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.

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:

+ *iR*

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

= 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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