rule of thumb ????

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charlie

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Is it still 3/4 ( GN 3 ) or now 0.8 ( 17th edition ) ????

 

Mr Sworld

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Is it still 3/4 ( GN 3 ) or now 0.8 ( 17th edition ) ????
Now that we've harmonised with the EU with UK electrics via the 17th Edition the rule of thumb is now tied to the Euro.

So what is 'our' thumb is actually now 1.265 thumbs under the 17th Ed.

 

charlie

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What ??

Lost me there.

Need to know which value to use in 2391 exam which is now to 17th ed.

Is it still 3/4 (GN3) or 0.8 (bs7671 17th ed. )

 

Admin1

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What ??

Lost me there.

:|

Need to know which value to use in 2391 exam which is now to 17th ed.

Then I'd use the 17th Rule Of Thumb? (Is it possible to check with your provider?)

Is it still 3/4 (GN3) or 0.8 (bs7671 17th ed. )

I was going to say that GN3 is published to the 16th, but apparently - there is one test added / altered on the 17th? So it may still be usable for the 17th?

The experienced chaps will advise. Sorry I couldn't be of any proper use to your OP. :(

Bumped it back up though. :)

 

steptoe

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IT WOULD HELP IF YOU asked rule of thumb for what exactly.

(sorry for caps but not typing again, too lazy)

Zs

RCD times

cable lengths

MCB ratings

cable current carrying capacities

bit more concise than rule of thumb would be handy.

or,

if a man says a horse is 10 yr old, rule of thumb is to eat 3 shredded wheat.

makes about as much sense.

 

Mr Sworld

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What ??Lost me there.

Need to know which value to use in 2391 exam which is now to 17th ed.

Is it still 3/4 (GN3) or 0.8 (bs7671 17th ed. )
Oh! This is a serious question?

I thought it was a joke! :eek:

 

The Godfather

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My boys often bring fingers home with them, never seen any thumbs yet.

Don

 

jay_t

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Just state in your answer which edition you are answering to. They are both correct and as GN3 hasn't been updated yet, they are both relevant.

I would suggest sticking to one or the other throughout the paper though rather than changing back and forth.

(If you can remember them, why not answer to both editions)

Remember these are marked by a person, unlike the multi guess exams.

 

jay_t

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Anyone that has done the 2391 course should know what the rule of thumb refers to.

The tabulated values of Zs need multiplying by 0.75 (or 0.8 if working to the 17th) and the measured values must be equal to or lower than the corrected value.

I think the OP is just assuming that people here would know that.

 
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.8 of what exactly?of the measured, of the calculated,

seems to me , and Im really trying to be good here,

(ask anyone)

that you really dont have a f*ing clue what you are even asking the Q about.!
Yes, he IS trying (to be good) :^O :^O

Don`t be pedantic Steptoe!

That`s supposed to be MY job; remember?

Admin asked ME to do it. You`re supposed to be cantankerous!

:^O :^O

 
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Is it still 3/4 ( GN 3 ) or now 0.8 ( 17th edition ) ????
BOTH valid 16th & 17th:)

What ?? Lost me there.Need to know which value to use in 2391 exam which is now to 17th ed.

Is it still 3/4 (GN3) or 0.8 (bs7671 17th ed. )
where is this 0.8 17th reference coming from?

passed my 17th in April (100%) have no recollection of this mentioned? ?:|

IT WOULD HELP IF YOU asked rule of thumb for what exactly.(sorry for caps but not typing again, too lazy)

Zs

RCD times

cable lengths

MCB ratings

cable current carrying capacities

bit more concise than rule of thumb would be handy.

or,

if a man says a horse is 10 yr old, rule of thumb is to eat 3 shredded wheat.

makes about as much sense.
Very true sir... pick your thumbs wisely? ; \

Anyone that has done the 2391 course should know what the rule of thumb refers to.The tabulated values of Zs need multiplying by 0.75 (or 0.8 if working to the 17th) and the measured values must be equal to or lower than the corrected value.

I think the OP is just assuming that people here would know that.
still unsure where this 0.8 = 17th idea is coming from? ?:|

I do have my 2391 along with 2360, 2381 & 2382..

and I'm still struggling to find reference to back up this idea? :(

sorry where's my manners... Hello Charlie.. :D

I have been looking at this thread and I think there is a bit of confusion...

which hasn't yet been addressed..

been trying to figure out best way to put it..

without tying myself up in knots..

