Lightning has long fascinated the technical community. Ben Franklin
studied lightning's electrical nature over 2 centuries ago and Charles
P. Steinmetz generated artificial lightning in his General Electric
laboratory in the 1920's. Anyone concerned with premises data
communications needs to worry about lightning. Few worry enough!
High voltage transients caused by lightning and other electrical
disturbances have the easy ability to destroy semiconductor junctions
and thus cause equipment failure. A lightning strike in the vicinity of
data lines can cause induced ground currents that can also do serious
However, coming right down to it, a lot can be done as far as protection
is concerned. In particular, typical leading edge wave-fronts of a
lightning strike rise at the rate of 1,000 Volts per microsecond.
Adequate protection from this particular lightning threat can be
provided by the Model 22NX Lightning Sponge. This unit is intended to
provide protection to data equipment having RS-422/RS-485 interfaces.
The Model 22NX Lightning Sponge provides protection for 4 independent
data communications circuits, each operating at a rate up to 1 MBPS and
each possibly exposed to such lightning induced transients.
The protection provided by the Model 22NX Lightning Sponge is
particularly robust. The protection circuitry incorporates 3 separate
stages for limiting high voltage lightning transients. These stages are
respectively realized from gas tubes, avalanche diodes and MOV
thyristers. Each is matched to a different rise time and surge level. In
particular, the first stage of protection, the first line of defense,
the gas tube, is capable of accommodating 1,000 A. Voltage is limited to
It would be a good idea to have a Model 22 NX installed on each end of
the 4 wire transmission line for RS-422/RS-485 data devices found in the
premises data communication environment.
The illustration above shows the Model 22 NX in a typical application.
Notice the designations of 'Earth Ground.' This unit and other similar
lightning sponges operate by directing lightning induced currents to
'Earth Ground.' It is important that the Model 22 NX be grounded
properly. This should be done with a wire that is, at least, 12 AWG and
is as short as possible. Connection to cold water pipes probably
provides the best ground. However, make sure that they are metal not
plastic. Connection to a plastic pipe doesn't help. Also beware of
connecting to a ground stud in a wall which, itself, is not connected to