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Lead wire on a surge arrestor

Lead wire on a surge arrestor

Lead wire on a surge arrestor

(OP)
Hi,

We have a power pole with a surge arrestor on it.  The surge arrestor will redirecting a lightning strike to ground to prevent our electrics from being damaged. I am trying to determine the maximum length the lead wire can be so that its inductance doesn't interfere with the surge arrestor.  This is what I have found:

PM = Protection Margin: set to 0.1
Vp = Voltage drop per a foot of the lead arrestor wire =6KV/FT
BIL = Basic Insulation Level of the power cable = 8KV
Va = Arrestor switch voltage = 18.5 KV



The equation I was using was

PM = (BIL - 2*VA - L*VP)/BIL

I solved for L with the above values and got about 5ft.  Is this correct or is there a table someplace I should be using?

RE: Lead wire on a surge arrestor

Seems to me that

PM = (8 - 2*18.5 - 5*6)/8 = -1.125

As long as your arrester switch voltage is more than half the BIL, you have no protective margin no matter how short the lead is.

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