On 9/18/2012 10:46 AM, Dale Svetanoff wrote:
Pete,
Two questions: 1) That 40 feet of bare #2 wire between the entrance panel
and power line ground - I assume it is run outside, and if so, how many
ground rods are on it? 2) Does the wire provide all of the grounding for
the entrance panel, or do you have one or more ground rods tied to the
panel?
This brings up a number of thoughts and questions.
Around here, code requires the ground from the panel AND meter are
relatively short. I don't remember the required minimum distance but
IIRC it's less than 8 or 12 feet and if additional rods are required
they must be within that distance. Even with our soil which is wet most
of the year 3 ground rods are required. I had to put in 5/8 X 8,
however the required entrance rods are skinny 3/8 X 8.
While I understand and agree with your comment about non-ideal
installations, I think the point has to be made that trying to accomplish
as good of an installation as possible should be a prime goal of every
radio installation design. The two most important factors, in ANY case,
are: A) Keep the lightning current on the outside of your
shack/house/vehicle;
Induced voltage may be higher inside the house than out if a strike is
close and on the side opposite the entrance panel. But hopefully all
runs are the same length and voltages rise together.
B) Design for as low an impedance as possible between
the entrance panel and earth ground and between tower (or antenna support)
and ground. Naturally, net Z will be the total of Xsub-L + wire resistance
+ bonding resistance + earth coupling resistance. Net Z should be figured
for frequencies in the range of about 100 kHz to 10 MHz, the main spectral
distribution of lightning energy. (Yes, it extends upwards to nearly 100
MHz, but at greatly reduced energy levels.)
Here they "specify" the grounding and it's nowhere near what I'd use.
It has to be a #6 green insulated wire and it must be continuous which
means scraping insulation, yet they use mechanical connections which are
usually loose enough to just lift off the rods after a year or two.
OTOH you can add additional wires of larger size and there is an
additional ground out at the power pole.
But when looking at total Impedance and rise times, with the proper
conditions you can have far more voltage induced into the wiring in the
house than comes into the panel under some conditions.
The goal is not only grounding, but wiring in such a fashion that all
equipment connections will rise at close to the same rate, to the same
potentials. The outlets in a room should be on the same circuit, or
parallel circuits where the wiring follows the same path with the goal
of all connections rise and fall together. IE if you have an internet
connection to the rigs or to the computer that connects to the rigs and
a phone line they all are the same length and follow the same path after
entering the home through a grounded panel.
With older homes wiring is often "daisy chained" from room to room.
Often where the wiring runs for a particular circuit is a mystery.
Using my house as an example, I discovered that the outlets on the S
side of my den where the computers connect, roughly follows a path from
the panel, up through the wall and over the ceiling to the living room.
Then down the wall to an outlet, back up and over about 12 feed where it
descends to another outlet. Some where "up there" is a junction box
where it feeds two outlets on the West wall and also goes to the back
bedroom on the West side of the house. From there it goes into the
bathroom to a switch box with 3 switches and feeds the overhead fan and
lights. How it gets to the outlets in the den I don't know but the den
is on the N side of the bathroom. Every thing else in the den is fed
with relatively short, direct runs from the panel including the phone
and cable/internet. IE it's about the worst cable routing I could get if
I did it on purpose. With all those connections, plugging in a small
canister type vacuum cleaner will dump all the computers so there has to
be a poor connection in there "some where".
I have an electrician coming this week to remedy the mystery path into 3
circuits that will be a short as possible. Were I 10 years younger I'd
do the job myself.
Often we don't have a choice such as here the coax cables (TV and ham),
and rotator cables enter through a common, grounded bulkhead at the back
of the house while the cable and telephone come in with the power at the
front of the house. Now the entrance panel and the bulkhead are
connected with a common #2 ground wire. Unfortunately the house ground
and bulkhead are almost 30 feet apart. The ground tie does parallel the
feeds to the den so the rise time better known as Dv/Dt(voltage/time)
can lead to substantial differences for strikes with rapid rise times.
I agree that just because what we have is less than ideal we still
should strive to put in the best system possible within our constraints.
73
Roger (K8RI)
The latter is essential during the strike because of the huge currents
during a strike. Since all of the station equipment is tied (bonded) to
the one ground system (or should be), the lower the net Z to ground means
the less voltage developed differentially between grounded connections and
power feeds, antenna inputs, and so forth. (This is in addition to the
common mode induced currents from a strike.) Keeping the lightning
current on the outside of the building is key to minimizing what I call the
"Kingsford Effect" (conversion of a building into a pile of charcoal) and
is usually handled well by a good entrance panel, well-grounded, and
equipped with protective devices for each and every I/O line used in the
station. I can personally vouch for the effectiveness of that approach.
73,
Dale Svetanoff, WA9ENA
Sr EMC Engineer
E-N-A Systems, LLC
Specializing in shielding applications, system grounding, and lightning
protection
[Original Message]
From: Pete Smith N4ZR <n4zr@contesting.com>
To: <rfi@contesting.com>
Date: 9/18/2012 6:36:02
Subject: Re: [RFI] ISOBAR
I use a piece of 3/4 inch copper pipe running along the rear of my desk
as a ground bus. All of my radios, computers etc. are fed off a single
branch, and each is grounded to the pipe by a short, direct copper
wire. The copper pipe is connected to my aluminum entry panel, and from
there #2 solid copper goes to my power-line input ground.
Unfortunately, that wire has to be around 40 feet long, so I know it's
not ideal. Not all of us can have purpose-built shacks with perimeter
grounds and copper strap galore.
73, Pete N4ZR
The World Contest Station Database, at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000 and
arcluster.reversebeacon.net, port 7000
On 9/17/2012 10:47 PM, Jim Brown wrote:
On 9/17/2012 7:14 PM, Pete Smith N4ZR wrote:
I guess what I was asking was whether the snubbing at the power entry
would allow MOVs to work for the branch circuit protection a
cost-effective share of the time. I have experienced problems with
Ethernet EMP pickup, but so far my cascaded MOVs have done the job,
even when I took a direct hit on my tower.
Not if there are the multiple ground point issues with stuff plugged
into the MOV box.
73, Jim
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