"Ground loops" are an interesting question. The problem is a "single
point ground" is often not feasible and is impossible at my QTH. My
decision was to connect everything together. At my QTH the conductor
paths are:
1. Buried HV feed to mains transformer which has a Ufer vault ground,
which then feeds
2. 200a house service with 2 ground rods at entry
3. 400a shop/shack entry panels with 2 ground rods and bonding to
structural steel
4. Charger and heater at a backup generator on a concrete pad,
which then feeds back underground to 200a transfer switches at house and
shop, next to mains entry panels
5. The shop/shack foundation is a perimeter Ufer and the structural
steel is grounded to it, also the main shack ground is to the Ufer
6. The 3 towers have Ufer bases and ground rod+radial fields
7. The shack coax entry panel is bonded to the steel structure, coax is
bonded to towers top and for the largest tower to a shed at base entry
panel, all cables are in buried conduits to the shack
8. Underground control and coax to a 160m wire vertical T with a ground
rod and shunt inductor from 8 elevated 125' long radials
9. Ethernet, RG6, and control cables are in conduit from shop to house
(not protected at either end - needs fixed)
10. The Comcast cable entrance is tied to ground rod at house, telephone
entry is there also, so the house on its own is near single point grounding.
11. The equipment shed at the base of the largest tower has the coax
entry panel bonded to the rod and radial field. The shed mains power
panel is also bonded to the radial field.
Thus, there are a large number of "loops". This is probably more
complex than most, but I think it is not uncommon to have loops. Code
requires much of the above, and white (neutral) is connected to yel/grn
(earth) at several places. So my strategy was to bury a large ground
conductor below the conduit runs when I could, to tie stuff together. I
didn't do this initially but wish I had as later DC ground resistance
measurements showed the buried wires were about 1 ground rod equivalent
at about 100' of buried bare copper wire (#6). I note that the Andrew
lightning protection guide advises against tying towers to building
entry panels with a separate buried conductor. Rather they let the
voltage surge be equalized on the coax and control wires, I think the
theory is that the differential voltages are less as a result. If my
code knowledge is correct, it requires towers to be bonded to "house
ground", if there is AC at the tower as is true for my motorized crank ups.
I think my shop foundation Ufer plus structural steel frame makes a low
inductance path from the shack on the opposite corner from the mains
entry and transfer switch panels. The tower foundation Ufers and
rod/radial fields are other "good" low ohms and L grounds with lots of
surface area. The code required pair of mains entry panel rods are poor
grounds in comparison (180 sq ft of concrete per tower in earth contact
vs 0.1 sq ft per rod. 1000 sq ft of concrete surface in the shop Ufer).
So if "SPG" means a single point ground at shack entry panel for coax,
control cables, and rigs, then what I have might qualify. Otherwise, it
is impossible to achieve.
What happens if a nearby strike induces a large current in the several
100'+ diameter loops is a concern, but given the reality that loops were
unavoidable, a ground wire mesh seemed to me to be the best strategy.
As I have progressed re this topic, I now consider #2 bare wire buried
below every conduit run as the way to go. Solid is better for longevity.
I also harken back to the days of large "glass house" many refrigerator
sized box computers which had to have multiple power feeds. The only
way to manage the neutral/frame/ground loop problem was a stiff enough
ground mesh to swamp the frame ground voltage offsets so the single
ended control signals would be reliable. Later, differential control
signals and serial interfaces lessened the loop problems and hardened
the systems for EMI/static discharge. Some big systems used M-G set
isolated 3p delta 400Hz power and differential control signals, yet one
I worked on always crashed with a nearby strike. A solution at another
installation I worked at was that the entire computer room power grid
had to be on isolated M-G power so nearby strike surges didn't couple
into the system signals from the lighting and general purpose circuits,
as Jim noted.
An "RF ground" for lightning as the Andrew guide explains is a ground
rod + radials field that distributes the RF energy of a strike over a
large area both capacitively and via conduction. So the 5 ohm DC ground
resistance target has merit as does the radial wire and ground rod field
size. Both are needed. While buried conductors benefit from the shunt
earth conductivity, the wire inductance limits the useful length of the
radials to about 50ft according to Andrew. The K1TTT analysis shows why
tower top and bottom coax shield bonding is needed and why elevated coax
should be avoided if at all possible.
Fortunately, Western Washington has a very low strike frequency, but
with a tower top 40' above the 110' tree line and on a ridge, my
attention to lightning protection has significantly increased.
"There is no such thing as ground" from Vonada's Engineering Maxims.
Grant KZ1W
Redmond, WA
On 7/24/2016 9:04 AM, Dick Blumenstein wrote:
From everything I've read, the prevailing overall grounding technique
is to run a heavy duty copper wire from the grounding system around
the tower, back to the ground rod outside the shack wall as well as to
run it to the ground rod under where your AC power enters the house.
It just occurred to me that the AC ground wire, besides going into the
house and connecting to the chassis ground in the breaker box (where
also all the neutral white wires are connected) then proceeds
throughout your house and also to your ham radio shack equipment. It
is here that the ground wire also connects to all the chassis in your
shack as well as the shields on your coax connectors that also finds
it way back outside your shack wall to the ground rod; one huge ground
loop. I know that there are 2 issues here; RF grounding and
lightning protection. Any comments about that?
Thanks,
Dick, K0CAT
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|