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As an approximation, the earth resistance of a strip earth The potential of the earth´s surface decreases as the distance
electrode with earth rod can be calculated as if the strip earth from the earth electrode increases (Figure 5.5.1).
electrode was extended by the drive-in depth of the earth rod.
The inductive voltage drop across the earth electrode as the
R E lightning current increases only has to be taken into account
A l +l for extended earth-termination systems (e.g. in case of long
stripearthelectrode earthrod surface earth electrodes in poorly conducting soil with rocky
surface). In general, the earth resistance is defined by the ohm-
Ring earth electrode ic component only.
In case of circular ring earth electrodes with large diameters
(d > 30 m), the earth resistance is calculated as an approxima- If isolated conductors are led into the structure, the full earth
tion using the formula for the strip earth electrode (where the potential U E occurs.
circumference π ⋅ d is used for the length of the earth elec- In order to avoid the risk of puncture and flashover, such con-
trode): ductors are connected to the earth-termination system via
isolating spark gaps or in case of live conductors via surge pro-
d tective devices (see DEHN surge protection main catalogue) as
R = E ln
A 2 part of the lightning equipotential bonding.
d r
The magnitude of the earth resistance must be limited to mini-
r Radius or the round wire or quarter width of the strip
earth electrode in m mise touch and step voltages.
The earth-termination system can be designed as a foundation
For non-circular ring earth electrodes, the earth resistance is
calculated by using the diameter d of an analogous circle with earth electrode, a ring earth electrode and, in case of buildings
the same area: with large surface areas, as a meshed earth electrode and, in
special cases, also as a single earth electrode.
2
R = E In Germany, foundation earth electrodes must be designed in
A 3 d accordance with DIN 18014.
Conventional foundation earth electrodes are designed as a
closed ring and arranged in the foundations of the external
A 4
d = walls of the building or in the foundation slab according to
DIN 18014. In case of large structures, foundation earth elec-
trodes should contain cross-connections to prevent that the
A Area encircled by the ring earth electrode in m 2 maximum mesh size of 20 m x 20 m is exceeded.
Foundation earth electrodes must be arranged so that they are
Implementation enclosed by concrete on all sides.
According to the IEC standards, each installation to be protect- In the service entrance room, the foundation earth electrode
ed must have its own earth-termination system which must must be connected to the equipotential bonding bar. Accord-
be fully functional without using metal water pipes or earthed ing to IEC 62305-3 (EN 62305-3), a foundation earth electrode
conductors of the electrical installation. must be provided with terminal lugs to connect the down
The magnitude of the earth resistance R A plays only a minor conductors of the external lightning protection system to the
role for protecting a building or installation against lightning. earth- termination system.
More important is that the equipotential bonding is estab-
lished consistently at ground level and the lightning current is Due to the risk of corrosion at the point where a terminal lug
safely distributed in the ground. leaves the concrete, additional anti-corrosion measures should
The lightning current i raises the object to be protected to the be taken (PVC sheath or preferably stainless steel e.g. of mate-
earth potential U E rial No. AISI/ASTM 316 Ti).
The reinforcement of slab or strip foundations can be used as
1 di a foundation earth electrode if the required terminal lugs are
U = i R + L
E A 2 dt connected to the reinforcement and the reinforcements are
interconnected via the joints in such a way that the can carry
with respect to reference earth. currents.

126 LIGHTNING PROTECTION GUIDE www.dehn-international.com