Re: I National Electric Code

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 3/17/01 7:45:13 PM Eastern Standard Time, tesla-at-pupman-dot-com 
writes: 







>
> 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. 



I guess there is some difference in the rules for the "innards" depending on 
whether a TC is considered as an energy-using appliance (transmitter, X-ray 
machine, TV, etc.) or an energy-converting device (distribution transformer, 
generator, etc.) or am I reading it wrong? 
Matt D.