However, if
you have the good fortune to live in the countryside in a building
served by overhead telephone lines, there’s an obvious risk of very high
voltages being induced on the lines during thunderstorms. While we
have lost count today of all of the modems, fax machines and other
telephones that have been destroyed by a ‘bolt of lightning’,
surprisingly you only have to invest a few pounds to get a remarkably
efficient protection device like the one we are proposing here.
During
a storm, often with lightning striking near a telephone line, the line
carries transient voltages up to several thousands of volts. Contrary
to the HV section of television sets or electrical fences, on which
practically no current is running, in the case of lighting striking
current surges of thousand of amps are not uncommon. To protect oneself
from such destructive pulses, traditional components are not powerful
or fast enough.
As you can see on our drawing, a (gas-filled)
spark gap should be used. Such a component contains three electrodes,
insulated from each other, in an airtight cylinder filled with rare gas.
As long as the voltage present between the electrodes is below a
certain threshold, the spark gap remains perfectly passive and presents
an impedance of several hundreds of MW. On the other hand, when the
voltage rises above this threshold, the gas is very rapidly ionized and
the spark-gap suddenly becomes a full conductor to the point of being
able to absorb colossal currents without being destroyed.
The
one we are using here, whose size is of the same magnitude as an
ordinary one watt resistor, can absorb a standardized 5,000 amps pulse
lasting 8/20 ms! Since we are utilizing a three-electrode spark gap, the
voltage between the two wires of the line or between any wire and
ground, cannot exceed the sparking voltage, which is about 250 volts
here. Such protection could theoretically suffice but we preferred to
add a second security device made with a VDR (GeMOV or SiOV depending on
the manufacturer), which also limits the voltage between line wires to
a maximum of 250 volts.
Even if this value seems high to you,
we should remember that all of the authorized telephone equipment,
carrying the CE mark must be able to withstand it without damage. This
is not always the case however with some low-end devices made in China,
but that’s an entirely different problem. Since pulses generated by
lightning are very brief, the ground connection of our assembly must be
as low-inductance as possible.
It must therefore be short, and
composed of heavy-duty wire (1.5 mm2 c.s.a. is the minimum). If not, the
coil, composed of the ground connection, blocks the high frequency
signal that constitutes the pulse and reduces the assembly’s
effectiveness to nothing. Finally, please note that this device
obviously has no effect on the low frequency signals of telephones and
fax machines and it does not disturb (A)DSL signals either.
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