[TowerTalk] Fwd: Grounding of tower

[Jim Lux]

On 5/6/13 5:27 PM, K8RI wrote:
> On 5/6/2013 6:48 PM, Hans Hammarquist wrote:
>> Maybe you can count on a "nice" weld if/when lightning strikes. It,
>> sure, will jump that little distance.
>
> Yup, sure will and in the process apply many thousands of volts to
> equipment tied to that ground.  Poor grounds are the cause for many
> equipment losses.  It doesn't have to be a gap, just a few ohms.  Just
> think if a few ohmd resistance in the ground connection can cause a
> great deal of damage, how about a small gap?
>
> For the moment, let's look at that gap.  Say it only has 300 volts
> breakdown and lets ignore even that.  The lightning flashes across the
> gap creating a momentary plasma of just 1 ohm.  The lightning strike is
> not a strong one, but develops 5000 amps for only a millisecond. During
> that one thousandths of a second the wiring in the house is E=IR or 1 X
> 5000 = 5000 volts above ground
>


actually, the voltage rise due to ohmic loss is a tiny fraction of the 
rise due to L*di/dt.  Typically, a lightning stroke might have a di/dt 
of 10kV/us for a smallish stroke.  With 1 microhenry inductance (about a 
meter of any conductor, strap, wire, any size) that's 10kV.