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High Voltage Coax


Quoting Eugene Welcome, and talking about using coax for high voltage 
applications:

RQ> To say that simply because they are electrically equivilent
RQ> for signal transmission is one thing: we are using the material
RQ> for a purpose for which it was not designed or rated.

EW> There is a Technical information chart in the Belden Catalog that 
EW> gives the max RMS operating voltage of coax. Polyethylene (PE) 
EW> insulated RG-8 = 5000 volts. Foamed Polyethylene (FPE) insulated 
EW> RG-8 = 600 volts. Some RG-58 FPE insulated is good for only 200 
EW> volts as 1900 volts for RG58 PE. RG-213 PE insulated = 5000 volts. 

I would have to say that these ratings are much too conservative. Any 
neon shop will sell 15kv rated wire, and it will hold up to 15kv. But 
this wire is thinly insulated, leaving little safety margin. By
comparison RG-213 PE makes the 15kv rated wire look like bell wire
for a 24 volt buzzer circuit. My materials and electrical references
are in agreement that polyethylene has a dielectric strength of 1000
volts DC per mil. I halve this voltage rating when running AC across
the dilelectric, so I figure that this material has a 60 cycle break-
down voltage of about 500 volts per mil. Any guesses on how many mils
radius the PE insulation extends from the wire core on RG-213 coax?

Over 80 (closer to 100 mils). 

This gives me a practical HV rating between the center conductor and
the shield of about 40,000 volts minimum.

BTW, the insulation on RG-213 PE coax is over three times thicker than
the insulation on the 15kv rated wire used in neon work. This is also
in agreement with a value closer to 40kv minimum breakdown voltage.
Of course I could perhaps understand the descrepancy if we looked at
HV RF, which would require many more mils per volt to contain.

EW> I have seven 0.1 mfd 5000 VDCW extended foil Part# 0F50-104 and 
EW> two 0.1 mfd 4000 VDCW extended foil Part# 0F40-104 capacitors 
EW> made by Plastic Capacitors Inc. of Chicago Ill. They are assembled 
EW> in glass tubes of 1.25" diameter by 2 inches long for the 5KV and 
EW> 1X2" for the 4KV. There is a clear oil and the metal end caps are 
EW> solder sealed to the glass. There is a 1/2" X 8/32 thread stud on 
EW> each end. I have some lower cap values in higher KV ranges that 
EW> are built the same except they are 1/2 to 5/8 diameter by 5 or 6 
EW> inches long. Can these be of any use for your work? 

Doubtful, these sound like filter caps off of the X-ray equipment
you were describing. These caps are not designed to withstand the
severe pulsing that is normal in the Tesla tank circuit. It is my
understanding that these caps heat up rapidly when placed in the
Tesla tank circuit, and with no room for expansion, the glass cases
rupture explosivly.

Richard Quick



... If all else fails... Throw another megavolt across it!
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