Tesla chit chat

  Date: 04-03-95  11:35
  From: Terry Smith                           
    To: Richard Quick                             
  Subj: Tesla Chit Chat
EW> There is a Technical information chart in the Belden Catalog
EW> that gives the max RMS operating voltage of coax. Poly- 
EW> ethylene (PE) insulated RG-8 = 5000 volts. Foamed Poly-
EW> ethylene (FPE) insulated RG-8 = 600 volts. Some RG-58 FPE 
EW> insulated is good for only 200 volts as 1900 volts for RG58
EW> PE. RG-213 PE insulated = 5000 volts. 

My Belden catalog shows 300-3700 volts for these cables, RMS.  A
tiny cable like RG-174 shows 1,100 volts with 20 mils of di-
electric, while RG-8, 213, etc. show 3,700 for 100 mils of PE, or
600 volts for foamed. The voltage specs are NOT listed with
primary cable specs, but separately in a mil equivalency
related chart.  I interpret that to imply a worst case com-
bination of measured RMS carrier voltage in AM use, over a range
of -55 to +80 degree C and 0-100% humidity, with flexing and
sustained water exposure.  After all, we know that foamed poly is
much better than solid for RF, except when the holes fill with
water. The same logic about corrosive deterioration is why tinned
copper in some RG-58's exist.  They spec less well based on new
cable, but don't degrade as much in some applications.  Silver
platings exist for a combination of that reason, skin effect, and
tolerance of some teflon chemistries.  Voltage ratings may also
reflect a working safety margin, either Belden or mil chosen, and
a thermal limit of an extreme attenuation frequency translated
from power at the cable's nominal impedance.  

 RQ> I would have to say that these ratings are much too          
 RQ> conservative. Any 

I'd say there is unspecified fine print to explain them, mostly
not relevant to an assumed indoor non-water submersive DC or low
frequency application.  

 RQ> neon shop will sell 15kv rated wire, and it will hold up to  
 RQ> 15kv. But this wire is thinly insulated, leaving little      
 RQ> safety margin. By

If you mean the typical GTO-15, that's about 85 mils of
insulation (Plain single layer PVC?) around a 14 AWG stranded
conductor.  There are many thinner high voltage wires made, such
as TV anode wire, with a PVC jacket over a PE, insulator, or high
voltage test probe wire, which adds a conductive wrap for corona
distribution to the conductor inside a similar two layer
insulation and environmental protection jacket.  Belden rated
working voltages are a peak voltage of half the typical breakdown
voltage of the insulation.  Belden's 80,000 VDC breakdown 18 AWG
test wire uses 57 mils of PE between the corona guard layer over
the conductor, and a 15 mil protective PVC outer jacket.  They
use 46 mils of PE for an Underwriters Labs Recognized 60 KVDC
wire, 44 mils for 48 KVDC, and 27 mils for 35 KVDC.  Obviously
there are some other design art factors which make this a
nonlinear relationship.  

 RQ> comparison RG-213 PE makes the 15kv rated wire look like     
 RQ> bell wire for a 24 volt buzzer circuit. My materials and     
 RQ> electrical references are in agreement that polyethylene has 
 RQ> a dielectric strength of 1000 volts DC per mil. I halve this 
 RQ> voltage rating when running AC across the dilelectric, so I  
 RQ> figure that this material has a 60 cycle breakdown voltage   
 RQ> of about 500 volts per mil. Any guesses on how many mils
 RQ> radius the PE insulation extends from the wire core on       
 RQ> RG-213 coax?

 RQ> Over 80 (closer to 100 mils). 

98 mils.  The inner conductor is .089, and the dielectric OD is

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

Other than corona hot spots, that might handle 120 KVDC, since
it's PE.  

 RQ> BTW, the insulation on RG-213 PE coax is over three times    
 RQ> thicker than the insulation on the 15kv rated wire used in   
 RQ> neon work. This is also in agreement with a value closer to  
 RQ> 40kv minimum breakdown voltage.

The PE dielectric is less likely to track than the plastic used
in GTO-15. GTO-15 also has to be abrasion and chemical resistant,
without an outer jacket. In sign work, there are also handling
factors such as PK housings.  

I see little advantage of RG-213 over good quality PE RG-8/u for
your work. Belden 8237 (RG-8/U) has 100 mils of PE, the same
conductor guage and shield DCR as RG-213, and a 2 ohm impedance
difference (52 vs 50). The only benefits spec'ing RG-213 offers
are a non-contaminating additive in the outer jacket to lessen
fungus growth, and the lack of availability of that nominal cable
type in many garbage quality brands.  Uniform tolerances useful
in preventing voltage spike arcover points (and maintaining
uniform RF impedance for normal users), and enough copper to
reduce losses and improve RF shielding, are equally present in
Belden RG-8/U and RG-213, but lacking in many cheaper RG-8's
such as Rat Shack's (and some recognized commercial brands).  

Using a coax with a shield and outer protective jacket is much
like the electric utility practice of using sheathed underground
high voltage feeders. The shield, while not used as in RF to
maintain a constant impedance energy transfer, is a safety drain
if a spike punches through the primary insulation. 

Terry  (203)732-0575 BBS (1:141/1275)

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