Max Zs

[florider]

Hi, i have a question im a bit stuck on at the moment, should probably know but my brain is mashed on it!!! 

Max Zs- i believe the figure stated in fig 41.3 of BS 7671 is @ 70 degrees ambient temp?   - how to find correction factors for ambient temp for 20 degrees?

also, if i have the R1+ R2 reading, and Ze reading, can i use R1+R2 - Ze to give me an indication of what to expect for Zs once this is measured on the final circuit? 

i hope this makes sense!! 

many thanks 

 

[NozSpark]

Without having the regs in front of me.... I believe that the table that you're on about is for 70deg rated cable at an ambient temperature of 20deg... there is another table that shows the temperature correction factors for different temperatures

There are 2 ways to determine the Zs of a circuit...

1. measurement

2. calculation - i.e. Zs=Ze+(R1+R2)

You are not meant to reverse the calculation method to determine your (R1+R2) though

 

[phil d]

got my OSG in front of me and the tables are as follows

temp 0c   correction factor 0.96

temp 5c       "               "      0.98

 "      10c      "              "       1.00

"       20c      "              "        1.04

"       25c      "              "       1.06

"       30c     "               "        1.08

the correction factors are from table b8 which are different from table 12 because b8 corrects at 10c and table 12 at 20c

the appropriate ambient correction factor is applied to the earth fault loop impedance of tables B1_B6 if the ambient temperature is other than 10c when the impedances are measured.

e.g if the ambient is 25c the measured loop of a circuit protected by a 32A type B breaker to BS60898 should not exceed 1.16 x1.06 =1.23 ohms