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Topic Title: "Max Zs as permitted by BS7671" - What values to use?
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Created On: 30 June 2010 06:03 PM
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 30 June 2010 06:03 PM
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Smith249

Posts: 353
Joined: 09 September 2004

Guys,

Whats the right thing to put in the max Zs field, is it the corrected values that appear on the OSG or the higer values listed in BS7671.

I was told on my 17th edition update course that it should be the OSG yet my inspector and my software state the BS7671 values.

Thanks...
 30 June 2010 06:09 PM
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perspicacious

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"I was told on my 17th edition update course that it should be the OSG yet my inspector and my software state the BS7671 values."

Not all lecturers walk on water

Regards

BOD
 30 June 2010 06:10 PM
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Smith249

Posts: 353
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So, the higher values in BS7671 then....
 30 June 2010 06:11 PM
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OMS

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What it says on the tin really - use the maximum value permitted by BS 7671 from BS 7671 - all the variations of derating factors applied are generally to account for temperature difference when testing a cold circuit and to allow for conductor resistance increase under fault (where neccessary) - just put the max permitted and measured numbers in the boxes and do your comparison

Regards

OMS

-------------------------
Failure is always an option
 30 June 2010 06:18 PM
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Smith249

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Thanks,

Where are the tables for 3871 breakers?
 30 June 2010 06:20 PM
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perspicacious

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"Where are the tables for 3871 breakers?"

In earlier editions of the WRegs

Regards

BOD
 30 June 2010 07:38 PM
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Martynduerden

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Originally posted by: Smith249


Whats the right thing to put in the max Zs field


I Think you may have answered your own question..."Max Zs as permitted by BS7671"

-------------------------
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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 30 June 2010 08:11 PM
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leckyles

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Originally posted by: perspicacious

"Where are the tables for 3871 breakers?"



In earlier editions of the WRegs



Regards



BOD


Would they not be based on 240 Volt ?...as opposed to current 230 Volt Uo.
 30 June 2010 09:05 PM
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michaelbrett

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Originally posted by: leckyles

Originally posted by: perspicacious



"Where are the tables for 3871 breakers?"







In earlier editions of the WRegs







Regards







BOD




Would they not be based on 240 Volt ?...as opposed to current 230 Volt Uo.



You are correct. Multiply the 240 Zs values by 0.96 & you will get the 230V Zs values.

Regards

Mike
 30 June 2010 10:02 PM
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leckyles

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Yes Mike.. Thanks
 01 July 2010 12:34 AM
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spinlondon

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On my 17th course, I was given this question:
A loop impedance test indicated a value of 1Ω for a ring final circuit protected by a BS1361. Would this circuit conform to the requirements of BS7671?

Refering to Table 41.2 the maximum permitted Zs value is?........

Applying the factor of 0.8 to the maximum permited Zs value is?........

Will this circuit conform to the requirement of BS7671?

 01 July 2010 03:54 PM
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SandyBoiler

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Refering to Table 41.2 the maximum permitted Zs value is?........


1.15Ω

Applying the factor of 0.8 to the maximum permited Zs value is?........


0.92Ω (1.15x0.8)

Will this circuit conform to the requirement of BS7671?


no, cos 1Ω is more than 0.92Ω...but the 0.8 factor assumes the temp of the conductors when the test is conducted to be about 8ºC, if the temp of the conductors was 33ºC or higher when the test was conducted, the measured Zs of 1Ω would conform with BS7671...but then if the cpc was only 1mm2...

-------------------------
Andy B
 01 July 2010 06:43 PM
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ACKS72

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Originally posted by: perspicacious

"I was told on my 17th edition update course that it should be the OSG yet my inspector and my software state the BS7671 values."

Not all lecturers walk on water

Regards

BOD


Maybe not BOD, I prefer to part the seas.

Acks
 01 July 2010 10:44 PM
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spinlondon

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There is a note with all the tables in BS7671 stating that the values should not be exceeded when the conductors are at there normal operating temperature. The note goes on to state that the reading should be adjusted as per Appendix 14.
Statistics

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