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Potential grading earth electrode
earthing busbar in the station, (closed ring), e.g. StSt (V4A),
e.g. St/tZn, 30 x 3.5 mm, 30 x 3.5 mm, installed around
closed ring
the station, spaced at intervals
between 0.8 m and 1 m, buried
at a depth of about 0.5 m
trans-
MV switchgear
LV distribution board main earthing cabinet
former
installation
cable
busbar (MEB)
cabinet
with earth
disconnecting
earth rod, e.g. StSt (V4A),
clamp If required, additional
cable ∅ 20 mm, about 5 m
cabinet
transformer

insulated earth Additional earthing conductor,
electrode bushing e.g. StSt (V4A), 30 x 3.5 mm
installed in the cable trench,
in every direction about 30 m

Figure 5.9.3 Schematic diagram of the earth-termination system at a transformer station (source: Niemand / Kunz; “Erdungsanlagen”,
page 109; VDE-Verlag)

where according to Table A 54.2 of the standard the material voltage side. According to the transformation ratio n of 50,
factor k (insulated, thermoplastic) is 143 in case of a copper the short-circuit current would be transformed to the high-
line, I is the short-circuit current and t the duration of current voltage side with about 450 A and trip the HH fuses according
flow (Figure 5.9.2). It is extremely difficult to calculate the to the fuse characteristic curve at a nominal current of 31.5 A
actual flow of fault current since it depends on the nominal in about 25 ms (on all poles). According to the equation
power of the transformer S N , the driving voltage, the short- I
circuit voltage u k and the relevant loop impedance (which can S = t
only be determined by measurements). Fast analysis is only k
possible to a limited extent by considering the initial sym- the copper protective conductors / protective bonding con-
metrical short-circuit current I’’ k (~three-pole short-circuit as a ductors in the station would have a minimum cross-section
2
2
defined state) which can be calculated by the nominal power S min = 25 mm . In practice, this value is rounded up to 50 mm .
of the transformer, the nominal voltage and the short-circuit It must be observed that in case of larger transformers and
voltage according to the following equation: consequently higher currents in conjunction with the relevant
S disconnection times the cross-sections for the protective and
I '' = N earthing conductor can be considerably higher. The earth-ter-
k
( 3 U u ) mination system itself (namely the part in direct contact with
N k
earth) is not stressed in case of this fault. On the low-voltage
In case of a 630 kVA transformer with u k = 4 % and side, currents only flow through the earth-termination system
U N = 400 V, the initial symmetrical short-circuit current I’’ k in case of an earth fault outside the station. The current
would be e.g. 22.7 kA. In our example with a 20 kV installa- U
tion, the transformer would have to be protected by means of I =
E
HH fuses with a nominal current from 31.5 to 50 A on the high- (R + R )
B
E

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