Page 228 - Electrical Safety of Low Voltage Systems

Safety Against Overvoltages
211
FIGURE 12.11 Closed metal loop formed by the down-conductor and the gas
pipe.
in air, of the loop, per unit of length given by Eq. (12.3):
0 a
L l = ln (12.3)
2 r
5
where 0 is the magnetic permeability of the air, r is the radius of
the down-conductor, and a is the distance between the surface of the
vertically developing metal part (i.e., the natural gas pipe in Fig. 12.11)
and the center of the cross section of the down-conductor.
If the induced overvoltages exceed the impulse withstand capa-
bility of the separation means between the down-conductor and the
natural gas pipe, which we can consider as electrodes, a lateral spark
may occur across the gap a that separates them. This sideflash may be
extremely dangerous, as it can have sufficient energy to trigger fires
or ignite explosive atmospheres eventually present.
The dangerous discharge between the aforementioned electrodes
can be prevented if they are separated from each other by a safety
distance s. In fact, the intensity of electric fields existing between two