[TowerTalk] STEPPIR QUESTION

[Rick Stealey]

Dick -
Your scenario sounds plausible to me.  From the standpoint of MOVs.
You have 82 v MOVs at the base of the tower and at the shack on a good ground.  Fine.
Your tower takes a hit and a portion of the current flows on each SteppIR wire, dividing up.  Let me take a stab at some numbers, say 24,000 amps of strike curent, 2000 amps on each wire.

Now, whether it would survive that or not I don't know, it depends on the rating of the MOVs but if the continuous rating of the MOV is 80 volts then with a surge it might clamp at an even higher voltage because there is a continuous voltage rating and a surge voltage rating on an MOV.  Let's say some portion of the current goes toward the house and that's what blows the fuse.  Then the MOV at the house lets voltage up to 82 volts get through to the controller and zap !


If you were using a Littelfuse V39ZA20P MOV (look up in Mousers catalog
and follow the "data sheet" link to get specs) rated at 2000 amps (strike
current and duration ~20 microseconds) and 30 volts continuous it
would clamp at about 80 volts.  My understanding of that spec is it will clamp at 30 volts with a small current continuously, and as much as 80 volts if it is being slammed.  So the MOV at the tower might survive and maybe not, but it has taken most of the brunt of the strike and a much smaller current heads toward the house which the next MOV can handle - hopefully clamping it at 30 volts.

I can't see any reason to use 80 volt MOVs on the SteppIR cable.
I don't know, but in this forum collective wisdom sometimes prevails, so let's see what we get.  We probably should examine the 80 volt (spike) rating on the MOV I mentioned and see what it means exactly.  Hopefully it means it survives that, or possibly degrades slightly.  If so it saves the controller, and lives to do it again.

The steady current performance of the above MOV can be easily measured by limiting the current to 1 ma and reading the clamp voltage.  On the above MOVs that would be ~30 volts. 

Rick  K2XT

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