A timely subject for me, what with Spring just beginning here in the
Northeast.
I agree that the cables coming from the tower should be grounded to the
bulkhead panel.
I have a related question about the equipment inside:
I'm working on two basement patch panels, one for the shack and one for the
family entertainment center, that let me quickly disconnect *all* equipment
from *all* lines entering from outside the house, including coax, control
lines, phone lines, TV antennas and satellite TV. I'm also installing
several AC pull-disconnect boxes next to the patch panels so I can cut off
the AC feeds to the shack and entertainment center as well. Although not
ideal, the pull boxes have plastic insulation between the conductors and a
larger air separation between the conductors than a circuit breaker. The
reason I'm using the patch panels and pull-disconnects is that there are
simply too many cables in the shack and entertainment center - about 60 of
'em -- to quickly disconnect or unplug, and many of them are hard to access
-- behind furniture, etc.
But although I can cut off every cable entering from the outside, I can't
easily disconnect the ground leads. This applies to the AC ground (the
pull-disconnects are designed to leave the AC ground connected), the telco
ground, and the heavy 1/0 lead coming in from my single-point ground. So,
all of the equipment will still be connected to the ground system, mostly
through three-prong power plugs and/or ground straps to the chassis.
Seems to me the main risk is that my ground system won't be able to absorb a
really big surge. In that case, the voltage rise on the equipment grounds
could cause damage. I have a pretty good ground system. The towers have 12
eight-foot ground rods each, connected with radials, and I have a bulkhead
panel and single-point ground outside the shack bonded to the AC and telco
grounds.
How much risk is there that I'll get a surge large enough to overwhelm my
ground system? Is it worth devising a method to disconnect the AC ground and
single-point ground leads running into the shack?
Those who are interested can read below how I've converted from relying
solely on suppressors to disconnecting everything.
73, Dick WC1M
My sad lightning story:
Last year I took a lightning hit that caused about $10,000 worth of damage.
All signs are that it wasn't a direct hit, but a nearby strike that induced
a surge in my 110' Rohn 55 tower. Although there was one vaporized PCB trace
in the tower-mounted stack switch, and a carbon trail inside one feedline
connector at the top of the tower, there were no other cases of
vaporization, melting or burn marks. A number of devices were taken out by
invisible damage to semiconductors directly connected to control leads from
the tower. By far most of the damage was from the surge propagating from
those pieces of equipment, down their RS-232 cables to my computer (killing
the motherboard and most of the I/O cards), and from there back out the
other RS-232 cables to the rest of the devices connected to the PC. I lost a
Ten-Tec Orion, a rig RS-232 interface (saved my FT-1000D, an Alpha 87A, an
Acom 2000A, a late-model computer, three SteppIR controllers, two Green
Heron controllers and a Wri. various rotor and antenna controllers, a stack
switch, etc, etc, and that was just the stuff in the shack. Also lost a CD
player, a postage scale, even a garage door opener. In all, about two dozen
pieces of equipment had to be repaired or replaced. Insurance covered it,
but I don't want to go through that nightmare ever again!
This was the first time I've taken a bad hit in over 10 years of having
towers at this location. I have two main tower locations, each of which has
12 eight-foot ground rods connected with 1/0 radial wires. I have Polyphaser
coax suppressors on every RF run and MOVs on every control line. Identical
set of lightning suppressors are installed at both the tower and shack end
of the cables (225 and 265 feet of separation for the two tower locations,
respectively.) In all that time, only one device got damaged by lightning --
a homebrew switching matrix that had CMOS chips connected directly to the
control lines for SO2R switches located at the towers. I had to replace
those chips just about every time we had a lightning storm.
However, during that 10-year period, the towers were all under 70 feet high
and surrounded by tall trees. Last year I put up 110' of Rohn 55 in the
middle of a cleared field. Not surprisingly, it's more of a magnet for
surges. Despite having dual sets of suppressors on all coax and control
lines between the new tower and shack, I took a lot of damage from the big
hit. One reason is that some of the new controllers I've installed have
delicate semiconductors connected directly to the control lines from the
towers, and those devices can't tolerate voltages much above their operating
norms. It's unlikely that the MOVs can protect these devices. Second, I
believe one of the controllers I use has a path from the control lines to
its RS-232 interface, which is how the surge propagated to my computer,
killing it, and thence to many other devices connected to the computer via
RS-232. I guess my previous equipment was much more tolerant of whatever
voltage snuck past the MOVs, and didn't have pathological connections to
other devices.
After that experience, I decided to reverse my long-held philosophy that
it's possible to rely on a superior ground and suppressor system. I decided
to implement master patch panels for my shack and entertainment center
equipment, so I can quickly and easily disconnect every cable coming into
the two main equipment rooms. My plan is to leave the station disconnected
most of the time during lightning season, and disconnect both the shack and
entertainment center whenever we leave the house overnight. If, for some
reason, I forget to unplug, or a storm comes up suddenly, the suppressor
system will be the second line of defense.
> -----Original Message-----
> From: Kimberly Elmore [mailto:cw_de_n5op@sbcglobal.net]
> Sent: Monday, April 07, 2008 3:30 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Lightning...
>
> It's severe storm time in the Southern plains and, while I've taken,
> and am still taking, steps to protect my shack from lightning damage,
> I began to wonder...
>
> I always disconnect all transmission and control lines from equipment,
> but what should I do with them after that? Should I make connectors so
> that I connect everything to the station earth ground or should I let
> them float? Especially if I disconnect all *equipment* from earth
> ground, should I still connect all transmission and control lines that
> enter the shack to earth ground when I'm not operating?
>
> Kim Elmore N5OP
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|