Zs Calculations

[r10ley]

Hi im writing this post as ive read loads of information regarding adjusting Zs values due to temperature. If I go wrong anywhere in my explination please correct me : D

sooo....

When doing inital verification Ze should be measured aswell as R1+R2. The R1+R2 value should then be adjusted for temperature. Firstly the ambient temperature should be recorded and then using the values from table 9B from the OSG as dividers you adjust the R1+R2 value from ambient temperature to 20oC. Once this is done you must calulate for the worst case scenario which is at operating temperture of 70oC. This is done using table 9C from the OSG and multiplying the value by 1.2. This value can then be directly compared to value found in tables 41 in the BRB.

For periodic inspections a loop impedance tester is used to obtain a value for Zs, this must then be compared to an adjusted value for Zs obtained from tables 2A - 2D in appendix 2 in the OSG. The ambient temp is taken then a correction factor from table 2E in appendix 2 in the OSG is used to find the maximum permissable Zs.

If any of this is not correct please tell me because my head is fuzzy : P

Just a couple of questions about the whole thing. For intial verification the R1+R2 values are adjusted for temperature why is Ze not adjusted also?

Also the rule of thumb of 80% excists regarding adjusting the values in the BRB to ones taken at ambient temperature when testing, I assume the values found in the OSG tables 2A to 2E are meant to be these adjusted values roughly?

Im pritty new to this game and my head is aching now lol.

Thanks for your help

r10ley

 

[telectrix]

you have it pretty spot on there, r10. as for correcting Ze, i'll venture a guess that due to the large size of the cables ( incomers) that determine Ze, temperatue increase is very small and so resistance would not be affected enough to cause an increase in Ze.

 

[r10ley]

Thank god for that : D One other question is the tables in the OSG in appendix 2, 2A-2D are calulated at a temperature of 10oC which I guess is considered to be a common ambient temperature. Is this why 80% is used to adjust the values from 70oC when (60 x 0.004) + 1 is 1.24 which when divided by will give your a tiney bit over 80% ?

Thanks

r10ley

 

[telectrix]

yes, it's to allow for resistance increase with temperature.