Polyphaser after lightning strike(s)

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Bob W
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Polyphaser after lightning strike(s)

Post by Bob W »

Well, it's getting to be that time of Year again. Lightning Season. We all know about the importance of using lightning suppression on feedlines. What we may not know, and what I found out the consequences of, is whether that surge suppressor - a PolyPhaser in my case - is any good. I've learned that these have a finite, unspecified life expectancy.
This looks like a happy, healthy PolyPhaser. It's not.
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high rez picture here: http://n2qew.dyndns.org/Polyphaser/Pix1.jpg

Inside, you can see that the isolation capacitor has flashed over.
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high rez picture here: http://n2qew.dyndns.org/Polyphaser/isocap.jpg

Note the zorch mark from the brass strip on the center conductor of the equipment side to the case. That went on to adjust the front end of the attached radio.
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high rez picture here: http://n2qew.dyndns.org/Polyphaser/Flashover.jpg

This all likely occurred because the gas tube took too many shots, and opened. Something that you'll never know until you take the next shot, or do some periodic testing. You can use a low current megger that can do 1kv to find the breakover voltage. I think a new one is around 400 volts. One with a broken gas tube won't breakover with a 1 kv megger. Of course, this means removing it from the transmission line to test. A broken gas tube has no effect on RF match, so you won't find it that way.
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high rez picture here: http://n2qew.dyndns.org/Polyphaser/Gastube.jpg
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kcbooboo
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Post by kcbooboo »

Interesting.

I have an Alpha Delta lightning arrestor on my 100w UHF repeater and it, among other things, was generating noise in the system. If I unscrewed the element from the aluminum block, I'd get scratching noises while doing so. Since the entire unit is only $50, I bought a new one. It does NOT have this problem, so all I can figure is that something must have happened to the first one, although it had no effect on received or transmitted signals if you just left it alone. Wiggling the screw-in element or even unscrewing it WOULD generate noises. Weird ! There was no other evidence that my system took a hit or even a nearby discharge, although the unit had been in service for about 9 months.

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poly

Post by repeaters-r-us »

I'd like to know what type of antenna and how far from the top of the tower it was.
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kcbooboo
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Post by kcbooboo »

Super Stationmaster PD455-5 for 440-450 MHz, installed in early 2002. Sidemounted about 6-10 feet from the tower with an anti-sway bar at the top. The antenna is mounted 400 feet up a 600 foot tower whose base is at 700 feet ASL. The tower is VERY well grounded and there's an FM broadcast antenna on a 30 foot pole atop the tower. 500 feet of 7/8 Cellflex coax runs into the building and is grounded at the base of the tower. A short piece (8 feet) of 1/2 inch Heliax connects the 7/8 to the lightning arrestor at the repeater which, unfortunately, has NOT been directly/securely connected to the building's internal ground system. It receives its ground through the power cord right now.

Bob M.
Bob W
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Post by Bob W »

Mine was attached to a high split - 173 - Vhf Super Stationmaster. Top mount, 400 foot tower, no lightning rod above it, 7/8" hardline. It's been up there for over 20 Years. Antenna is now toast. This tower takes lightning strikes on a regular basis - as do most in the Carolinas. Generally, lightning strikes are detectable as marks on light metal to metal contact in the buildings. Everything entering our buildings is bonded to via sheath ground to an outside window, and via polyphaser to an inside window. Tower is bonded to the grounding system via it's own path, building windows both go to that same point via their own cables. My point of posting this was Polyphaser's reaction. Basically, if you know the tower has taken a strike, replace the Polyphasers. We'd be doing nothing but replacing them during May, June, July, August, and September. I asked about hipot testing them, and they must have been thinking of their surge generator when they said I'd damage it testing it. I clarified that I was talking about the normal, low current (<1ma) variety. They said that would work ok.
RFdude
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Hi Pot Tester almost Free

Post by RFdude »

Poor man's discharge tube tester:

Take apart an old photo flash unit. The disposable camera kind work well. With alligator clips, connect the Device Under Test across the large capacitor. DANGER! Its OBVIOUS: Don't touch HV leads when capacitor is charged. Then simply connect DC power and let her go!!!! When it gets to the breakdown voltage, you will know! You can hear the pop.

Many DVMs have a 1000V range on them now. Use MAX / MIN and you will know exactly and with repeatability the voltage your device will break down at and what voltage it quenches. Useful also for testing neon tubes, telephone surge devices, etc. Maximum working voltage is only about 420V.

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tvsjr
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Re: Hi Pot Tester almost Free

Post by tvsjr »

RFdude wrote:Poor man's discharge tube tester:

Take apart an old photo flash unit. The disposable camera kind work well. With alligator clips, connect the Device Under Test across the large capacitor. DANGER! Its OBVIOUS: Don't touch HV leads when capacitor is charged. Then simply connect DC power and let her go!!!! When it gets to the breakdown voltage, you will know! You can hear the pop.

