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 MIL-HDBK-419A
If
is 25 ohms while
is only 1 ohm or so, then a lightning strike as indicated could easily cause the
potential of the overhead ground wire to become high enough to produce an arc across the transformer windings
and insulators. Since the low voltage secondary side offers a lower impedance to earth, it is the preferred path
for the discharge.
Figure 3-11. Capacitive Coupling of Lightning Energy
This type of lightning threat can be minimized by (1) reducing
to approximately the magnitude of
(2)
the installation of appropriate lightning arresters at the transformer to keep the potential difference between
the power conductors and the ground wire and between the primary and secondary windings to within the stress
ratings of the transformer, and (3) interconnecting the earth electrode subsystem (to include the water and
other utility pipes) with a 1/0 or larger buried copper cable as illustrated by the dotted line in Figure 3-12.
Interconnecting the ground electrodes of the building and transformer pole to form one effective earth contact
does not eliminate the lightning threat to the buried cable between the two buildings. As shown, the cable
shield is connected to the cabinet, i.e., the building ground. In the event of a lightning strike as shown, Building
1 and its power supply system will be elevated in potential relative to Building 2. In particular, if the distance
between the two buildings is more than just a few meters, the inductance, primarily, of the cable shield will
prevent the cable from providing the low impedance necessary to keep the two buildings at the same potential.
In addition if the shield of the cable is insulated from the earth, as is usually the case, the potential of the cable
shield can become high enough with respect to the earth to exceed the breakdown of the insulation.
3-22
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