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Re: [TowerTalk] questions about lightning suppressors

To: "Tower" <>
Subject: Re: [TowerTalk] questions about lightning suppressors
From: "Roger (K8RI)" <>
Date: Tue, 18 Sep 2007 23:12:04 -0400
List-post: <>

> I'm a little confused about lightning suppressor theory.
> Many of us have installed lightning suppressors from Polyphaser, ICE, 
> K5FD,
> etc. to protect control lines running from towers to the shack.  The
> Polyphaser units contain MOVs that turn on at 82V. ICE offers two MOV

I don't have my GE manual at hand but IIRC:
Something to remember about Metal Oxide Varistors. They are a whole 
conglomeration of diodes mechanically mixed and packed together. Unlike 
Zener diodes that have a more or less set voltage, they do as you say, start 
to conduct. Normally when an MOV  actually does something they receive 
*some* damage so they tend to age over time, depending on how hard and often 
they are used. If they take a healthy whack from a nearby strike (or closer) 
they may end up a one-time-use. Polyphasers and the GE MOVs are like this. 
I'm not familiar with the other brands.

> voltage levels, 50V and 20V.  K5FD offers three voltage levels: 65V
> (standard), 35V and 17V. I have the Polyphaser and K5FD standard units in 
> my
> suppressor network.
> So, if components in a control box are connected to these suppressors, the
> voltage on the pins can rise as high as 65V-82V with respect to ground

It's the job of the suppressors in this case to bypass most of the voltage 
spike while absorbing the energy within it,  before it gets to the unit 
connected to it. It may or may not do anything for other things in the 
shack. If you are using DC the MOV can be as little as 20% above the 
regularly applied voltage. With AC you need at least 1.414 times the RMS 
value plus about 20%. Each conduction will tend to lower the starting 
voltage a little (or a lot) depending on how hard it has to work. So you end 
up having to decide just how much slack (the 20% in this case), you are 
willing to give between your operating voltage and what will do damage. As 
far as a direct hit, the MOV is usually replaced by a scorch mark and a pair 
of bare wire leads.  I've been within 6 feet of one rated at 50,000 jouls 
when it left leaving a pair of #8 wires sticking straight out.  It's quite 
spectacular, but a bit hard on the ears...and nerves... and attached SCRs.

> before the suppressor starts to do its job. Some bipolar transistors can
> take that kind of voltage but many FETs and ICs can't. While the 
> suppressors

Also that rated voltage is the knee of the curve. It isn't a cut off as with 
a Zener diode. The MOV is rated in volts and jouls or the amount of energy 
it can absorb.  As the voltage continues to rise the resistance basically 
goes down until the MOV is destroyed. Most, but not all I have seen fail did 
so shorted. OTOH a few did fail open (not counting the ones where only the 
leads were left)

If the strike is close enough (not necessiarily a direct hit) to impart 
enough energy the MOVs  will probably do little to save the equipment, but 
they do increase your odds from lesser strikes.

> probably keep things from melting and vaporizing, isn't it probable that
> semiconductors attached to the line will be damaged?

That's one of those, "It all depends" but in general the answer is probably 

However, I do not use them in the lines to the rotator and with all the 
verified hits since it's been up, there has been no damage to any rotator or 
control box due to lightning. I should note that more than likely that tower 
has taken more direct hits since it's been up than most ham's towers do 
druing the hams entire life. I did have one Polyphaser short out (in the 
coax from the 440 array) and I had one F to F N-connector fail at the 75 
meter half wave sloper.
> I ask because we took a massive lightning hit here about three weeks ago
> that damaged at least a dozen boxes in my shack. I've got an extensive
> ground system and suppressor network. The only way I can explain some of 
> the
> damage is that the suppressors allowed the voltage to rise too high for 
> the
> semiconductors connected directly to the control lines. Does this make
> sense?

MOVs are not infallable.

> I'll post a complete report on the damage, which was extensive, my theory 
> on
> the path taken by the surge(s) and the recovery when I get a chance. Too
> busy fixing things right now.
> 73, Dick WC1M

73 and good luck on the restoration.

Roger (K8RI) 


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