Page 148 - 35_DS702_E_2014_Lightning_Protection_Guide

Previous examination results allow the conclusion that ϕ A is weeks, they are neutral to other (more precious and non-pre-
much smaller than ϕ C . cious) materials. Stainless steels should consist of at least 16 %
chromium, 5 % nickel and 2 % molybdenum. Extensive meas-
The following applies for ϕ C : urements have shown that only high-alloy stainless steels with
Steel in the ground approx. 1 Ωm 2 material No. AISI/ASTM 316 Ti / AISI/ASTM 316 L, for example,
Copper in the ground approx. 5 Ωm 2 are sufficiently corrosion-resistant in the ground. Stainless
Steel in concrete approx. 30 Ωm 2 steels without molybdenum are not suited for use as earth
electrode material and are not permitted by the standard.
From the area rule, however, it can be clearly seen that high
corrosion effects occur both on enclosed steel pipes and tanks Other materials
with small defects connected to copper earth electrodes and Other materials can be used if they are particularly corrosion-
earthing conductors made of galvanised steel connected to resistant in certain environments or are at least equivalent to
extended copper earth-termination systems or extremely large the materials listed in Table 5.5.8.1.
reinforced concrete foundations.
The risk of corrosion for earth electrodes can be avoided or
reduced by choosing suitable materials. To achieve a sufficient 5.5.7.4 Combination of earth electrodes made
service life, the minimum material dimensions must be main- of different materials
tained (Table 5.5.8.1).
The cell current density resulting from the conductive combi-
nation of two different buried metals leads to the corrosion of
the metal acting as anode (Table 5.5.7.4.1). This cell current
5.5.7.3 Selection of earth electrode materials density basically depends on the ratio between the size of the
Commonly used earth electrode materials and their minimum cathodic area A C and the size of the anodic area A A .
dimensions are listed in Table 5.5.8.1. The German “Corrosion behaviour of earth electrode mate-
rials” research project has found that, when selecting earth
Hot-dip galvanised steel electrode materials particularly regarding the combination of
Hot-dip galvanised steel can also be embedded in concrete. different materials, a higher degree of corrosion only has to be
Foundation earth electrodes, earthing and equipotential bond- expected in case of the following area ratio:
ing conductors made of galvanised steel in concrete may be
connected with reinforcing bars. A C >100
A
Steel with copper sheath A
In case of copper-sheathed steel, the comments for bare Generally, it can be assumed that the material with the more
copper apply to the sheath material. Damage to the copper positive potential will become the cathode. The anode of a cor-
sheath, however, presents a high risk of corrosion for the steel
core. Therefore, a completely closed copper layer must always Material Material with a large area
be applied. with a Galva- Steel in
small area nised steel Steel concrete Copper
Bare copper Galvanised +
Bare copper is very resistant due to its position in the electro- steel + zinc removal – –
chemical series. Moreover, when connected to earth electrodes Steel + + – –
or other installations in the ground made of more “non-pre-
cious” materials (e.g. steel), bare copper additionally provides Steel + + + +
cathodic protection, however, at the expense of the more in concrete
“non-precious” metals. Steel with
copper + + + +
Stainless steel sheath
Certain high-alloy stainless steels according to EN 10088-1 Copper / + + + +
are inert and corrosion-resistant in the ground. The free corro- StSt
sion potential of high-alloy stainless steels in normally aerated + combinable – not combinable
soils is mostly close to the value of copper. Since the surface of Table 5.5.7.4.1 Material combinations of earth-termination systems
stainless steel earth electrode materials passivate within a few for different area ratios (A C > 100 x A A )

www.dehn-international.com LIGHTNING PROTECTION GUIDE 147