Maximum Zs values and the rule of thumb

gareth020 · Mar 31, 2011

Hello, can someone explain to me the 'rule of thumb figure' (0.8) when calculating max Zs value's from BS7671!?
Currently doing a PIR in a foundry where i measured the ambient temperature at 21 degrees at ground level, i bet you can add another 10 degree's higher up ...and that's on a good day. Anyway surely i don't need this 0.8 rule in this situation and i'll use the values given in the regs!??? Or should i be calculating the values myself!???

Thank You
Gaz

telectrix · Mar 31, 2011

on your cert. you enter the value given in brb. you hope to achieve a ZS value of below the 80% temp. corrected figure. e.g. 32AQ type B is 1.44 in brb. 1.15 corrected. the lower the measured value, the better.

gareth020 · Mar 31, 2011

what temperature is the 80% of!? the ambient temperature!?

Deleted member 26818 · Mar 31, 2011

The 80% rule is to allow for the higher resistance, when the conductor is at working temperature (70ºC).

gareth020 · Mar 31, 2011

how do i know what that is!?

Chr!s · Mar 31, 2011

Zs values in the BRB are take at an operating temp of 70 C.

When your testing the circuit wont be at 70 C Obviously.

Copper has a coefficient of .004 per 1 degree.

So if your conductor is the same temp as your ambient lets say 20 C, we need to make an allowance for testing.

So 70 C - 20 C = 50 C so 50 x .004 = .2

If we take the Zs values as a factor of 1, then 1 - 0.2 = .8

So 32 amp type B mcm Zs at 70 = 1.44, at a conductor temp of 20 C we need to make sure the resitance is no more than 1.44 x .8 = 1.152 ohms.

gareth020 · Mar 31, 2011

Thanx.
Another question then ...where i'm working has above average ambient temperatures, do i need to factor this into my maximum Zs values!? Or is this just a factor that effects my Zs reading!?

Deleted member 26818 · Mar 31, 2011

The maximum Zs values as per BS7671 will remain the same whatever the ambient temperature.
You would apply any corrections to your measured value.

Chr!s · Mar 31, 2011

Thanx.
Another question then ...where i'm working has above average ambient temperatures, do i need to factor this into my maximum Zs values!? Or is this just a factor that effects my Zs reading!?

Well if you assume your conductor temp is the same as your ambient, so lets say 35 C

so 70 - 35 = 35 35 x .004 = .14

so 1- .14 = .86 so x the Zs values in BRB by 0.86.

Appendix 14 give you a method which is appropriate which also takes into account the load(operating temp may be lower)

telectrix · Mar 31, 2011

your readings will be affected by the ambient temperature. they will be fractionally higher at 30deg. than at 20. the corrected values in the osg are determined so as to be acceptable at 70deg. in other words, say your Zs is 1.15 at 20, 1.44 at 70. it would be around 1.25 at 30.