you dont need to double the length. Just use the length of the run.

The 1.83mΩ/m is at 20 degrees.
Temperature correction is 4% increase in resistance per 10°Cincrease in temperature, i.e. 20% for a 50°C difference.

so 1.83 x 1.2 (20%) = 2.2mΩ/m (at 70 degrees)

mV/A/m is the same as mΩ/m