AN579 Datasheet, PDF (4/10 Pages) STMicroelectronics – PROTECTION CONCEPTS IN TELECOMMUNICATIONS EQUIPMENT

AN579 APPLICATION NOTE
Figure 6. Primary/secondary protection topology
V peak
>
A
20kV
t > 50ms
B
1 to 4 kV
t < 1ms
Few 10V
C
A: Overvoltage across the line without protection.
B: Remaining voltage after the primary level action.
C: Remaining voltage after the secondary level action.
Figure 6 shows the goal of both protection levels.
In this example the surge across the line without protection will be several 10kV peak value for several
10ms duration (Figure 6A).
After the primary protection the major part of the energy is cancelled (Figure 6B). The remaining overvolt-
age may be a few kV (depending on the dv/dt of the surge and the surge arrestor technology used).
Across the second level protection the voltage does not exceed a few 10 Volts.
Primary protection:
Actually two kinds of primary protection are used:
– Carbon gaps.
– Gas tubes.
Figure 7. Carbon gap based primary protection
Zl
LINE
Zl
EXCHANGE
Carbon gaps: These components are made by two carbon electrodes. The carbon gap is a low cost pri-
mary protection but it has two major disadvantages:
– its short life duration
– its variable spark threshold.
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