In the case of long transmission lines, or extensive networks



I04 Three-Phase Transmission

of extra high-pressure cables, we have considerable capacity
and self-induction, and when such circuits are carrying heavy
currents at the instant they are interrupted, the total energy
stored electro-magnetically in the field surrounding the con-
ductors of the circuit will appear as potential energy, charging
up the circuit to a very high voltage.
Taking as an example the case of an overhead transmission
line, we may assume the self-induction L of one line wire per
mile to be .0037 henry, and its capacity 0.8 x lO"^ farad. If
a short circuit occurred, causing the interruption of the line
when the current had an instantaneous value of 150 amperes,
we get for the value of iLC^ the electro-magnetic energy of the
circuit : —

.0037 X ^150; =42.4 watt-seconds.
2

Now upon the interruption of the line this energy must
appear as a charge increasing the potential of the circuit ; in
other words

JKV2 = 42.4

= V ^ .8
= 103,000 volts.
Experiments made in America upon a line 100 miles in
length, short circuited at the far end, resulted in a power surge
which jumped spark gaps 4J in. in width at the generator end
of the line. Allowing 24 kilovolts as an ordinary value of the
dielectric strength of air, this sparking distance would in itself
correspond with an instantaneous pressure rise of 108,000 volts.
