AN579 APPLICATION NOTE

2/10

Figure 2. Lightning phenomenon

Figure 3. Direct lightning strike

Figure 3 shows the first case which is produced mainly on overhead lines.

Induced shock is more frequent than a direct shock. Lightning strikes the ground and a current flows in the

cable shield. This current produces a voltage gradient which in some places is above the insulation capa-

bility of the cable material (see Figure 4).

Figure 4. Direct lightning strike

Proximity and crossing with AC mains lines:

For these kinds of surges two cases may be seen:

The first one is due to the falling of an AC mains cable on a telephone line.

The second case is produced by the proximity of a subcriber line with an AC mains line or equipment

(mainly capacitive coupling). It is interesting to note for these types of disturbances a RMS value of a few

Amps for a duration of between 1s and 15min.

+ + + + + + +

- - - - - - -

GROUND

IONOSPHERE

CLOUD

12

3

45

6

78

9

*

0#

CENTRAL

OFFICE

SUBSCRIBER

CENTRAL

OFFICE

12

3

45

6

78

9

*

0#