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AN579 APPLICATION NOTE

PRIMARY AND SECONDARY PROTECTION

Figures 2, 3 and 4 give us an idea of the energy which may appear on the lines. In the field these surge

values are lower due to the losses of ground resistance, the capacitive coupling and so on, but are signi-

fiant nevertheless.

We have to divide these disturbances into two families:

– High peak value and short duration (lightning).

– Short peak value and long duration (crossing with AC power).

For both cases the present state of the art of silicon protection devices does not permit the suppression

of these levels of energy.

the line interface. This forces the designer to use a protection solution with silicon (fast response time/low

clamping factor).

High energy values and low clamping factor impose two protection levels.

Note: 1. The clamping factor is the ratio of the normal operating voltage over the maximum clamping voltage.

Figure 5. Primary/secondary protection topology

The first level called primary protection (see Figure 5) located on the connecting terminal of the exchange,

suppresses the major part of the disturbance. The second level called secondary protection reduces the

remaining overvoltage.

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