Don (resqcapt),
I have to agree with you on the distance from the lines and the chance of flash-over.
15" sounds more like it, there is no way that an arc could bridge a gap like that, at a low voltage like that.
To be honest, the RH can have an effect on the Flash-over distance, but, you should not be using plant that close to the lines that Flash-over is an issue.
Over here in NZ, you are required to maintain a distance of 4 metres (12ft) from any O/H lines, when using plant that could contact the conductors.
As a side-line, the PoCo I work for will sleeve any lines for free, as in, we don't want our Network damaged either.
Mind you, most accidents involving HV lines from trades staff, the PoCo are never even asked what the risk is, until something goes wrong. [Linked Image]