LIGHTNING PROTECTION USING FRANKLIN RODS

Abstract:
Under normal conditions, the electric field at ground level remains at a value of several
hundreds of volts per meter. Under thunderstorm conditions, the electric field could
reach as high as several thousands of volts per meter. The increase in the electric field
is attributed to the charge separation in the cloud.
The lightning stroke is initiated by the formation of a negative downward leader at the
storm cloud, which advances in steps toward the ground. This results in enhancement
of the field at the tip of the Franklin rod, with a subsequent formation of corona
streamer discharge, which may turn into a positive upward leader. As the upward leader
progresses, it influences the path of the downward leader propagating toward the
ground. Both downward and upward leaders converge until interception occurs, where
a conducting path between the cloud and ground is completed to convey the lightning
current to ground.
For a given rod, the upward-directed streamers start from the rod at downward leader
heights that depend on the lightning current. The higher this current, the higher is the
location where the upward- and downward-directed leaders meet each other and the
more is the protection area around the rod. Energizing the rod influences the onset
condition of upward streamers and the radius of protection. This improves the protection
efficiency of the rod in capturing the downward leaders