Many DVMs have a 1000V range on them now. Use MAX / MIN and you will know exactly and with repeatability the voltage your device will break down at and what voltage it quenches. Useful also for testing neon tubes, telephone surge devices, etc. Maximum working voltage is only about 420V.

Educational value! Not a play thing. Keep away from children or facsimilies thereof. Have someone close by who knows CPR.

RFDude
You're kidding, right? Let's see, you're using clip leads that, at best, have a breakdown voltage of 300V, and you're going to run 600+V through them? Plus leave HV out in the open?

This sounds like a Darwin Award waiting to happen.
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kcbooboo
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Post by kcbooboo »

The gas discharge tube should flash over at or below 400 volts, so the fact that the test leads aren't good for much higher than that is of no major concern. Plus it's a ramped DC voltage that will go back to zero when the arc happens. I think you'll find that the test leads are a lot better than you rate them for. You won't be holding anything in your hands during the test, so even if you put everything on a metal benchtop and it arcs to that, you will still be safe.

Now, if he had been using an old 21 inch round color TV high voltage supply (25kV), THEN I'd be worrying.

Bob M.
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batdude
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Post by batdude »

Generally, lightning strikes are detectable as marks on light metal to metal contact in the buildings


could you expound on that?


i guess i never see such things as most buildings i deal with are the cement block style

i have only seen one lightning damaged "radio" - it was a 100kw FM station that had visible damage all over the inside of the final PA area...blew up 1/2 the transmit combiner too.... all this on a 1500 foot tower.


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kcbooboo
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Post by kcbooboo »

I hope that kind of damage never happens to the facility I'm at. They've had nearby lightning hits that have taken out phone line protectors and a remote control phone line interface, but the RF stuff has been unaffected.

I can't say that my equipment was hit, but several pieces of coax, including the jumpers on the duplexer, were all generating noise when I flexed them. Nothing else seemed to be wrong however. Lightning really is a strange beast.

Bob M.
Bob W
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Post by Bob W »

batdude wrote:Generally, lightning strikes are detectable as marks on light metal to metal contact in the buildings

could you expound on that?

doug
Sure. Waveguide comes into the building, and has a ground clamp on it at the point of entry bonding it to the entrance ground window. Waveguide continues on and a pressure window / connector is on the end of the waveguide. Connector (huge chunk of brass) is in contact with the ladder rack. After a good shot to the tower, you can pick the connector up and look at where it was touching the ladder rack, and see arc marks. Both items were bonded to the same point a few feet back which removes most of the voltage difference, but there's still some current available.

Cool pictures......
Everyone seems to think of lightning in terms of voltage, but there's a bunch of current available too. A #6 wire can carry the entire current of a lightning strike without burning open. If you do make it jump a gap - like a poor connection - the results can be interesting. The transformer below is normally about the size of a 55 gallon drum. The walls are about 1/8" thick steel. It's typical of the type of transformer that's on a pole to serve around 5 houses. It had an external lightning arrester that apparently failed open. Next shot flashed over in the can. The normal primary is fused at 4 amps (13,200 volts), so there's not enough energy available from the normal voltage source to do anything violent. This is purely due to a lightning strike. (We contemplated asking ABB if it was still under warranty ;-) )

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kb0nly
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Post by kb0nly »

:o

Wow!
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mr.syntrx
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Post by mr.syntrx »

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spareparts
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Post by spareparts »

Now consider the cleanup if that transformer was filled with PCB based oil. I would bet that there are still a few in service in some markets.
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mr.syntrx
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Post by mr.syntrx »

20 years ago back when my old man was a broadcasting tech, PCB cleanups meant putting on a pair of overalls. Quite a scary thought.

There are still PCB filled transformers in use in remote locations etc.
Bob W
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Post by Bob W »

spareparts wrote:Now consider the cleanup if that transformer was filled with PCB based oil. I would bet that there are still a few in service in some markets.
Most of the grey ones out there are not. I can tell you that the one in the picture had none. Where I work, all of the new ones that the manufacturer has certified has none in it get a blue dot. All in service transformers were looked at for certification, and a blue dot was applied. All transformers for which certification could not be verified were tested. Those that are below the limits got a blue dot. Those that were above didn't and their locations noted for replacement. ALL capacitors were verified as being below, or replaced. Capacitors split their guts open alot more often than transformers. Also, while PCB oil is toxic, you really don't want to be anywhere near it if it's burned. It turns into dioxin - agent orange.
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Post by nmfire10 »

I love this video. I gave it to one of the guys I know at the UI and was big hit in his office.

Video: Substation goes nuclear
http://205.243.100.155/frames/mpg/XfrmBlast1.mpg


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