Figures 1 and 2 show the voltages a person can be exposed to in a substation. There are many definitions related to these voltages, but the following six are the most important.
These definitions are:
- Ground potential rise (GPR)
- Mesh voltage
- Metal-to-metal touch voltage
- Step voltage
- Touch voltage
- Transfer red voltage
The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. Ground potential rise is the product of the magnitude of the grid current, the portion of the fault current conducted to earth by the grounding system, and the ground grid resistance.
The maximum touch voltage within a mesh of a ground grid.
The actual mesh voltage, Em (maximum touch voltage), is the product of the soil resistivity, ρ; the geometrical factor based on the configuration of the grid, Km; a correction factor, Ki, which accounts for some of the errors introduced by the assumptions made in deriving Km; and the average current per unit of effective buried length of the conductor that makes up the grounding system (IG/LM):
The difference in potential between metallic objects or structures within the substation site that can be bridged by direct hand-to-hand or hand-to-feet contact.
Important Note //
The metal-to-metal touch voltage between metallic objects or structures bonded to the ground grid is assumed to be negligible in conventional substations.
However, the metal-to-metal touch voltage between metallic objects or structures bonded to the ground grid and metallic objects inside the substation site but not bonded to the ground grid, such as an isolated fence, may be substantial.
Step voltage is actually the difference in surface potential experienced by a person bridging a distance of 1 m with the feet without contacting any other grounded object.
Touch voltage is the potential difference between the ground potential rise and the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure.
A special case of the touch voltage where a voltage is transferred into or out of the substation, from or to a remote point external to the substation site. The maximum voltage of any accidental circuit must not exceed the limit that would produce a current flow through the body that could cause fibrillation.
Assuming the more conservative body weight of 50 kg to determine the permissible body current and a body resistance of 1000 V, the tolerable touch voltage is:
and the tolerable step voltage is:
- Estep – Step voltage, V
- Etouch – Touch voltage, V
- rs – Resistivity of the surface material, V-m
- ts – Duration of shock current, in seconds
Since the only resistance for the metal-to-metal touch voltage is the body resistance, the voltage limit is:
The shock duration is usually assumed to be equal to the fault duration. If re-closing of a circuit is planned, the fault duration time should be the sum of the individual faults and used as the shock duration time ts.
Reference: The electric power engineering handbook – L.L. Grigsby (Purchase hardcover book from Amazon)