Substation ‘earthed’ metal work (consisting of switchgear enclosures, supports, fencing, etc.) and overhead line steel supports all have impedance to true earth. When fault current flows through them to earth, a voltage rise will occur.
This ‘earth potential rise’ (EPR) is the maximum voltage that the earthing system of an installation may attain relative to a remote point assumed to be at true (zero) earth potential. The EPR is the product of the current that returns to its remote sources via the soil and the earthing system impedance.
The overall (gross) fault current calculated will normally include amounts that may return via metallic routes such as the other (un-faulted) phase conductors, the metal screens/sheaths of buried cables and the earthwires (if fitted) of an overhead line. These current flows are subtracted from gross value to leave the amount that will return via the soil.
This current will flow through the local electrode system and any other electrodes connected in parallel to it. These include sheet steel foundations, the steel legs of transmission towers, large pipes and the lead sheath of cables that have a conductive outer covering (such as Hessian).
Whilst the EPR exists, voltages will occur in and around the installation. A number of voltage definitions are used to characterize the situation at any point. They include:
• The ‘touch voltage‘ which is the potential difference between the EPR on a structure and the surface potential at a point where a person is standing (normally 1 m away), whilst at the same time having one or both hands in contact with the structure.
• The ‘step voltage’, which is the difference in surface potential experienced by a person bridging a distance of 1 m with their feet.
• The ‘transfer potential’ is that between steelwork (physically distant from the installation, but bonded to it) and the remote local earth. This could occur at any point along a cable, pipe or steel fence. The design normally seeks to prevent dangerous transfer potentials occurring, by limiting the EPR or removing the electrical connection between the steelwork and installation earth.
• The ‘mesh voltage’ is a quantity used in the American Standard, IEEE 80, and is the touch voltage seen at the centre of a mesh of the substation earthing grid.
Calculations seek to find the worst case value of touch and step voltage for the design and compare it against tolerable voltage limits.