2391 exam in 3 hours need help with a question!!!

[tommy.dale]

sorry guys need to rack your brains again just want to make sure i get the right answer as im goin over my notes


State the two conditions that need to be taken into account when comparing measured Zs value and the maximum values quoted in bs7671



PLEASE HELP IM SOO NERVOUS IM PULLING MY HAIR OUT!!!!!!!


thanks again!

 

[Deleted member 26818]

I would imagine that temperature would be the two conditions. The ambient temperature at the time of testing and the conductor operating temperature when in service.

 

[tommy.dale]

it is worth 4 marks so i presume 2 answers and 2 statements to back them up??

 

[newbie111]

agree, then use the 0.8 rule of thumb as its measured not calculated..

 

[tommy.dale]

sorry im a bit confused so how would i answer that question....i really appriciate your feedback sooo much!!!!

 

[SirKit Breaker]

I would say,

1, ambient temperature, if the temp is high, this will increase the resistance and so increase maximum measured Zs, if low it will have the opposite effect.
2, The tabulated values in BS7671, these need to be corrected using the 0.8 correction factor to ensure that the measured Ze is acceptable.

That is 2 conditions and 2 reasons, but i am not saying that this is the correct answer.

Cheers.........Howard

PS a bit of tip with the 2391. When asked to show all calculations, show them, even if you get the result wrong you can still pick up points for showing the correct method.

Oh and good luck, lets us know in due course how you get on.

 

[tommy.dale]

will do fella cheers!

 

[newbie111]

good luck tommy.dale, where are you sitting the exam?

 

[Deleted member 26818]

When measuring Zs, the conductors will be at the ambient temperature, not at their operating temperature.
An adustment to the measured values must be made to accomodate this difference.
The 80% rule of thumb is applied for operating temperatures of 70°C, and measuring when the ambient temperature is 20°C.
A different percentage will have to be used if the operating temperature is lower than 70°C, or if the ambient temperature when measuring is either higher or lower than 20°C.
GN3 Appendix B, OSG Appendix 2 and BS7671 Appendix 14 have information relating to this.
Another factor to consider is the temperature rise in the conductor operating temperature that will occur when conductors and circuits are bunched together, and those caused by installation methods. These are usually associated with current carrying capacity, but as they will cause temperature rises, should also be considered when assessing conductor operating temperature.

 

[Sparky Ninja]

My answer would be very similar to that of spinlondon's.
Applying reference to appendix 14 of BS7671 shows a greater understanding than the commonly phrased 'rule of thumb'.
I would maybe also write down an understanding between the effects of a conductors resistivity/conductivity according to ambient and operating temperatures and the reason the two exist.

I'm just setting my class up for the exam now.

Good luck to you.