Specifications

T4.47
Surge arresters
Different earthing systems
require different SPD’s
Depending on how the earthing of the power
distribution system is implemented, single pole or
multipole SPD’s are required in order to fully protect
the installation and the hooked-up equipment against
destructive overvoltages. For an in-depth explanation
about the different existing earthing systems, please
refer to page T2.4.
For the explanation below, we will always take the
worst case example: a direct lightning-strike-hit on only
one of the conductors of a 3-phase energy distribution
system, discharging through this one conductor only.
We have also simplified the drawings by only
showing a varistor, and not the complete internal
circuit including the thermal fuse, fault-indicator-
and auxiliary-contact-circuit.
TT and TN-S earthing systems require multipole
SPD’s
Figure 2 shows a TT
-earthing system
, with varistors
installed only between each live conductor and
protective earth (PE), and also between the neutral
and the PE.
Right after the direct lightning strike hit, the
tremendous amount of free charges injected in to
the conductor, generates a very strong electrical
field, pushing these free charges as far apart as
possible. As a result, an impulse-wave-shaped
current travels away from the point of impact, in
both directions along the conductor towards the PE,
generating a voltage-drop across the conductor
given by the law U = -L x (di/dt). Typically, a 10kA
8/20 current-impulse generates a voltage of 1250V
across a wire with a length of 1m.
The varistor installed on the hit-by wire will clamp
this generated voltage to a value corresponding to
the instantaneous value of the current, given by the
U-I-plot of the varistor (see table 3), and will deviate
the current (I
2
) towards the local PE.
Because of the relative high local PE-impedance
(typically Z
2
= 10…30 ohm), the voltage-drop U
2
generated by I
2
could easily reach the level at which
the varistor between local PE and Neutral starts to
clamp, and therefore also starts to conduct current
towards the PE of the energy supplier (I
1
).
Once this happens, the bulk of the current will flow
through this parallel path, since on the side of the
energy supplier, the earthing as well as the
generator itself (or secondary of an intermediate
step-down transformer) has a very low impedance
(typically Z
1
= 0.3…1 ohm).
As you can easily see, the clamping voltage
between live and neutral is U
P1 + UP2, which is
roughly twice the clamping voltage of a varistor and
not once as may be expected. This results in a very
poor degree of protection. Therefore, in this case an
additional varistor between each live conductor
and the neutral is necessary to guarantee full
protection (see fig.3).
Based on the above explanation, you can easily
see that in the case of a TN-S earthing system
,
multi-pole SPD’s are required in order to fully
protect the installation and hooked-up equipment
against over-voltage-surges (fig.4).
Here however, since the impedance towards earth
via the neutral-conductor is roughly the same as
the one via the PE-conductor, both conductors
will share the current–surge, roughly equally.
Nonetheless, again the varistor between the neutral
and PE will conduct current, because it will clamp
the voltage across itself to its U
P and therefore
again the clamping voltage between the live and
the neutral becomes roughly twice U
P.
T4
fig.2
fig.3
fig.4