Re: [TCML] surge arrestor for pole pig

["J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>]

"25+" is fairly amusing :-)  Nice job repainting it!

Lots of high voltage arresters have series spark gaps built into them, so "gap" is for real.  In the old SiC days (1930's to 1970's), it appears that this was mostly to deal with the durability of SiC, which didn't like being continuously exposed to power frequency current.  But spark gaps have their own durability issues; very high fault currents could "fry" them and change their characteristics such that they would trip under normal operating conditions and/or fail short.  That's why gapless MOV arresters came into being.  The ZnO MOV elements can withstand power frequency current continuously without significantly reduction in lifespan, and then you don't have to worry about gap failures.  But it turns out that gaps improve the clamping characteristics under certain circumstances, which is why arresters with series air gaps are still used in certain applications.

I would be reluctant to use ceramic arresters in place of a safety gap for the same reason ceramic arresters have generally been abandoned by the power industry in favor of polymers:  Pressure caused by arcing inside the ceramic kind may cause them to shatter violently!

Back when I first started collecting and reading about arresters, I thought the idea of using one in place of a safety gap was a cool idea.  The more I read about them and ponder this application, however, the more skeptical I become.  As a secondary measure, maybe.  Like in a Terry Filter.  But without a safety gap?  I don't know.  A properly-set gap ought to clamp down faster.  Plus you'll see it operate.  If the arc fails to die of natural causes, it won't destroy your safety gap as it might an arrester.  The fault-clearing capability of the SiC or MOV elements doesn't seem like a big advantage to a Tesla coiler; if you're worried about clearing faults in your safety gap as quickly as possible, you've probably got other design issues you should be confronting instead!  My $0.02, anyway.

Cheers,
Aaron, N7OE

--- On Thu, 9/18/08, Yurtle Turtle <yurtle_t@xxxxxxxxx> wrote:

> From: Yurtle Turtle <yurtle_t@xxxxxxxxx>
> Subject: Re: [TCML] surge arrestor for pole pig
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Date: Thursday, September 18, 2008, 3:36 AM
> When I "liberated" my Westinghouse 25 kVA 14.4 kV
> pig:
> 
> http://www.hot-streamer.com/adam/garage/pig_14.4kV_25kVA_1.jpg
> 
> http://www.hot-streamer.com/adam/garage/pig_14.4kV_25kVA_2.jpg
> 
> http://www.hot-streamer.com/adam/garage/pig_14.4kV_25kVA_3.jpg
> 
> it was mounted on the pole with a "Surgemaster LV
> Arrester USA 7-78 18 kV Gap 1":
> 
> http://www.hot-streamer.com/adam/garage/gap_18kV_1.jpg
> 
> http://www.hot-streamer.com/adam/garage/gap_18kV_2.jpg
> 
> Since 14.4 kV RMS is over 18 kV Peak, I'm assuming the
> arrester is also rated in RMS. I know we use 15 kV arresters
> at work for 12.47 kV.
> 
> I thought it was interesting that the arrester calls itself
> a "gap". A quick Google search indicates that
> "Surgemaster" was apparently never copywrited,
> making it difficult to find anything on it.
> 
> However, I'm using mine on my coil without any problems
> so far, even when pushed to 280 volts.
> 
> Adam
> 
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