[TowerTalk] Lightning Protection

[K8RI]

On 7/6/2013 9:22 AM, James Wolf wrote:
>> Lightening protection is pretty well standardized and there is plenty of
>> information available.  There is no 100% guaranteed, fail safe
>> protection even for the 24 X 7 stations, but they are well protected
>>from most strikes.  It's just that there is no guaranteed protection
>>from the so called, "super strikes" that can be many times more powerful
>> than your run of the mill, every day strikes.
>
> Roger,
>
> I totally agree.  All you can do with lightning is do your best to manage
> it. Twenty some years ago, after just putting up the tower I took a strike
> and toasted most everything in the shack as well as burning a hole in a
> water pipe touching rebar in the slab.  I read Polyphaser's ( Now
> Transector)  "Grounds" booklet and installed as good a ground system as I
> could. I have a 120 ft Rohn 45 about 100 ft from the house with 27 stainless
> steel ground rods under it. My entrance window (panel) has around 8 more
> ground rods connected to it.  I don't know how many strikes it has taken
> since, but in one night, my neighbor saw the tower take two direct strikes.
> I have a DX spotting cluster so I want to have it connected while I am not
> here.  I have never since disconnected anything - there is just too much to
> manage without getting complacent.  All the equipment is connected with 2"
> copper ground straps connected to the entrance window. The complete shack is
> powered through a Polyphaser AC line protector - also grounded to the
> entrance window.  The house entrance electrical box is a few feet from the
> radio entrance window and is connected to the ground system.  The HDR-300
> rotor is at the 60 ft level and the control lines go through a box of MOV's
> and is grounded at rotor level as well as at the ground window and
> additional MOV's mounted inside the rotor control box.  Believe me, you
> don't want to buy repair parts for that controller - don't ask me how I know
> that.
> Since then,  I have had damage only to the antennas.  One of the loading
> coils on the Cushcraft 40 was just missing after a strike
>
> Through my occupation, I talk frequently to the Transector Rep.   He tells
> me that over time  (< 5 years) MOV's lose their effectiveness.  I've not
> heard this before.  If this is true, need to get on that.
>

MOV's are as the name implies, a metal oxide Varistor, but the actual 
construction is a huge number of tiny metal oxide diodes.  For the MOV 
to work, it has to reach the break down potential of a bunch of these 
tiny diodes. When it does, at least a few of them short out reducing the 
break down voltage of the device, slightly even if it was only static 
buildup.

Over time there are usually many events of short duration, but each 
takes a toll. Although each toll is tiny, even miniscule, they add up, 
slowly reducing the breakdown voltage from the original until the device 
begins to draw current at the circuits operating voltage.  Needless to 
say, once this point is reached, the degradation proceeds at an ever 
increasing speed (avalanche)with the device being destroyed.  Generally 
they short, but if enough power is available they can fail open, 
sometimes in spectacular fashion.  It depends on the device's rating in 
joules and the current available.
The breakdown current, unlike a diode with a sharp knee is a gradual, or 
rounded knee with the current increasing at a relatively gradual pace as 
the voltage rises.

If you can find a copy, GE wrote a very good manual(the bible so to 
speak) explaining the operation of MOVs back in the 70's, "I think"
A search returns many references to this book, but I could find none 
that work or don't charge.  I'm sure there must be free downloads 
available "that work" and if you wish to find out how MOVs work, there 
is no better source.  I still have my original copy and it's a large 
format a good half inch thick.

BTW I have seen, USED MOVs for sale. Purchasing a used MOV is definitely 
not a good idea, but most purchasers "and sellers" are not aware of how 
the MOV operates.

73,

Roger (K8RI)


> Another note:  KA9A, when he lived in town had his shack on the second floor
> of his garage with the tower next to the garage.  He took a strike on the
> tower and everything was disconnected from the antennas and the AC power.
> Yet almost everything was destroyed.  I had loaned him a 24V power supply
> which was sitting on the floor at least 2 ft from anything and nothing
> connected to it.  The fuses in the DC power line (again, not plugged into
> anything) were blown and the supply trashed.  I suspect that the EMF from
> the proximity of the tower was the culprit as the strike found its path down
> the tower to a single ground rod.
>
> Jim, KR9U
>
>
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