Re: I National Electric Code

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>

One of the reasons why the clearances are so big for the higher voltage
stuff is probably so you can work on it (potentially while energized)
without worries of flashovers.  When I started looking at the OSHA rules, I
couldn't figure out why they wanted such big clearances for seemingly low
voltages.  Then, I found an "opinion letter" in the OSHA web site (such
letters are issued to clarify or interpret an OSHA reg for a specific
instance).  It indicated that the guidelines were developed considering a
nominal length for a worker's arm, how far they might move inadvertently,
etc.

As David points out, switching surges and lightning transients also enter
into it.  Even when operated totally indoors most commercial gear running at
>2 kV is connected to what's outdoors (i.e. distribution network) by a
pretty low impedance path.

Another place to look for regulatory guidance is the rules surrounding
things like Xray machines.  They use clearances much less than 2" for 70 kV
relative to ground.