challenge from boss

[andy120]

My boss has set me a little bit of homework. A cable calculation I've found Ib,In,Iz Selected cable Ect. I now want to calculate the r1+r2 for x2 240mm2 swa cables in parallel using the swa as the CPC. I'm assuming the table in regs, giving the mV/A/m value, is not applicable when using the swa as the earth. any ideas ??

 

[Guest55]

you need the cable manufacturers data that provides resistance values for each size of swa to calc R2.
that precise info is on this forum if you look harder ;-)

 

[andy120]

Thanks, I found a table values are in ohms per kilometre, so would I need to first convert these to milli ohms per meter then add the copper conductor and swa values together then this gives me the - milli ohm /m value

 

[telectrix]

i find it easier to calculate R1 from the copper ( figures in osg ) and separately calculate the R2 from figures obtained for the armor. then add the two together.

 

[andy120]

I may be looking at the wrong table, but only goes to 50mm, the figure I found online was 0.0754 ohms per kilometre so would be only 0.019 ohms for my 260 meter run this seems really low even for a cable as large as 240mm, I got 0.124 over 260 meters for the r2 which seems realistic

 

[Sintra]

Have you remembered to apply the formula for resistances in parallel.

 

[Guest55]

resistance of swa for 240mm 4 core is 0.54 mO/mtr*
resistance of 240mm conductor is 0.075 mO/mtr*

* at 20 deg C

i've given you all you need to finish your homework ;-)

 

[andy120]

Thanks a lot !!