Re: flashover

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Herwig,

	High voltage RF is attracted to insulators due to the dielectric effects.
Even though the PVC is in insulator it can still easily attract arcs.  The
current will be very low but that arc can then travel further from there or
eventually breakdown the insulator (corona).  In general, it is best to let
air do as much of the insulating as possible and then let the insulator do
the rest.

	Often, RF insulators have to have holes in them to lower the current flow
to ground.  I have seen various insulating plastics attract RF arcs and
burn up just from the current flowing into them due to charging of the
dielectrics and how those charges flow to the space capacitance around the
insulator.  It is a little hard to explain in text, but the effects are
very real and not obvious unless you really get into how dielectrics behave
in high voltage RF fields.  Air is the best insulator you have.  that's the
best I can do to explain it here with a keyboard.  It really takes lots of
diagrams and hand waving ;-))

You should round off the top of the insulator too so there are no sharp
edges.  The currents can really concentrate at sharp edges and dramatically
increase the change of arcing.  All this is especially true for CW coils
that can really heat insulators.

Cheers,

	Terry

	

At 07:55 AM 1/6/1980 +0100, you wrote:
>
>Terry Fritz wrote to the list:
>> Thus letting the air and distance do most of the insulating work. 
>
>Terry,
>
>Because I'd like to understand the physical background of your 
>advice:
>
>The punch through voltage of insulators like PVC is much higher 
>than that of air. Wouldn't the screening effect be better at a place 
>where the E-field is stronger (closer to the secondary)?
>
>Could you please enlighten me?
>
>Regards,
>
>Herwig
>