Roger,
Just a few remarks:
1. YES, all I/O cables between the house and outside world need to pass
thru a common grounding panel. The fact that I have 2 such panels, at
opposite ends of the house, does not seem to be an issue with respect to
lightning. I think that is because both panels are securely bonded to that
perimeter ground wire system. The point here is that no I/O cable should
be unprotected.
2. Since you have a large, distributed system, have you considered using
either isolation transformers/baluns between parts of the house or other
buildings? An alternative would be to go wireless with wi-fi. Either
method would get around the long conductive path issue. I think the wi-fi
would be a better way to go, as baluns or transformers could arc over
internally.
3. I totally agree with your last paragraph. You can be assured that
"ground" will not stay at 0 volts during a lightning strike. It will
bounce, a lot, and what matters is that everything must bounce together.
Remember, if there is just 1 ohm of resistance/impedance in the strike
path, and allowing for the strike currents to divide into 2 or more paths,
even a modest 20,000 amp flow in some part of the ground system will put
that ground at 20,000 volts above and below 0 volts reference. (Lightning
strike current follows a decaying sine waveform once it is coupled onto
conductors.)
I had the privilege of working alongside one of the aerospace industry's
top lightning experts when I was at Rockwell Collins. I showed him the
damaged antennas and cables from my tower after the big blast I took in
2008, and his estimate was that my system had taken a heavier than average
hit - probably 40 to 60 kA. A "normal" strike is 20 to 40 kA, and
super-cell blasts can exceeed 100 kA.
73, Dale
WA9ENA
> [Original Message]
> From: Roger (K8RI) <k8ri@rogerhalstead.com>
> To: <rfi@contesting.com>
> Date: 7/1/2012 6:07:58
> Subject: Re: [RFI] Single-point grounding
>
> 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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> >
> >
>
>
>
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