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Topic Title: Max Zs tables in 7671 17th
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Created On: 02 August 2008 01:49 PM
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 02 August 2008 01:49 PM
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nickperkins

Posts: 162
Joined: 25 July 2008

I am used to reducing the tabulated values max Zs in 7671 16th by 80% to alow for heat in the circuit. Now that the tables are calculated differently in the 17th, do i still need to reduce the new values by 80%?

NP
 02 August 2008 01:59 PM
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Testit

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yep....

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 02 August 2008 02:35 PM
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Legh

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By measurement
Zs(meas) <= 0.8 Zs(tab)

or by calculation
Ze + 1.2*(R1+R2)*L = Zs(Calc) <= Zs(tab)

(R1+R2 are taken from the tables in GN3 (and I assume, OSG 17th))
Legh

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 02 August 2008 11:31 PM
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Pauldf1

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Just to confuse the matter, the figures in the back of guidance note 3 (2008 ed) appear to have been calculated by dividing the tabulated figure by 1.24.

(The 1.24(F) is given by the calculation F = 1 + (60 x 0.004) where 60 is the difference of conductor temperatures between max and measured and 0.004 is the simplified resistance coefficient of copper or alu.)
 02 August 2008 11:44 PM
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alancapon

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Originally posted by: Pauldf1
Just to confuse the matter, the figures in the back of guidance note 3 (2008 ed) appear to have been calculated by dividing the tabulated figure by 1.24.


Yes, but dividing by 1.24 is the same as muliplying by 0.8065, which is almost 0.8. That is where we started.


Regards,

Alan.
 02 August 2008 11:50 PM
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Pauldf1

Posts: 54
Joined: 09 May 2007

Yep, x 0.8 is almost right. I just thought I'd add the / 1.24 as the figures don't match up with the ones in GN3 when you do the x 0.8 thing.
In the old GN3 they just used x 0.8.
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