From: martin@datacomm.ucc.okstate.edu
Newsgroups: comp.dcom.telecom
Subject: Re: Phone Line Surge Suppressors
Date: 12 Apr 92 14:04:28 GMT
Organization: TELECOM Digest
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 315, Message 4 of 11

After some recent discussion of the merits of surge suppressors, it
would be good to remind those readers who might be new to electronics
what black magic surge suppressors can perform.

     Surge suppressors take a couple of common forms.  One type is the
solid-state suppressor which consists of some ceramic material that
acts as an insulator at whatever voltage is normal for the circuit but
starts to conduct if the voltage exceeds the threshold.  A surge
suppressor for a telephone line, for example, would act as if it isn't
even there at 48 volts or during ringing voltage, but will start to
conduct at, say, 200 volts.  The surge suppressor for a RS-232 line
might look very much like the one used for the telephone line, but it
would spring into action at, perhaps, 15 or 20 volts.

     The other common type of surge suppressor is the spark gap.  This
is a device which is sort of like the spark plug in an automobile
engine.  Two pieces of metal are mounted so that they are close enough
that if the voltage across them reaches a critical point, the air will
ionize and form a miniature lightening bolt.  Refined versions of this
type of zapper have the gap inside a tube filled with an inert gas
such as xenon, but the idea is the same.

Since lightening strikes and other electrical catastrophes don't
always neatly happen the way one might think, a good surge suppressor
must have components that could handle a voltage which might appear
between any external connection and any other.  On a telephone line,
this means that the suppressor must zap any surge which appears
between tip and ring or both tip and ring to Earth.

     If one has a surge suppressor that will zap any and all
overvoltages between any and all external connections, there is still
one more consideration.  How long does the destructive voltage have to
be there before the suppressor will conduct?  The answer to that
depends upon what material the suppressor is made of and the wire
length between the zapper and the device it is protecting.  The
shorter the wire length between the surge suppressor and the
electronics being protected, the more likely it is to do its job.  No
matter how good the surge suppressor is, it can only draw a finite
amount of current over a period of time before it overheats and
self-destructs.  The sickening description by a recent poster of what
happens when a high-voltage primary feeder contacts a secondary
residential service drop is the kind of situation for which no surge
suppressor was designed to survive.  Another impossible situation is
the case of a direct lightening strike.

     For years, electric utility companies have been using both the
spark gap and the solid-state type surge suppressors on commercial and
residential power lines.  These devices aren't much different than the
ones in your computer's power supply except that they are designed to
survive much bigger hits.  Sometimes one of the ceramic types will
fail and start conducting continuously.  When this happens, the heat
sets off a 22 caliber cartridge inside the suppressor which blasts the
ground terminal away and breaks the path.  Power line repair folk need
only drive along the road looking for dangling ground terminals to
know which lightening arresters have joined the enemy and need to be
replaced.

     In short, (See a pun if you find it), surge suppressors are great
things to have on your telephone and power lines.  They are probably a
little over sold because most quality electronic equipment has such
devices already built in, but they surely don't hurt anything.  The
amateur radio magazine "QST" published by the American Radio Relay
League has had many good articles about serge suppressors over the
last few years.  Consult your local library.


Martin McCormick WB5AGZ   Stillwater, OK
Oklahoma State University Computer Center Data Communications Group