Touch / Step Voltage

[IndianTechEngg1001]

My general engineering knowledge says that touch potential should not increase 50Volts for AC system. But I am seeing that normally in grounding/ earthing calculations we get this voltage much higher that is tolerable and is around 367V/976 V (Touch/Step) (it is not constant and varies with the bar arrangements, Soil resistivity etc).

Can someone clarity?

Thanks in advance.

 

[dpc]

I don't think a touch or step potential of those magnitudes would be tolerable to someone actually experiencing it.

David Castor

http://www.cvoes.com

 

[jghrist]

Tolerable step- and touch- voltages in substations depend on fault duration and the resistivity of the surface layer. See IEEE Std 80-2000. What is the basis for you "general engineering knowledge"?

 

[IndianTechEngg1001]

My general engineering is from codes only. I think I must clearity more for my raised point.

For fault times greater than 600ms(Heart beat time) the affordable touch Potential should be around 50V. But for 0.5Second fault current I am getting the values stated in OP (367V) in ETAP calculations.

 

[cuky2000]

The following clarification may be appropriate:
• Tolerable voltages of 367V and 976 V are within ranges for usual allowable step and touch potentials.
• The grid arrangement will affect the current injected into the ground and the allowable voltages.
• Check the applicable code for earthing (grounding) in substation. Most programs including the ETAP may be set to calculate the allowable voltages in accordance with the IEEE Std 80 or IEC/CENELEC or similar.
• IEEE Std 80 provide an allowable step and touch potentials of ~164V for o.5 sec for 50 kg person neglecting the footage resistance and surface resistance (crushed rock). Those resistances provide larger allowable voltage comparable to the one calculated by ETAP program.

 

[cuky2000]

CORRECTION:

• The grid arrangement will affect the current injected into the ground BUT NOT the allowable voltages.