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# rule of thumb ??

Discuss rule of thumb ?? in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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#### charlie

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

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#### wayne

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

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#### kevint

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

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#### wayne

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

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#### MacSparky

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

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#### Shakey

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

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#### wayne

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

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#### 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. ) ???

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#### Shakey

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)

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

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#### PAUL M

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.

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#### charlie

Thanks for reply shakey cos i've just done my course to 16th notes even though as you say exam to 17th. All very confusing at times and a poor show by the powers that be who release all the reading material i.e. BS7671, GN3, + on site guide.

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#### Shakey

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.
and there lies the problem - no 17th GN3 (as you know)

personally, i am going to teach the theory to 17th, the practical hasnt changed

C&G have made it clear that as long as you state what you are answering to, it 'should' be ok

However, if you are asked a question about methods of basic or fault protection (for example) , these are clearly 17th edition questions, you would not need to refer to GN3 to answer these, and I cant see how you could possibly answer them to the 16th

I think a few people could be on a sticky wicked sticking 'too rigidly' to their 16th edition answers

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#### PAUL M

Just Out Of Interest Shakey What Is The Percentage You Need To Achieve To Make Sure You Pass ,we Been Told To Aim For At Least 70 Percent But I Have Heard Others Say Lower Not That Im Aiming For Any Less Than 70 Percent Myself.lol

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#### Shakey

Just Out Of Interest Shakey What Is The Percentage You Need To Achieve To Make Sure You Pass ,we Been Told To Aim For At Least 70 Percent But I Have Heard Others Say Lower Not That Im Aiming For Any Less Than 70 Percent Myself.lol
well C&G wont release it

and they actually change it AFTER marking the exam acording to "the qulaity of the candidate"

there are 150 marks on the paper, and i have 'heard' that you need 80-90 marks to be safe

#### jeremy

##### -
Mentor
Arms
I've just read your reply about 2391 now being done to the 17th. I hope not because I have my 2391 theory tonight

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