[Sneap] RG58C/U max dc voltage?

[Klaus Bahner]

Hjalmar,

RG 58C/U is usually not specified by the vendor for high voltage
applications. Typically you will find some maximum voltage specs for such
cables at for example 600V, because then it is easy to meet required
approvals (like UR, VDE, CE etc.). Imagine that 99.99% of all coax cable is
used at very low voltages, so why should any vendor go through a
qualification process for higher voltages. They will never get their
investment back.

That being said, coax cables are intrinsically safe to use for low power HV
applications as long as their outer shield is intact *and* connected to a
good ground source. This way, you don't have to care much about a vendors
specification from a safety point of view. This is probably the reason, why
coax cable has been chosen as high voltage cable in the first place. As a
practical matter I've seen RG 58 C/U been used up to voltages of 30kV DC.
This can be done because the insulator of a good coax cable is PE, which can
withstand high DC voltages rather reliable. (It's the preferred for high
power, high voltage cables nowadays).

On the other hand, you are using these cables completely beyond their specs,
this is, it's you and only you who can be blamed for it, if it goes boing.
And beware, there is some truth in the HV power supply vendors opinion that
you should *not* use ordinary coax cable for high voltage applications, but
buy their specifically designed and priced(!) high voltage coax cables. The
truth in it is that there are some cables which actually will fail over time
or at very low voltages, especially since the electronics components market
has been swamped with crap made in China.

I suggest to use your own validation process, buy cable from a trusted
source/vendor and then make a few test as to breakdown voltage of this cable
and maybe keep a log about long time performance. Once you've found a cable
that meets your requirements, make this cable your in-house standard up to
your in-house standard max. Voltage.

I use RG-58C/U up to maximum 15 kV, but with respect to lifetime I feel
better if I can limit it to 10 kV. For higher voltages I prefer using
thicker (higher imepedance) coax cable. For pulse applications I would lower
these limits further, but I think RG58 should be OK for your purpose.  


Although you are probably aware of those problems, I want to add that there
are two potential problems associated with using coax cable for such HV
pulse applications. First of all your cable must be properly terminated
(which I think is hard to do, at these voltages), if not you risk having a
reflected 8kV pulse of opposite polarity, which can easily kill your
pulser's driver stage. Secondly, remember that coax cable has a rather high
capacitance (~100pF/m). Just having 1m cable, would require 8kV *
100pF/100ns = 8A pulse current, which means your pulser must be of the hefty
league if you don't want the cable to affect your rise and fall times. Also
consider that such high ripple currents may easily overload connectors
and/or the cable itself (conductance of the inner lead).

Regards,
Klaus Bahner

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> -----Original Message-----
> From: sneap-bounces at tunl.duke.edu 
> [mailto:sneap-bounces at tunl.duke.edu] On Behalf Of Hjalmar Bruhns
> Sent: 12. marts 2008 14:46
> To: sneap at tunl.duke.edu
> Subject: [Sneap] RG58C/U max dc voltage?
> 
> The host for the 2008 SNEAP annual meeting is the University 
> of Michigan at Ann Arbor.  The meeting dates are: October: 10 
> - 16, 2008.
> The meeting web link is:
> http://www-ners.engin.umich.edu/research/Mibl/Sneap/2008.htm
> 
> 
> Dear SNEAPers,
> 
> Does anyone know how much DC voltage a RG58C/U cable is 
> spec'd to handle?
> 
> SHV-10 connectors like the Kings 1065-1 
> http://www.kingselectronics.com/HighVoltage/connectors/tabid/5
> 2/laproduct/351/default.aspx
> are rated to 10kV "for use with RG58C/U cable". But the bit 
> of info I find on RG58C/U concerning voltage is "Max. Voltage 
> 1900 Volt" - although that probably applies to RF 
> applications, not DC.
> 
> We intend to switch our ~8kV on and off with up to 5kHz using 
> HV switches that have ~100ns switching times.
> 
> Can we trust RG58C/U to handle that?
> 
> Thanks,
> 
> Hjalmar
> 
> --
> Hjalmar Bruhns
> Columbia Astrophysics Laboratory
> Nevis Laboratory/Cyclotron building
> 136 South Broadway
> Irvington, NY 10533
> tel: +1-212-854-1449 (Campus)
>       +1-914-591-2874 (Nevis)
> fax: +1-212-854-8121
> email: hbruhns at astro.columbia.edu
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