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.
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