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Some types of earth electrodes and their classification accord- Conventional earthing impedance
ing to their location, shape and profile will be described below. R st is the resistance as lightning currents flow from one point
of an earth-termination system to reference earth.
Classification according to the location
of earth electrodes Voltages in case of current carrying earth-
Surface earth electrode termination systems, potential control
is an earth electrode that is generally driven into the earth Earth potential
at a depth up to 1 m. It can consist of round material or flat U E is the voltage occurring between an earth-termination sys-
strips and can be designed as a radial, ring or meshed earth tem and reference earth (Figure 5.5.1).
electrode or a combination thereof.
Earth surface potential
Earth rod ϕ is the voltage between one point of the surface of the earth
is an earth electrode that generally extends vertically into the and reference earth (Figure 5.5.1).
earth at great depths. It can consist of round material or mate-
rial with another profile, for example. Touch voltage
U B is the part of the earth surface potential which can be
Foundation earth electrode bridged by persons (Figure 5.5.1), the current path via the
consists of one or several conductors embedded in concrete human body running from hand to foot (horizontal distance
which are in contact with earth over a large area. from the touchable part of about 1 m) or from one hand to
the other.
Control earth electrode
is an earth electrode whose shape and arrangement serve Step voltage
more to control the potential than to maintain a certain earth U S is the part of the earth surface potential which can be
resistance. bridged by persons taking one step of 1 m, the current path via
the human body running from one foot to the other (Figure
Ring earth electrode 5.5.1).
earth electrode that forms a closed ring around the structure
underneath or on the surface of the earth. Potential control
is the influence of earth electrodes on the earth potential, par-
Natural earth electrode ticularly on the earth surface potential (Figure 5.5.1).
is a metal part in contact with earth or with water either direct-
ly or via concrete which is originally not intended for earthing Equipotential bonding
purposes, but which acts as an earth electrode (reinforcements for lightning protection systems is the connection of metal in-
of concrete foundations, pipes, etc.). stallations and electrical systems to the lightning protection
system via conductors, lightning current arresters or isolating
spark gaps.
Classification according to the shape and profile
of earth electrodes Earth resistance / earth resistivity
A distinction is made between strip earth electrodes, crossed
earth electrodes and earth rods. Earth resistance R A
The lightning current is not conducted to earth at one point via
the earth electrode, but rather energises a certain area around
Types of resistance the earth electrode.
Earth resistivity The shape of the earth electrode and type of installation must
ρ E is the electric resistivity of the earth. It is specified in Ωm be chosen to ensure that the voltages influencing the surface
and represents the resistance of an earth cube with 1 m long of the earth (touch and step voltages) do not assume hazard-
edges between two opposite sides of the cube. ous values.
The earth resistance R A of an earth electrode is best explained
Earth resistance with the help of a metal sphere buried in the ground.
R A of an earth electrode is the resistance of the earth between If the sphere is buried deep enough, the current discharges
the earth electrode and reference earth. R A is practically an radially so that it is uniformly distributed over the surface of
effective resistance. the sphere. Figure 5.5.2a illustrates this case; in comparison,

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