Specifications

T4
T4.48
GE Power Controls
Comfort Functions
IT and TN-C earthing systems require single-pole
SPD’s
As can be seen in figure 5, the main difference
between a TT- and an IT
-earthing system is the high
impedance Z through which the generator or the
secondary of the step-down-transformer is
grounded in an IT-system.
Therefore, the low-impedant current-path towards
the PE of the energy-supplier which exits in a TT-
system, no longer exists in an IT-system, and for
this reason will never conduct current. So no
additional varistors between the live conductors and
the neutral are required to guarantee full protection.
In case of a TN-C-earthing system
, the Neutral-
and PE-conductor are combined in to one PEN-
conductor (fig.6). Therefore there is no alternative
parallel current path as it exists in a TN-S-system
and thus the highest possible voltage between the
neutral and a live conductor is equal to the
clamping voltage of only one varistor.
TN-C-S earthing systems
Last but not least, in a TN-C-S-earthing-system
,
always use multipole SPD’s where the neutral is
separately available and the equipment requires the
Neutral to be connected. Use single pole SPD’s only
if you are sure that the neutral is not separately
available or if the neutral doesn’t need to be
connected to the equipment (i.e. for a 3-phase 400V
delta motor).
fig.5
fig.6
Cascading of SPD’s
In areas where the exposure to lightning is very
high, SPD’s with a high I
MAX, must be installed (see
below). In general, the U
P of those devices is too
high to protect sensitive equipment like i.e. TV-,
VCR-, and computer-equipment.
Therefore, besides these high I
MAX / high UP front-
end SPD’s, devices with a lower U
P are to be
installed in cascade (parallel) in order to bring the
protection voltage down to a reasonable level.
Special care must be taken when two SPD’s, both
based on MOV-technology, are connected in
parallel, especially when their electrical
characteristics differ a lot from one another.
As can be seen in the graph of figure 7, when
putting two MOV’s direct in parallel, thus without
any substantial wiring in between, the one with the
lowest clamping voltage and lowest I
MAX will
conduct the bulk of the current.
This set-up is completely missing its goal, since the
MOV with the highest and not the one with the
lowest I
MAX should conduct the largest portion of the
current.
In order for this set-up to be effective, the
interconnecting wire between both SPD’s should
have at least a length of 1m (the longer, the better)
introducing a series inductor. If this is practically
impossible, a real inductor should be installed
between both SPD’s (fig.8).
fig.7
fig.8