Negative Grid resistance by IEEE 80 ?

[IEEEcontrol]

Per IEEE 80 grounding,

I got a negative resistance by equation 53, Rg=(R1*R2-Rm^2)/(R1+R2-2*Rm), R1=0.697 ohm, R2=0.4012 ohm and RM=0.531 ohm.

But when use equation 52, Rg = 0.693 ohm which is fine. Why?


This happens lots when I am doning the grounding desing by IEEE 80. Any professor can help at convenience?

CODE: IEEE 80-2000

 

[Phil Corso]

IEEEControl:


Please provide the Page numbers for the Equations cited!

Regards, Phil Corso

 

[steve66]

That equation is pretty sensitive to small changes in the numbers. I'd say you are rounding off during some intermediate step, and that's throwing the result off by enough to go negative.

When I look here at the equations for R1, R2, and Rm:

http://www.openelectrical.org/wiki/index.php?title=Earthing_Calculation

there seems to be lots of chances for rounding errors to enter the equation, and those equations are probably also very sensitive to rounding errors.

Personally, I wouldn't try to calculate these by hand on a calculator - I would type the equations into an excel spreadsheet. Less chance for rounding and other errors.