[TowerTalk] Rotator Lightning Protection

[Roger (K8RI) on TT]

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)

.
On 5/15/2015 10:38 AM, Paul Christensen wrote:
> N4CC and I have finally completed our remote Internet station in 
> Hilliard, FL.  Probably half our time spent before and during 
> construction was studying and implementing lightning abatement.  
> Ground rings were created around each tower, and the communications 
> shelter.  All grounds, including the adjacent electrical service 
> ground are brought together at an external ground bus (EGB).  In 
> total, 55+ ground rods are used with four rods each 24 ft.  Lines are 
> bonded at the top and bottom of the towers.  We do not disconnect 
> anything.  Everything stays up and running 24/7 regardless of the WX.
>
> Poyphaser rotator MOV protection devices are installed at the base of 
> each tower.  At the EGB, we're using an Array Solutions model. In 
> looking at all the failure modes, I missed one:  If the MOV on the 
> rotator return lead was to short to ground, it creates a disastrous 
> situation where the prop pitch will turn freely beyond the electrical 
> stop point.  The MOSFET is controlled by a PWM circuit and the 
> duty-cycle is what varies motor speed.  A shorted MOV will cause the 
> lines to break apart up the tower at the coax loop.  The prop pitch 
> would stop at nothing and keep turning.  Do the MOVs in these units 
> generally fail open or closed?
>
> We're using a pair of M2 PCX2800 controllers.  To help protect the 
> MOSFET device from a similar "short-to-ground failure" during a 
> lightning event, I designed a circuit that engages a vacuum relay such 
> that the MOSFET is only exposed to the outside world during 
> rotation.   It's not a 100% guarantee against MOSFET failure, but 
> should help to mitigate damage.  This is an add-on that's similar to 
> what K7NV did with the Green Heron prop pitch controller model.
>
> Here's my thought for a potential fix:  While another vacuum relay 
> could be added on the +48V supply side to the prop pitches, I could 
> isolate circuit ground from chassis ground on the secondary side of 
> the controller's power transformer.  The primary would still be safety 
> protected and meet UL. Isolating circuit ground from chassis ground 
> would inhibit rotator turning in the event of a MOV failure on the 
> return line.
>
> Anyone been through this?  I welcome comments on any better ways to 
> manage this.
>
> Paul, W9AC
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-- 

73

Roger (K8RI)


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