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2000
copper galvanised steel
G = short-circuit current density [A/mm 2 ] 400 (EN 50522,
1000
800
600
300
Figure D.1)
200
150
100
80
60
316 Ti) determined in
tests (test report EPM
20
Figure 5.9.4 Connection of an earth rod to the ring earth electrode 40 StSt (V4A, AISI/ASTM
No. 6337 of 16/12/1993)
of the station
10
0.02 0.04 0.08 0.1 0.2 0.40.6 1 2 4 6 10
which flows back to the neutral of the transformer via the t F = duration of the fault current [s]
earth-termination system of the station, occurs at the fault lo-
cation. In case of a line-to-earth voltage of 230 V, a resistance Figure 5.9.5 Current carrying capability of earth electrode materials
R E of some ohms and an earth resistance of the station R B of
about 2 Ω, this current is uncritical. The current will not exceed
some 10 A so that overload is not to be expected if the maxi-
mum earth resistance is observed. IEC 61936-1 (EN 61936-1) standard must be observed in low-
voltage systems. Therefore, isolated earth-termination systems
Practical implementation of earth-termination sys- may be required in special cases, particularly in overhead line
tems for transformer stations systems or in case of dead-end feeders. In such cases, case-by-
The earth-termination system of a transformer station (Figure case examination is required. This outdoor earth-termination
5.9.3) must be designed according to IEC 61936-1 (EN 61936-1) system enters the station in an isolated way to prevent con-
and EN 50522 considering the local system data from the dis- tact with the building reinforcement, which would negatively
tribution network operator. An earth-termination system typi- affect a measurement result. The outdoor earth-termination
cally consists of several horizontal, vertical or inclined earth system is connected to the main earthing busbar by means of
electrodes which are buried or driven into the soil. In Germany, a disconnecting clamp. If the disconnecting clamp is closed, the
the use of chemicals to improve the earth resistance is not com- total earth resistance can be measured. If the disconnecting
mon and is not recommended. Surface earth electrodes should clamp is open, the earthing conditions of the relevant instal-
be typically buried at a depth of 0.5 m to 1 m below ground lation can be measured. As already described before, the total
level. This provides sufficient mechanical and frost protection. earth resistance of the station R B of about 2 Ω is sufficient.
Earth rods are typically buried below the surface of the earth Distribution network operators often refer to this value in
(Figure 5.9.4). Vertical or inclined earth rods are particularly the German Technical Connection Conditions. Therefore, it is
advantageous since the earth resistivity decreases in relation often helpful to roughly determine the total earth resistance
with the depth. Typical values of the earth resistivity depend- before installing the earth-termination system. Table 5.5.1 of
ing on the type of ground can be found in Figure 5.5.4 of chapter 5.5 includes formulas for roughly determining the to-
section 5.5. In general, a ring earth electrode (potential grad- tal earth resistance of different buried earth electrodes. When
ing earth electrode) is installed at a depth of about 0.5 m at a selecting the materials for earth electrodes, not only their cur-
distance of about 1 m around the station building. The earth rent carrying capability (Figure 5.9.5), but also the corrosion
resistance is often improved by driving an earth rod (typical behaviour must be considered, which will be described below.
length of about 6 m) into the ground. In addition, a strip earth
electrode of some 10 m is frequently routed along the cable Selection of earth electrode materials considering
routes in the cable trench. In practice, a common earth-ter- the corrosion behaviour
mination system should be preferred on the high-voltage and If adequate materials are chosen, corrosion hazards for earth
low-voltage side. In this context, the requirements concern- electrodes can be reduced or even prevented. To ensure a suf-
ing touch voltage and voltage rise specified in Table 2 of the ficient service life, the minimum material dimensions must

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