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Voldatge Supression • Modem
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MODEM
AND POWER-LINE PROTECTION
Admittedly,
I have a unique situation when it comes to lightning
and surge protection. A simple lightning rod and single
buried ground rod (like Jeff’s) are more typical.
Once
you solve the direct lightning threat, protecting
a building from secondary invasions via the phone,
cable, and power lines is the next order of business.
There are two methodologies to this second line of
defense.
The
first method is to create a protective barrier using
suppression devices. The second method is just a simple
equipment-usage rule. When not in use, if the computer
(or any piece of equipment) isn’t plugged in,
then nothing can hurt it. You have to decide which
method is more practical for you.
All
the discussion about various suppression components
could lead you to believe we should use all of them.
There are many exotic combinations of these devices,
but those are generally intended for specialized applications.
Typically,
a liberal sprinkling of MOVs provides a high level
of protection at a reasonable price. The clamping
voltage and physical placement of the MOVs are the
only real issues.
There
are two basic types of surges—common mode and
differential mode. Common mode is when the surge potential
is between the incoming line and the earth ground.
A differential-mode surge is between two incoming
lines with no reference to earth ground. All lines
entering a building are susceptible to both.
Figure
1 illustrates a typical telephone/modem protection
circuit. Photo 2 shows how I installed this circuit
where my phone lines enter (the phone and cable companies
provide the equivalent of a GDT connected externally).
Figure 1—The telephone
line can serve as a lightning conduit. This
circuit (shown in Photo 2) provides signi-ficant
protection for both the phone equipment and
user. |
 Photo 2—The four phone lines entering the
house have both differential and common-mode surge
protection as outlined in Figure 1. |
Given
the currents usually associated with phone communications,
fusing might seem unnecessary. There isn’t much
that can help you in a close or direct lightning hit.
If such an event occurs, the fuses are intended to
simply disconnect the phone lines.
Interestingly,
a telephone line is an isolated signal. Devices attached
to it only require differential-mode protection. The
two common-mode connected MOVs are there not to protect
the modem or phone, but to protect the user.
While
most phones use high dielectric plastic, a 10-kV common-mode
surge could easily make the user be the path of least
resistance to ground. The two common-mode connected
MOVs prevent this.
Protecting
the AC power line uses the same three-MOV configuration.
Large MOVs, affectionately called doorknobs, are used
at the power-line entry. Smaller MOVs (e.g., the 130K20)
are used in the individual circuits or directly at
the equipment power source.
