The Fig.3 shows the first case which is produced
mainly on overhead lines.
Induced shock is more frequent than a direct
shock. Lightning strike the ground and a current
flows in the cable shield. This current produces a
voltage gradient which in some places is above
the insulation capability of the cable material
(Fig.4).
II.2 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 telephoneline.
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 1 s and 15 mn.
III PRIMARY AND SECONDARY PROTEC-
TION :
The figures in chapter II give us an idea of the
energy which may appear on the lines. (so in the
field these surge values are lower due to the
losses of ground resistance, the capacitive
coupling and so on, but are signifiant
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.
A second parameter to keep in mind is the very
low clamping factor (1) needed by the IC’s used
to realize the line interface. This fact
necessitates 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.
The first level called primary protection (fig.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.
(1) the clamping factor is the ratio of the normal
operating voltage over the maximum clamping
voltage.
Fig.4 induced strike.
SUBSCRIBER
CENTRAL
OFFICE
Fig.5 Primary/secondary protection topology.
LINE
PRIMARY
PROTECTION
SECONDARY
PROTECTION
SLIC
Fig.3 Direct lightning strike.
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789
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CENTRAL
OFFICE
APPLICATION NOTE
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