[TowerTalk] Rotator Lightning Protection

[Paul Christensen]

Roger,

Tnx for the MOV info.  I'm also considering the addition of a second vacuum 
relay inside the M2 prop pitch controller.  That would actually be the 
easiest solution and not be subject to an accidental chassis to circuit 
ground path during a strike event if I were to do as earlier discussed 
(i.e., isolate circuit from chassis ground).  So, with two vacuum relays: 
(1) the MOSFET is protected by the existing vacuum relay; and (2) the second 
vacuum relay would apply +48V to the winding only during the time that the 
rotator turn is engaged.  In fact, the two vacuum relay coils can be 
paralleled from the same source.

Tnx to all who provided ideas.

Paul, W9AC


> On May 15, 2015, at 9:11 PM, Roger (K8RI) on TT <K8RI-on-TowerTalk at tm.net> 
> wrote:
>
> Having worked with literally thousands of MOV,s from little ones up to 
> 50,000 Joul ratings, They will typically fail shorted, BUT I have seen 
> many fail open.  It depends on the power contained in the "final spike" 
> that causes them to fail.  From power line spikes and nearby lightening 
> strikes the ODDS ARE it/they will fail shorted.  If enough power/energy is 
> contained in that final spike at failure time, it becomes likely they will 
> fail open and  that anything on that line will fail as well.
>
> Based on experience, you can expect the MOV to fail shorted and it will 
> PROBABLY  protect any electronic device connected to it. That being the 
> case, the prop pitch motor will PROBABLY need some sort of "fail safe" to 
> prevent over rotation.
>
> That's a lot of probables, but it's unfortunately a fact of life, leaving 
> you playing the odds which favor the MOV failing shorted, but with no 
> guarantee it will do so.   If the MOV works as we all hope, then you will 
> need some sort of "fail safe"
>
> Since the big ground system was installed my tower has been hit at least 7 
> times that can be verified with nothing disconnected and NO MOVs in the 
> rotator lines.  No damage was seen.
>
> I can not say the same for my computers and network.  A nearby strike did 
> nothing to either station, but it sure cost me a lot in network and 
> computer damage.  The CAT-5 lines go through protection at the UPS for 
> each computer.  The connectors were melted and the wire contacts welded 
> together at one UPS.  The router, 8 port switch and most NICs were shot!
>
> So, lightening and MOVs are a crap shoot.  They will protect you from the 
> smaller stuff, but cause other problems. They are unlikely to protect you 
> from the larger stuff.
>
> In the end, I'd use them, plan on the MOVs failed as a short, and hope for 
> the best  There is no guarantee, but you have tilted the odds in your 
> favor..
>
> 73
>
> Roger  (K8RI)