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Author Topic: Gas Discharge Tube Surge/Lightning Protectors After They Go Poof  (Read 931 times)
NY2KW
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Posts: 34




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« on: November 04, 2017, 06:51:03 AM »

Long introduction - please bear with me then 2 brief questions.....

I was moving QTH's and removed my copper ground rod mounting plates with attached Polyphaser IS-50-UX-C0 protectors for the new home.  I had several antenna's (dipoles and verticals) and as polyphaser recommended, the coax connection AND body of the surge protectors were also covered with their waterproofing tape for our harsh New England weather.  At the new QTH I will have a remote tuner with T bias, so I checked to see if these models had DC pass through.  All were blocked but one had a resistance reading of about 500k ohm !  I opened that one up and found a nice blown up cap and gas discharge tube with a charred interior.   All good, it did its job..... BUT I had no clue operationally that the protector was no longer doing its job.  On the OCF dipole this protector was protecting, I had no SWR clue that there was a problem.

How often do you routinely check these surge protectors for their integrity?  That requires pulling off the weatherproofing, coax and metering it.  Even doing it monthly, it seems you run the risk of being unprotected for signficant length of time.  Are there better protectors without gas discharge tubes?


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WB4SPT
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Posts: 477




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« Reply #1 on: November 04, 2017, 07:04:12 AM »

First thought. Put these in a box. Mine are.  Second thought.  I design quite a bit of equipment involving these GDT’s.  They have a unique characteristic of having an almost perfect open circuit with about 1pf of capacitance.  And some many hundreds of momentary current amps rating.
Dont expect lightning protection little parts to survive strikes.
I’ve opened other designs using neon lamps and toroids to earth and both of those failed too.  That did give vswr feedback though.  The problem with shunt lightning protection is that you need low z from dc to 1mhz while presenting an open circuit at your lowest oper freq. so a linear at 160 m is an issue for a non spark gap type of shunt element.   GDT’s are much like air spark gaps but more stable and more predictable. But they do fail if hit hard enough. Btw, you can test if you have a hi pot tester.
 Hi power models have a 800 or 1000v GDT.  usually a 10% tolerance.
« Last Edit: November 04, 2017, 07:08:07 AM by WB4SPT » Logged
NY2KW
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Posts: 34




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« Reply #2 on: November 04, 2017, 10:04:50 AM »

Weatherproof box would make that all easier including the coax connections but still have to go out and test them.  How often is neurotic and how often is reasonable.  In my sarea T-storms are fairly common in all 4 seasons.  I was hoping there was a more durable solution.  Would a true air spark gap be better?
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WB4SPT
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« Reply #3 on: November 04, 2017, 10:30:58 AM »

Tough to beat air gaps for reliability. Blitz bug in your future?
Cant do hi pot for some reason from indoors?  Works except for grounded antennas.
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DL8OV
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« Reply #4 on: November 04, 2017, 11:20:18 AM »

Why not use an outside box with a clear cover?

Peter DL8OV
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NY2KW
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Posts: 34




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« Reply #5 on: November 04, 2017, 02:09:41 PM »

Ideally, the lightning/surge protector should be as close to the ground rod as possible.  Best if it's mounted right onto the ground rod as Polyphaser and Alpha-Delta show in their literature.  I'm tempted to just keep it exposed to the weather and check it periodically.   Ignorance was bliss back 50 years ago when I was a novice and and just brought the coax or ladder line right into my bedroom window hihi

Jerry NY2KW
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W6EM
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« Reply #6 on: November 04, 2017, 05:42:52 PM »

Tough to beat air gaps for reliability. Blitz bug in your future?
As long as the air gap hasn't been widened by a high-current flashover, such as from a nearby strike.  The rating is a function of the gap, which may grow, if flashover current is high.
Quote
Cant do hi pot for some reason from indoors?  Works except for grounded antennas.
Hi potting can be dangerous in and of itself.  First, you should have some idea of acceptable insulation resistance as a function of test voltage.  In AC systems, generally, about 1 Megohm per kV of rating is deemed acceptable.  Also, everything else connected with be subjected to the test voltage, so if it can't take the typical test voltages of 250 or 500VDC, don't try a system test.....and obviously, one has to be very careful.

Since most won't have a decent hi-pot test set at their disposal, a Megger or equivalent high voltage DC ohmmeter should suffice.  Set the test voltage just under the flashover value.  For example, if the Megger test voltage selected is 500VDC, 500K ohms would be acceptable.  And, no, measuring a smoked gas tube with a multimeter isn't the same thing.  I'd be willing to bet that the smoked gas tube would not indicate 500K when 500V is applied.....probably a short.

Lee
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WB4SPT
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« Reply #7 on: November 05, 2017, 01:20:10 PM »

Lee. I think u are describing insulation resistance test. Typically done at 500V.  I’m describing a current limited test to prove the GTD still works.  For a 1kV tube, it should arc at 700 some rms volts.  I just saw a fluke dmm that had this feature.
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W6EM
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« Reply #8 on: November 05, 2017, 05:35:38 PM »

Lee. I think u are describing insulation resistance test. Typically done at 500V.  I’m describing a current limited test to prove the GTD still works.  For a 1kV tube, it should arc at 700 some rms volts.  I just saw a fluke dmm that had this feature.
Yes, but one that is stressed, per se.  A Megger typically allows you to choose the test voltage.  250, 500, and 1000V are typical.

A Hi-Pot test set would have higher discharge current available than would a megger's internal generator and probably would allow you to ramp up the voltage..  You could breakdown the tube, which is what you want to do, with a megger's generator, but you couldn't ramp it up to determine that actual flashover voltage.



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G3RZP
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« Reply #9 on: November 06, 2017, 01:50:37 AM »

A proper ionisation tester has a very limited current capability but is sensitive to the ionisation that precedes breakdown. They appear occasionally at hamfests and the like - mine only goes up to 5kV, but needing rebuilding, cost under $10. It consists of a power oscillator at around 240kHz using a 6F6, an EHT rectifier as used in tube TV sets and a three tube AF amplifier - two 6SJ7s and a 6J5, which feeds a speaker so you can hear the start of breakdown. Well worth $10, a few hours work and some paint!
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WB4SPT
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Posts: 477




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« Reply #10 on: November 07, 2017, 06:09:19 AM »

A proper ionisation tester has a very limited current capability but is sensitive to the ionisation that precedes breakdown. They appear occasionally at hamfests and the like - mine only goes up to 5kV, but needing rebuilding, cost under $10. It consists of a power oscillator at around 240kHz using a 6F6, an EHT rectifier as used in tube TV sets and a three tube AF amplifier - two 6SJ7s and a 6J5, which feeds a speaker so you can hear the start of breakdown. Well worth $10, a few hours work and some paint!

That's impressive sounding!  does it come with Tesla's autograph?   Wink
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KG7A
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« Reply #11 on: November 07, 2017, 06:42:40 AM »

Thanks for this topic!!!  Very Interesting.  I'd better go check mine...
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