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Figure 5.9.6 Corrosion of a galvanised earth rod after 7 years Figure 5.9.7 Corrosion of a galvanised earth rod (below) and a stain-
less steel earth electrode (above) after 2.5 years

be observed. The exact values are specified in Table 3 of the Nowadays numerous power supply systems are operated for
German DIN VDE 0151 standard. 50 years or even longer, that is frequently far longer than the
service life of earth-termination systems made of conventional
Bare copper materials. Therefore, the earth-termination system must be di-
Due to its position in the electrolytic series, bare copper is ex- mensioned for this operating time. It is advisable to use stain-
tremely resistant. In addition, it is cathodically protected when less steel (V4A). Figure 5.9.6 clearly shows localised corro-
connected to earth electrodes or other buried systems made of sion of an earth rod after only seven years.
more non-precious materials (e.g. steel), however, at the cost Figure 5.9.7 shows that high-alloy stainless steel does not
of the more non-precious materials. corrode in the ground.
Reliable and correctly dimensioned earth-termination systems
Hot-dip galvanised steel are vital for a functioning power supply to ensure personal
When using galvanised material for buried earth electrodes, and operational safety. However, their correct operation is
the corrosion behaviour must be especially observed. In case often taken as given without any question. In case of earth-
of transformer stations, galvanised steel is typically embedded termination systems for transformer stations, the technical re-
in concrete (in the foundation slab of the building). This earth quirements of the high-voltage and low-voltage systems must
electrode embedded in concrete is connected to the ring earth be considered in context. A global earth-termination system
electrode. This direct connection forms a concentration cell. The provides considerable advantages with regard to the hazard
steel embedded in concrete has a higher potential (like copper) potential of a possible earth potential rise U E . According to the
and thus the more non-precious metal (galvanised steel in the standard, dangerous touch voltages will typically not occur in
ground) corrodes like a battery in the ground. The area ratio of this case. To ensure that personal protection requirements are
the two earth-termination systems is decisive for this electro- met in the system parts connected to the earth-termination
chemical corrosion. This is described in detail in chapter 5.5.7. system even under fault conditions, a total earth resistance
R B of the individual earth-termination systems of less than
Stainless steel 2 Ω has proven its worth in practice. The minimum cross-
When using high-alloy stainless steel, the before mentioned ef- sections of the earthing conductor and the earthing busbars
fect can be virtually excluded. According to EN 10088-3, high- of the installation must be observed with regard to the cur-
alloy stainless steel is passive and corrosion-resistant in the rent carrying capability in case of possible faults in the sta-
ground. In the majority of cases, the free corrosion potential of tion. In case of a fault, the stress on the earth-termination
high-alloy stainless steel in conventionally ventilated grounds system is reduced depending on the neutral point treatment
is similar to that of copper. Since stainless steel earth electrode (e.g. compensated system). In practice, the principles of the
materials passivate at the surface within a few weeks, they before mentioned standards and important notes by local dis-
show a neutral behaviour with regard to other (more precious tribution network operators must be observed. When design-
or non-precious) materials. Stainless steel should consist of at ing and dimensioning the earth-termination system, it often
least 16 % chromium, 5 % nickel and 2 % molybdenum. Ex- makes sense to assess the total earth resistance in advance to
tensive measurements have shown that only high-alloy stain- define all necessary measures before installing the installation.
less steel (V4A), e.g. AISI/ASTM 316 Ti, is sufficiently protected It is vital to pick a corrosion-resistant material for the earth
against corrosion in the ground. electrode of the earth-termination system. The examples de-

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