[TowerTalk] RG-213 Hi Pot ?

[jimlux]

On 5/10/19 3:56 PM, Gedas wrote:
> Have a look at some of these cables.
> 
> https://www.awcwire.com/productspec.aspx?id=high-voltage-coax-15kvdc
> 

At 15kV kinds of levels, the cheap solution is what is called GTO-15 - 
it's used for wiring neon signs, and easily takes 15 kV RMS (so about 
20kV peak).  No shield.  It looks and feels like RG-59 or RG-58 solid 
dielectric with no shield (black outer jacket, PE insulation, stranded 
copper core).

There's some much nicer silicone GTO-15 that is more flexible. There's 
also a variety of HV cables used for applications like CRT anode 
voltages back in the color TV days.  Sometimes you can find a surplus 
spool of 30kV or 45kV rated silicone insulated wire.  You want to avoid 
the old surplus synthetic rubber insulation - by now, it's almost 
certainly got microscopic cracks in the dielectric, and tiny cracks in a 
wire carrying 30kV can lead to "exciting" failures, especially if you've 
got any stored energy in the system.


Shielded cables are a curse and a blessing - if the shield is grounded, 
it can take more voltage because of the field control. On the other 
hand, if you've got lots of stored energy, that shield can provide the 
return path for an exciting energy dissipation event.  Furthermore, the 
coax itself stores significant energy - at 30pF/ft, a 10 foot length is 
300 pF, and at 40 kV that's about 6 Joules - enough to make a nice snap 
or burn something out.

If you're working with fast pulses, you also have to worry about 
reflections from an impedance discontinuity inadvertently doubling the 
voltage (or worse). There's a whole literature on HV pulse generators 
using pieces of coax as transmission line capacitors. (see, e.g. 
Blumlein)  You can do some very cool stuff in terms of very fast, short 
pulses with HV, coax, and spark gaps (or Krytrons and similar triggered 
gap devices) as switches.



> Gedas, W8BYA
> 
> Gallery at http://w8bya.com
> Light travels faster than sound....
> This is why some people appear bright until you hear them speak.
> 
> On 5/10/2019 12:55 PM, Steven Katz wrote:
>> The mil spec rating for RG-213/U is 3700Vrms max; this is 5232Vpk.
>>
>> I wouldn't use it at 7.7kVdc.
>>
>> -----Original Message-----
>> From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of Jim 
>> Thomson
>> Sent: Friday, May 10, 2019 9:52 AM
>> To: towertalk at contesting.com
>> Subject: [TowerTalk] RG-213 Hi Pot ?
>>
>> Does anybody know how much max V is on  RG-213 ?   Like what are the 
>> results if a hi-pot tester used..
>> and say cranked up to the point where  just a few ma occurs .     I 
>> have an application where I need a  HV
>> cable, less than 10 ft, that will  easily handle 5700 /  7700  vdc.   
>> It will be hardwired at both ends, no HV connectors.
>> And with braid peeled way back from the center conductor, like  2-4 
>> inches .
>> The plan was to also wrap some red electrical tape  every few inches  
>> along its  entire  length, to readily identify it as a HV cable.
>>
>> Tnx....  JIm   VE7RF
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