AN579 APPLICATION NOTE

4/10

Figure 6. Primary/secondary protection topology

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

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.

t < 1ms

1 to 4 kV

B

t > 50ms

V peak

>

20kV

A

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.

Zl

Zl

LINE

EXCHANGE