On 7/1/2012 3:52 PM, Jim Brown wrote:
> On 7/1/2012 7:28 AM, Pete Smith N4ZR wrote:
>> I'm not sure exactly where I should post this, but suspect that this
>> group probably has more relevant expertise than most, so please bear
>> with me.
>>
>> In the last year, I have had two expensive episodes. In both cases, my
>> transceiver's RS-232 transceiver was fried during a lightning storm, and
>> several knowledgeable people suggested a difference in chassis potential
>> between computer and transceiver as the reason. Their prescription for
>> avoiding a repetition was simple - connect the chassis of all of the
>> units to a single-point ground.
Like Jim, I'm a strong proponent of the Single Point Ground (SPG) but
there are additional factors to be taken into account when other
services are taken into account, even if they all enter through a common
point. Three major ones are telephone, cable, and TV.
Even if they enter at the same point the telephone may connect to a TV,
Satellite receiver, and computers. Computers may be tied into the
common point ground, but if they have a network, or cable connection, it
likely follows a different path. TV antennas be they satellite, cable,
or plain off the air (OTA) antennas "should" enter the house through the
same grounding panel as the ham equipment.
The problem is with coax to the TV, the telephone lines, network cables,
and coax for the cable service are usually of different lengths and
follow different paths. The TV may be in a different part of the house,
or on a different floor. The coax to the TV may branch to many rooms,
and network cables may do likewise. I have three that go to my shop
that has its own electrical feed. Even a ground that traverses the
periphery of the house can have a huge voltage difference between one
corner of the house and another. It's that these lines don't just carry
the voltage from a nearby lightning strike into the house, they act like
an antenna themselves, or if you will, the windings in a transformer and
the voltage is directly induced into them, *INSIDE* the home.
BTW the antenna ground system connects to the grounds for both the house
and shop, forming a large network of ground cables and rods of over 600
feet of bare #2 cadwelded(TM) to 32 or 33 8' ground rods.
These voltage differences on ground are not normally a problem by
themselves, but when we also have network and coax connections running
direct between them, the voltages between say a network cable and
chassis ground can be thousands of volts. The wiring in older homes can
be a real crap shoot as who knows where the stuff runs or how long the
run. It's common to find the lights in one room and the outlets in
another sharing a common circuit. It saves a circuit, and saves money
from the permit. But as homes age these circuits seem to take on a life
of their one. I've mentioned it before, but I started tracing a circuit
while doing some rewiring. The circuit runs from the breaker panel, up
through the wall and through the attic to a junction box (where wires
are simply twisted together using wire nuts) Now the wiring *may* run
from outlet to outlet, outlet to junction box to outlet, for the room or
it may be a combination. Then it runs over to another room where one
Romex Runs down through the wall to several outlets and then "through
the wall to the bathroom. From there it goes to a common switch box
which feeds fans and lights, but from that box there is also a run to
the outlets AND from one outlet the wires go through the wall to feed
the outlets on the South side of my den. So this single circuit goes to
at least 4 different rooms through at least 3 (or more) junction boxes
and directly from one room to another through walls at least twice.
Now all the chassis are connected to the SPG, but the AC lines in the
Den are 3 circuits of much different lengths. So the chassis are all
common (more or less), but the electrical feeds may be many thousands of
volts different. The same is true of the network and USB cables.
I started rewiring to add circuits and to equalize circuit lengths. It
would take probably 4 more circuits, cost $300 to $400 with $80 of that
being to the township for permits. First will be to get the 3 circuits
into the den the same length and follow the same route. Following
backwards will be to isolate the bathroom into a single circuit. Then
the living room and back bedrooms will require an additional circuit so
the standard outlets and lights will cross between them and no wires
from either into the bathroom.
There is no 100% guarantee but I believe the more I do the more it puts
the odds in my favor. The idea of the ground (and equal length
circuits) is not to keep everything at ground potential, but to make
sure (or as sure as can be) that everything going into a piece of
equipment is at the same potential at the same time.
73
Roger (K8RI)
> I'm a strong advocate of that, but there's more to it. The fundamental
> problem is that we have many pieces of equipment that have unbalanced
> connections between them, and that are connected to ground at different
> points. A lightning event induces current in those ground wires, with a
> resulting IR drop in the grounding conductor. The DIFFERENCE between the
> voltage on one chassis and another appears on the low voltage connection
> between the equipment and smokes it. The difference between the power
> outlet and our shack ground can also smoke equipment,
>
> Exactly the same thing happens with leakage current on power wiring,
> causing hum and buzz to be added to unbalanced wiring between
> equipment. The solution is quite simple, and applies equally to your
> problem. 1) Bond chassis to chassis from all interconnected equipment
> using short, fat copper. I've generally recommended either braid
> stripped from transmitting coax or #10 stranded copper. 2) Get power
> from all interconnected equipment from outlets that are in the same
> electrical backbox, or that share the same green wire, or that have
> their green wires bonded together. 3) Bond from the collection of
> equipment chassis to the common point where your antennas are grounded,
> again by short fat copper. 4) Follow NEC requirements for bonding
> carefully. This includes bonding EVERYTHING to the power system ground
> by short, fat copper -- your station grounds, CATV, satellite, telco,
> cold water (if conductive), building structural steel, and lightning
> rods, if any.
>
>> My solution - which I'm asking people to critique - was to fabricate an
>> L-shaped, roughly 3x5 foot ground bus made from 3/4" copper tubing,
>> which I mounted on the back of my L-shaped operating desk. All joints
>> are silver-soldered. I then connected each of the affected units to the
>> bus with very short and heavy stranded wire, and connected the end of
>> the bus to my grounded shack entry panel (in a double-hung window).
>>
>> The DC resistance of the ground bus is very low, but the length
>> approximates a quarter wave on 10 meters,
> IEEE research says that the energy in lightning is very broadly centered
> around 1 MHz, with lots of energy at least a decade of frequency above
> and below. At these frequencies, impedance is dominated by inductance,
> not resistance. Thus short is better.
>
>> and with the units connected
>> near the ends of the bus, I wonder if I'm feeling a false sense of
>> security about the likely behavior of the bus during a nearby lightning
>> event.
> What you've done is very good, but I would ADD the short fat bonding
> between interconnected equipment. Contrary to popular myth about "ground
> loops," the added bond REDUCES noise and the ightning voltages between
> equipment that causes destructive failures.
>
>> Would I be better off (or no better) running heavy conductors
>> from each unit to the entry panel, even if they would have to be
>> similarly long? Or should I just give up and plan on disconnecting the
>> RS-232 connection whenever weather approaches?
> We MUST connect every AC outlet to the entry panel -- it's the "green
> wire" -- and it MUST be bonded to the chassis of all connected equipment
> that is required to have a 3-wire plug. The only AC-connected equipment
> that does not require that bond are products that are double insulated
> to prevent the possibility of shock. This equipment will have a 2-wire
> plug. Lots of wall warts and computer power supplies have this
> exemption. BUT -- the chassis of equipment powered by these power
> supplies still needs to be bonded.
>
> 73, Jim K9YC
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