On 9 Nov 2005 03:17:47 -0800, "SpamHog" wrote:

In my case, my shack is at the ground floor of a 9-story building, the
antenna is on top, and all grounds are pretty heavy duty and bonded -
but not low impedance.

I had assumed that you lived on the ground level and had the antenna
feed point somewhere further in the garden.

In your actual case, assuming that the separate grounding wires come
directly down from the roof and connected to the grounding of your
apartment at the ground level and assuming that the lightning bolt
rise time would be 1 kA/us. A thick wire has an inductance about 1
uH/m, thus, there would be a voltage gradient about 1 kV/m along the
grounding wire. If the antenna system grounding wire from the roof is
30 m long, the isolation transformer primary side potential would be
30 kV above the building neutral bar and also 30 kV above your
apartment potential as well as 30 kV above the isolation transformer
secondary. The building ground bar potential will be somewhere above
the average potential of the surrounding countryside.

With 50 kV isolation at the transformer, nearly 2 kA/us rise times
could be tolerated. To reduce the grounding wire inductance, several
grounding wires would be required well separated from each other.

Paul OH3LWR