rule of thumb ?? - Electricians Forum | Electrical Forum | An electrical forum for electricians in the UK

charlie · 17-05-2008

Is the rule of thumb for Zs still o.75 or is it now 0.8 ( from 17th edition ) ???

wayne · 18-05-2008

appendix 14 covers this question (17th issue),have a read and let us know if it helps

kevint · 19-05-2008

i just done the 2391 and there was some confusion on this through the course

wayne · 19-05-2008

what i understand is the tables quoted in the 17th(41.2-41.4) are for cables operating at their max working temp .however when you take a measurement the cables could be at lower temp thus giving a higher reading the 0.8 is to take this into account without further calculation.
please correct me if im wrong

MacSparky · 19-05-2008

Quote:

Originally Posted by wayne

however when you take a measurement the cables could be at lower temp thus giving a higher reading the 0.8 is to take this into account without further calculation.
please correct me if im wrong

Surely if the cable temperature is lower then the reading will also be lower

20-05-2008

Quote:

Originally Posted by MacSparky

Surely if the cable temperature is lower then the reading will also be lower

Yes, the reading will be lower, so you take 80% of the tabulated reading as the maximum, because at working temp the actual Zs will be higher
Wayne is basicallly right just got his temp the wrong way around

wayne · 20-05-2008

am i getting the laws of thermodynamics wrong ?if the temperature increases so does the resistance?

charlie · 02-06-2008

what i need to know is if on the 2391 exam they ask for the "rule of thumb" do you use 3/4 ( GN3 ) OR 0.8 ( BS7671 17TH ED. ) ???

02-06-2008

Quote:

Originally Posted by wayne

am i getting the laws of thermodynamics wrong ?if the temperature increases so does the resistance?

yes you are and yes it does (think of a 'superconductor' which must be at absolute zero or bloody chilly anyway)

Quote:

Originally Posted by charlie

what i need to know is if on the 2391 exam they ask for the "rule of thumb" do you use 3/4 ( GN3 ) OR 0.8 ( BS7671 17TH ED. ) ???

And the answer (from C&G) is

as long as YOU make it clear what you answering to

ie 3/4 ( BS7671:2001 as ammended 2004)

or

0.8 (BS7671:2008)

then either will be acceptable

you cant be 'wrong' either way, there is nothing wrong in saying both either

think of what you are saying to the examiner:

"Fella, I know what i am talking about"

and to answer any other query which is kicking about - yes the last 16th edition based 2391 exam has gone, and it is now 17th edition based

PAUL M · 03-06-2008

Im Doing My 2391 This Week,4 Day Course And They Still Teaching It To 16th And Will Be Until The New Gn3 Comes Out It Gets Bloody Confusing Right After Doing 17th Upgrade.