Step Potential

[ArnoldBraunschweiger]

Why does the tolerable step potential change with your soil parameters inside of ETAP? I would have thought that your maximum allowable step potential would be fixed and independent from the soil parameters. If my left foot is Va and my right foot is Vb, I am going to have (Va-Vb)/(resistance of me) current running through me. What am I missing?

 

[jbond]

If the top layer of the soil that you are standing on has a very high resistance, such as if you put crushed rock on the surface of the soil, then you can allow for a higher earth potential difference between your feet - the additional resistance underneath your foot is saving you!

Test it for yourself in your ETAP, by increasing the resistivity of the surface layer of soil you will find that you tolerable step potential limit is increased.

 

[cuky2000]

To meet the safe criteria, the surface voltage between the legs (Va-Vb), should be less than the allowable step voltage.

Va-Vb < E_step

In accordance with the IEEE Std 80, the allowable step voltage for a 50 kg person standing in bare foot can be considered as follow:

Va-Vb < (1000+6Cs.[&rho;]_s)x 0.116/[&radic;](tc)

For given fault clearing time (tc), the allowable voltage that a person can safely withstand is function exclusively of Cs and r

Cs = is the surface layer derating factor that account for the surface material thickness. As larger is the thickness larger will be the tolerable voltage

[&rho;]_s= soil resistivity ([&Omega;].m). As higher is the soil resistivity, the allowable voltage became larger.

_{(Ex. Crushed rock ~3000 [&Omega;].m, asphalt 10,000 [&Omega;].m)}