UFC 3 -520-01
June 10, 2002
for this configuration. Each parallel lead develops a voltage drop in addition to the
voltage drop across the sur ge protector. The total let-through voltage is the sum of the
three voltage drops.
Figure B -6. Lead Length Effect on Let-Through Voltage
Line
Voltage Drop
Lead
Across Leads
Inductance
Total Let-
Voltage Drop
Surge
Through
Across Surge
Protector
Voltage
Protector
Lead
Voltage Drop
Inductance
Across Leads
Neutral
B-8.2
As the lead length is increased, the added inductance increases the voltage
drop in proportion to the lead length, with the result that the let-through voltage
increases. For example, a surge protector connected by 305 millimeters (12-inch) leads
might allow an additional 200 volts of let-through voltage compared to an equivalent
surge protector with 152 millimeters (6 -inch) leads. The equation for voltage drop as a
function of surge current is given by:
di
V =L
+ iR
dt
EXAMPLE: At the typical surge current frequency, the inductance per foot is
near 0.25 x 10-6 henries. The surge current usually has a rise time of 8 x 10-6
seconds. In the above equation, the voltage generated by iR is negligible
compared to the voltage drop across the inductance. Assuming a surge current
of 4,000 amperes, the lead length voltage drop per foot is estimated by:
(
)
4,000
V = 0.25 10 - 6
= 125 volts per foot
-6
8 10
Notice that the voltage drop becomes linearly larger for larger surge currents.
The inductance per foot varies with wire gauge size, but this variation is not
significant compared to the increase in inductance with length.
B-13
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