Electric Power Generation, Transmission, and Distribution

5% of records at least 10,500 A

10% of records at least 6,000 A

50% of records at least 1,500 A

7. Percent of distribution arresters receiving lightning currents at least as high as in Col. 4 in
   Table 23.2.


8. Number of distribution arrester operations per year (excluding repeated operations on multiple
   strokes):
   a. Average on different systems—0.5 to 1.1 per year
   b. Maximum recorded—6 per year
   c. Maximum number of successive operations of one arrester during one multiple lightning
   stroke—12 operations

23.1.16 Arc Impedance

Although arcs are quite variable, a commonly accepted value for currents between 70 and 20,000 A has
been an arc drop of 440 V=ft, essentially independent of current magnitude:
Zarc¼ 440 l=Il¼length of arc (in feet) I¼current
Assume:
IF¼5000 A¼I
Arc length¼2 ft.
Zarc¼ 440 (2=5000)¼0.176V ;Arc impedance is pretty small.
Let us say you have a 120 V secondary fault and the distance between the phase and neutral is 1 ft. If the
current level was 500 A then the arc resistance would be (4401)= 500 ¼0.88V, which is significant in
its effect on fault levels.

TABLE 23.2 Lightning Discharge Current vs. Location

Col. 1 Col. 2 Col. 3 Col. 4

Urban Circuits (%) Semi-urban Circuits (%) Rural Circuits (%) Discharge Currents (A)

20 35 45 1,000

1.6 7 12 5,000

0.55 3.5 6 10,000

0.12 0.9 2.4 20,000

0.4 40,000

Approximate Saturation Curve

Core-Type Transformer

Times Normal Exciting Current

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1 2 3 4 5 6 7 8 9 1011121314151617181920

Times Normal Voltage (RMS)

FIGURE 23.11 Transformer saturation curve.