Campbell Scientific 036, 038 Spark Gapped Junction Box - Manual (Page 4)

036, 038 SPARK GAPPED JUNCTION BOX
2
5. THEORY OF OPERATION
Where there is more than a few feet of
separation between sensors and datalogger,
transients can occur because for the
tremendous voltage gradient in the ground as a
lightening strike is being dispersed. The sensor
lead provides a better path for dispersion than
the earth, so the current arcs to the sensor
leads where the voltage is highest and where
there is the least separation between the wires
and ground. Frequently this is through the
datalogger the sensor lead is connected to.
If a spark gap provides a path from the wire
back to ground, the current may flow through
the spark gap and not damage the datalogger.
Where lightening is severe, heavy conduit or
shielding around the sensor lead can carry most
of the current directly to ground, minimizing the
amount of current to be handled by the sensor
lead, the spark gap, and the circuit card.
If the conductor on the circuit card is burned off
by excessive current, the current will arc to
ground. The circuit card and box are designed
so that if the conductors are burned off, the
current should still arc to the ground connection.
The 036 or 038 must have a good ground
connection. If the connection is not good
enough to dissipate all of the electrical energy,
the next path to ground could be through the
instrument.