guidance note 3,help please

[RCBO]

page 122, 0 degrees has factor 0.96 table A6,and page 129 table B2 has 0 degrees at 0.92,
I understand that table A6 corrects from 10 degrees,and B2 from 20,but why not same value ,

 

[Strima]

What does BS7671 say?

 

[Wilko]

Hi - reasons given in Note 2 page 122, because the starting points are different by 10 degrees, then the factors will be a little different.

 

[Ian1981]

Because you are correcting from 10 degrees in table A6 and from 20 degrees from table B2.
Table A6 is based on site testing conditions at an assumed conductor temperature of 10 degrees,where B2 is the resistance/meter or R1+R2/meter For cables at 20 degrees in milliohms per meter.
2 different temperatures will not give the same correction factor to be applied.

 

[RCBO]

ok, so why not have the same values for cable if ur testing 10 c,=1.00
and b2 table at the same value,

 

[RCBO]

What does BS7671 say?

don't follow you,if you can point me in the right direction!lol

 

[Wilko]

ok, so why not have the same values for cable if ur testing 10 c,=1.00
and b2 table at the same value,

B2 calc'd from 20, so 20 has 1 multiplier.

 

[RCBO]

Because you are correcting from 10 degrees in table A6 and from 20 degrees from table B2.
Table A6 is based on site testing conditions at an assumed conductor temperature of 10 degrees,where B2 is the resistance/meter or R1+R2/meter For cables at 20 degrees in milliohms per meter.
2 different temperatures will not give the same correction factor to be applied.

thanks Ian,just trying to get my head round it,just don't get 2 values for same degree value

 

[RCBO]

is it the case then its assumed b2 values are stated as that's what the expected temp when doing install

 

[Ian1981]

is it the case then its assumed b2 values are stated as that's what the expected temp when doing install

Yes when designing a circuit to determine a calculation of the R1+R2 for said circuit you would use the values given in b2 unless you need to temperature adjust and then multiply by 1.2 to convert to max operating temperature of 70 degrees assuming a max operating temperature of 70 degrees for cables used to connect to similar rated accessories and switchgear

 

[RCBO]

What does BS7671 say?

regs,523.1 &2

 

[RCBO]

32 type B......76761 =1.37
gn3.p,122, 20c=1.04
p,129 20c=1.00
why do we have 2 values for same temp

 

[Christopher Bernard]

The values in 7671 are not the corrected values for Zs.

Zs = ((Uo x Cmin)/Ia) x 0.8
Meaning ((230 x 0.95)/160) x 0.8 = 1.0925 which the book rounds up to 1.1 which is the value you'll find in the on-site guide and GN3.

The values in 7671 are at 100% whereas 80% is to allow for rises in temperature, with every 5 degrees you get 2% more resistance.

A6 corrects from 10° whereas B2 from 20° hence the 1.04 because that's the decimal percentage of 104%

 

[RCBO]

The values in 7671 are not the corrected values for Zs.

Zs = ((Uo x Cmin)/Ia) x 0.8
Meaning ((230 x 0.95)/160) x 0.8 = 1.0925 which the book rounds up to 1.1 which is the value you'll find in the on-site guide and GN3.

The values in 7671 are at 100% whereas 80% is to allow for rises in temperature, with every 5 degrees you get 2% more resistance.

A6 corrects from 10° whereas B2 from 20° hence the 1.04 because that's the decimal percentage of 104%

thanks Christopher. but why are their two values 20c