I am going to split my explanation onto a couple of posts... otherwise it will get too long..

(welshy can't cope with too long posts... he falls asleep half way through:^ O)

But in a nutshell IMHO..

a] Assuming we are talking about Earth Loop Impedance max Zs values..

3/4 AND 0.8 are BOTH valid values... but this is nothing to do with 17th.. as far as I am aware.

b] Steptoe is absolutely right... there are numerous "Rules of Thumbs" (if thats the right plural) used within the electrical industry.

I shall put me neck on the block.. you can raise the guillotine,

and get the firing squad all armed up...

but check out my explanations first... and please if I am barking up the wrong tree in the wrong forrest... without me dog food...

can you give references to back up your statements..

this I think will benefit anyone else reading out there in luring world. :) ;)

 
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First...

"Steptoes Thumbs" :D

As far as I am aware the actual big regs book, doesn't actual mention the word

'Rule Of Thumb'...

It is other publications that offer quick guidance as to how to achieve compliance with various aspects of BS7671..

as "Rules Of Thumb" rather than doing complex or long calculations..

e.g. Various NICEIC guidance notes mention the following;

I have extracted and paraphrase to shorten the key points down.

 
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3/4 or 0.8 or Both?

2b or not 2b that is the question..

whether tis nobler.................. ooops' sorry ... Medication time again:p

My hunch is that confusion has crept in because of the fact that the Maximum permitted Zs values have changed in the 17th from the 16th.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

[16th - 17th Zs values]

The amended values in 17th Zs tables are due to voltage calculations.

(not a rule of thumb?)

Although we have used and are still using a "Nominal" 230v supply voltage..

230v -6% +10% (216v to 253v).

The actual Max permitted Zs in 16th edition

are calculated around an open circuit voltage of "240v"

see 16th ed' regs Pg 193 Appendix 3

Last but one paragraph open circuit voltage 240v for a nominal supply voltage 230.

Whereas the 17th are actually calculated to a 230v open circuit voltage.

The difference between 240v to 230v is 96%..

All of the Max Zs values in 17th are 0.96 (96%) of the 16th ed values. :)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

[ 3/4 or 0.8 ]

The Max Zs values in the "Big book" are prior to any correction factors allowing for things such as ambient temperature of conductors during fault condition.

The Zs values in the "On-site-guide" are all 0.8 (80%)

of the values in the "Regs Book"...

this to my understanding is the 0.8 "Rule of Thumb" figure.

allowing suitable correction for temperature,

generally used when testing your new installed work.

However the Guidance Note 3 Rule of thumb refers to a 3/4 Zs value.

see page 56 of GN3, point 4) "rule of thumb figures"

GN3 also mentions allowances for unknowns CPC conductor sizes..

which to my understanding is when tested unknown cables (e.g. PIR)

although a ring may have a 2.5 with 1.5mm CPC at the fusebox..

older wires with 1.0mm CPC may be present at other parts of the circuit!

So a 3/4 or 75% Zs value gives and extra 5% margin of safety compared to the 80% on site guide values!

O-S-Guide.. aimed at new work you are designing & installing (known cables)

GN3.. more aimed at other persons work, such as PIR type, unknown cables sizes.

both '3/4' and '0.8' apply to 16th edition

well they do in my books anyway? :)

and I think the same guidance will apply to 17th...

once the 17th OSG & GN3 are actually published..

Now theres something to look forward too:D:D

for example

32A type B 60898

16th=1.50 OSG=1.20 (80% of 16th) 17th=1.44 (96% of 16th)

15A 3036 0.4sec

16th=2.67 OSG= 2.14 (80% of 16th) 17th=2.55 (96% of 16th)

check them out yourself?

I had to type this in the daytime...

rather then nighttime with liquid refreshment slowing my typing!! :^O :^O:^O

ok Load your guns men!

Me neck is on the block! :eek: :O

 

lexusZe

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can i just say i sat 2391 june 2008 paper and that is the only paper u can answer both 16 or 17 edition, except when 17th has made 16th obsolete. ie, rule of thumb. id use 0.8 if asked. (unless u sit june 08)

 

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