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Topic Title: Ambient Temperature Correction Factors
Topic Summary: and finding the factor for 70C
Created On: 30 June 2007 07:52 PM
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 30 June 2007 07:52 PM
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andy4748

Posts: 130
Joined: 08 June 2006

I have been looking through my 2391 course books, and I understand about correction factors for cables, and the reasoning behind it - BUT I can't find the factor tables used. Are they in BS7671 or in the OSG, because I can't find them?

The OSG multipliers are on page 159 as Table 9B as well as the table 9C on page 160, but these factors used in my course book are not the same as these - so therefore I can't make sense of it.
GN3 on Table 2E page 115 makes more sense, but still not sure how these figures arrive on the following example.

Here is the scenario:-

Ambient temp is 25C for 2.5/1.5 cable with a reading of 1.3 ohms between the phase/neutral loop.

Solution

1.3 X 0.98 = 1.274 ohms

At 20C = 1.274/2 x 2.5/1.5 = 1.04

At 25C = 1.3/2 X 2.5/1.5 = 1.06

Maximum value R1 + R2 at 20C = 1.274/2 + 1.04 = 1.677ohms

So the factors that are bold, where did they get those from as I can't find them?

Also, (from another exercise) where can I find the table temperature correction factor of 1.2 for 70C to calculate the new Zs at 70C. Would this be from the table 9C in OSG page 160?

-------------------------
Andy Dunn. MIET

http://www.inspectrix.co.uk/

Edited: 30 June 2007 at 08:18 PM by andy4748
 30 June 2007 08:18 PM
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rocknroll

Posts: 8865
Joined: 03 October 2005

Also, (from another exercise) where can I find the table temperature correction factor of 1.2 for 70C to calculate the new Zs at 70C


OSG Table 9C page 150

Why have you used 0.98 when 1.02 is for 25 degrees C

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 30 June 2007 at 08:30 PM by rocknroll
 30 June 2007 08:52 PM
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andy4748

Posts: 130
Joined: 08 June 2006

That's what is throwing me - so it looks like the course notes supplied were wrong in the first place - and that is why I was doubting myself.

I gather you were looking at Table 9B on the OSG on page 159?

-------------------------
Andy Dunn. MIET

http://www.inspectrix.co.uk/

Edited: 30 June 2007 at 08:53 PM by andy4748
 30 June 2007 10:01 PM
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deleted_rossjenkin

Posts: 17
Joined: 20 December 2004

Hi Andy,

I assume the 0.98 (1/1.02) factor is to correct the resistance down from 25 degC to 20 degC. 0.98 is the reciprocal of 1.02.

I think waht is catching you (and me) out, is that the solution contains 1.04 and 1.06 as answers, they look like correction factors but are not!

They are resistance values!

The whole calculation takes the measured value of r1 + rn and tries to calculate R1 + R2 but actually makes some mistakes and leads us up the garden path.

Solution

1.3 X 0.98 = 1.274 ohms (correct 25 degC down to 20 degC)

At 20C = 1.274/2 x 2.5/1.5 = 1.04
(here r1 +rn divided by 2 to give r1, then multiplied by 1.67 to calculate r2 or the end to end resistance of the 1.5mm2 cpc which = 1.04 ohm)

At 25C = 1.3/2 X 2.5/1.5 = 1.06 (correct r2 to 25 degC(I don't know why!))

Maximum value R1 + R2 at 20C = 1.274/2 + 1.04 = 1.677ohms

(here the result is not R1+ R2 but r1 + r2, which the double loop resistance of the live cpc rings. to get R1 + R2 you divide r1 + r2 by 4. = 0.42 ohm


With respect to the temperature correction factors, the tables in the OSG are different to the ( BS7671 or GN3, I can't remember) one set adjusts from ambient to max working temp (for testing before use) the other form max working to ambient (for testing during operation). If you look closely they'll have slightly different descriptions.

I hope this helps.

-------------------------
Regards

Ross Jenkin
 30 June 2007 10:16 PM
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andy4748

Posts: 130
Joined: 08 June 2006

Thanks Ross for your explanation. And your last paragraph with comparison to BS7671, GN3 & the OSG through me too, as nothing is consistant. Its bad enough with BS7671 tells you what the maximum Zs readings you can get for protective devices, but OSG (which is also an IEE publication) allows for ROT. Now there are differences within three publications by the IEE with my example here!!!

-------------------------
Andy Dunn. MIET

http://www.inspectrix.co.uk/

Edited: 30 June 2007 at 10:17 PM by andy4748
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