[TowerTalk] Does prevailing grounding scheme promote large ground loop?

Tue Jul 26 21:35:12 EDT 2016

Jim,

Thanks for your comments.  The entrance panels for the two buildings 
have separate meters so they are correctly installed with local bonding.

NEC, if I understand it correctly, does clarify one important point, 
that is that permanently installed outdoor generators can be connected 
to an auxiliary grounding electrode .  Even if I didn't have a separate 
electrode the gen frame is bolted to a reinforced 60 sq ft concrete pad. 
So I have 3 code correct grounding electrode connections.

My logic for the tower shed panel bonding  was to shunt to the radial 
field a large amount of common mode RFI (S9+40) from PWM 240 VAC mains 
amplifiers driving the rotator 180v DC 1/2hp motors.  Even though the 
16KHz PWM amps are 240v sourced they have a bolted frame connection.  
These amps use IGBT's that switch really fast.  I have a shielded 
isolation transformer between them and the panel and balanced 50KHz 
cutoff filters I designed installed on the output of the amps.  This 
bonding is not per code, and if there is downside, I'd like to know what 
that is.   Fortunately, all this suppresses the rotator moving RFI to 
S1-2 for 20m and above, but I need another 60+ db on 40m.  The challenge 
is a filter design that doesn't cause the PWM amp protection circuits to 
trip. This problem is a drift off topic but if others have solved it, 
I'm eager to learn.  The configuration is Green Heron controllers with 
remote power amp turning K0XG rings.  The rotator cable is inside the 
tower, 16ga twisted pair, not shielded per manufacturer's advice (bad 
advice, they should be shielded).  The 180 VDC motors are filtered at 
their frames and can barely be heard on all bands when driven from a 
clean DC source.

Grant KZ1W

On 7/26/2016 13:41 PM, Jim Brown wrote:
> Hi Grant,
>
> I won't quote your entire post, but only some key elements that need a 
> comment.
>
> On Tue,7/26/2016 11:21 AM, Grant Saviers wrote:
>>  "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. 
>
> In general, that's the only right (safe) way to do it.
>
>> 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
>
> All of this is REALLY GOOD
>
>> 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)
>
> Yes, this is a recipe for destruction, needs serious attention.
>
>> 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". 
>
> Of course. The concept of "ground loops" is fundamentally WRONG, and 
> causes us to do dumb things. The ONLY context in which a loop is a bad 
> thing is magnetic induction.
>
>> 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. 
>
> TRAIN WRECK!  Good engineering practice, and virtually ALL building 
> codes in NA, require that neutral be bonded to ground ONLY at 1) the 
> service entrance (where power enters a premises); and 2) where a "new 
> system" is established. A new system is established by a transformer. 
> It is NOT established by a feed to another building from the main feed 
> to the premises.  The ONLY time there should be more than one bond in 
> two buildings is if 1) the two buildings have their own service (that 
> is, a separate metered connection from the power company; or 2) if the 
> second building is fed from the first, and ground is not carried 
> between the two buildings. This second scheme is no longer permitted 
> by NEC
>
> SO -- if you have neutral bonded to ground at more than one place, you 
> need to change that. It's a VERY big deal.
>
>> 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.
>
> NEC requires that the power system ground MUST be carried to all 
> outlets and loads, and it MUST be carried with the phase and neutral 
> conductors (in the same conduit or other cable).
>
>> 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.
>
> Yes, that's the real world.
>>
>>
>> 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. 
>
> Erase the words "RF ground" from your memory bank -- it is a fiction 
> that has no meaning. Connections to earth, and bonding between those 
> connections is ONLY for lightning safety. The only relationship 
> between those connections and "RF" is to realize that lightning is an 
> RF event, not a DC event, so that the impedance of those connections 
> and that bonding at RF, which is where the energy in lightning is, is 
> what matters.
>
>> So the 5 ohm DC ground resistance target has merit as does the radial 
>> wire and ground rod field size. Both are needed. 
>
> A radial field serves as a low resistance return for antenna current, 
> in place of earth, which is a big resistor.  That radial field SHIELDS 
> the antenna from the lossy earth. And from the point of view of 
> lightning protection, if it is bonded to facilities grounds, it 
> provides capacitive coupling to the earth to reduce the impedance to 
> earth at RF.
>
>> 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.
>
> Yes on all counts.
>
>>
>> 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.
>
> Lightning enters our premises on MANY conductors -- power line, TELCO, 
> CATV -- as well as antennas.
>
>> "There is no such thing as ground"  from Vonada's Engineering Maxims.
>
> Now we're getting closer to the real world. :)
>
> 73, Jim K9YC
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