Page 243 - 35_DS702_E_2014_Lightning_Protection_Guide

I t of a sinusoidial
2
arc voltage U
U (V) melting half-wave (10 ms)
400 mains integral I t
2
voltage of the fuse
200 in A s 250 A
2
0
100 000
- 200 U 0
- 400 100 A
I (kA)
70 prospective
short-circuit 10 000 63 A
current I kpros
35
32 A
0
1 000 25 A
0 5 10 15 20 25 20 A
t (ms) no follow 16 A
current
flowing
100
I (kA) follow current I f 0.1 1 10 50 100
prospective short-circuit current [kA rms ]
NH-gG fuse link nominal current
2
Let-through integral I t of the RADAX Flow spark gap,
e.g. in DEHNventil modular
0 10 15 t (ms)
2
Minimum melting I t values of the fuse link
Figure 8.1.7.10 Reduction of the follow current by means of the Figure 8.1.7.11 Follow current disconnection selectivity of
patented RADAX Flow principle DEHNventil M with respect to NH fuse links
with different rated currents

In Figure 8.1.7.8 it can also be seen that, during the melting the required impulse current carrying capability of the arrester
process, a voltage drop U S builds up across the fuse which can used cannot be reduced.
sometimes significantly exceed 1 kV. For applications as illus-
trated in Figure 8.1.7.9, the resulting voltage protection level Selectivity with respect to the protection of the
U S + U P can be significantly higher than the voltage protection installation
level U P of the surge protective device used due to the melting When using spark-gap based surge protective devices, it must
of the fuse. be considered that mains follow currents are limited to such an
extent that overcurrent protective devices such as cable pro-
Field 3: Explosion tection fuses and / or arrester backup fuses cannot trip. This is
If the energy of the lightning impulse current is so high that it called follow current limitation or follow current suppression.
significantly exceeds the melting integral of the fuse, the fuse Only technologies such as the RADAX Flow technology allow
strip can vaporise explosively. This often leads to the burst- the development of arresters and arrester combinations which,
ing of the fuse enclosure. Apart from the mechanical effects, it even in case of installations with high short-circuit currents,
must be observed that the lightning impulse current continues are able to reduce and extinguish the prospective short-circuit
to flow via the bursting fuse in the form of an electric arc. current to such an extent that upstream fuses with low rated
The lightning impulse current thus cannot be interrupted and currents do not trip (Figure 8.1.7.10).

242 LIGHTNING PROTECTION GUIDE www.dehn-international.com