The Grounding Circuit
The telephone pole
Power to a typical residence come from aerial (telephone poles) or underground service. The usual residential distribution lines consist of the three 60 cycle (AC) power phases, each 120 degrees out of phase with each other. The voltage of these distribution lines is usually 7,200 volts or 12,470 volts. A center-tapped transformer has one side of its input power windings attached to one of the three distribution lines. The other side of the input power windings will be connected to the fourth line that is on the pole – the ground. Thus the input power is taken across one of the three distribution power lines and the interconnecting pole ground.
The transformer has a 60:1 or 100:1 step-down ratio drops the voltage down the nominal 120 volts and delivers the two 180 degree phase of power lines for the home. The voltage between the two phases is 240 volts.
From the pole there are usually three lines: 120 volts phase 1 (black), 120 volts phase 2 (red) and neutral (white). Note that a 120 volt appliance will have current flowing down the black or red and returning on the white. If there is only a 120 volt, 10 amp appliance attached to the red phase, there will be 10 amps on the red wire and 10 amps on the white neutral returning it to the pole transformer.
If there is a 120 volt 10 amp appliance attached across the black phase and white neutral and a 120 volt 10 amp appliance attached across the red phase and the white neutral, there will be 0 amps flowing the white neutral. Why? Because the red and black are 180 degrees out of phase and their current flow will be in opposite, offsetting direction. Of course in the real world, the appliance loads on the two phases will never be exactly the same an so there will always be some current flowing on the neutral wire back to the pole’s step-down transformer. For example, if there is 10 amps on the black wire and 12 amps on the red wire, there will be 2 amps on the white neutral.
A 240 volt appliance will have current down the red and black wires only. For example, if there is only a 240 volt, 10 amp appliance connected to the circuit, there will 10 amps flowing down both the red and black wire and 0 amps down the white neutral wire. A 240 volt appliance will not contribute any current to the neutral white. Often, a 240 volt appliance will not even have a neutral while wire connected to it.
Telephone pole ground wire
In addition to the three distribution power phases on the pole, there is an interconnecting ground line. The line is usually seen above the three distribution power lines with the ground line at the very top of the pole. Or it may be below the three power legs which form a triangle near the top of the pole. The distribution lines will be differentiated by their heavy insulation (ceramics) from the pole. The ground line will have much less insulation and be very close to the wood of the pole.
Note that at each pole, a heavy solid copper line will connect to the ground line and run down the full length of the pole and disappear into the ground. This ground wire will not have insulation but will be physically protect for the bottom 10 feet or so of the pole. For many poles, this wire continues to run below the ground level – bare and in contact with the dirt — down the side of the pole and curls up on the bottom of the pole. In other cases, there is a ground rod placed at the base of the pole and the wire is connected to it. In all cases, the ground wire running between poles is connected the wire running into the ground at each pole.
Connection of the ground wire on the pole
On the pole, the pole-to-pole ground wire is connect to:
- The ground running down the side of the pole and to the pole’s ground rod or simply alongside the underground length of the pole
- One side of the input power winding of the transformer. Thus the power transformer operates between the power distribution phase and the ground wire.
- The center-tap of the output winding of the transformer. This becomes both a power return neutral white and the pole ground point.
In some special cases, the ground may not be connected to the center-tap point of the transformer. The neutral return runs to the house. However, the group of wires running to the house will include four wire: black, red, white and ground. At the house, there is ground rod which connects to the ground from the pole and the neutral and ground wires will all be connected together in the house power panel.
Per the National Electrical Code, at the primary/first breaker panel, where there is the main power disconnect, these wires are all connected together in that main panel:
- Ground from the telephone pole
- Neutral from the telephone pole
- Ground wires, usually on a ground buss bar in the panel
- Neutral wires, usually on a neutral buss bar in the panel
If there is a secondary fuse box, the neutral buss and the ground buss are not connected together. The neutral and ground are only connected together in the primary-main-first disconnect panel.
What is the reason for connecting grounds and neutral together only in the primary disconnect panel?
Power flows on the black, red and white-neutral wires in the house. The green-ground wire normally does not have any current flowing. If current flows in the green-ground that means that there has been a failure somewhere. Ground Fault Interrupters detect this flow and breaks the circuit. Furthermore, the green-ground assures that whatever item it is attached to remains a low voltage – essentially the voltage around the house, the voltage associated with the ground rod at the main panel.
What happens during a lightning strike?
In simple terms, lightning is essentially the breakdown of a capacitor formed by the earth’s surface and the clouds. During a storm, the motion of water particles in the air leaves excess, unbalanced charge on the moving water vapor. The earth’s surface reacts by having an equal but opposite charge. This forms a capacitor with an electric field running from the surface into the clouds. At some point the electric field becomes so intense that it breaks down the insulating air and a lightning strike occurs.
When the strike occurs, the electric field collapses and the charge on the earth’s surface dissipates, i.e. a huge current flow. Although soil is relatively highly resistive, this breakdown produces huge voltage differences in the soil.
During a strike, a house with only one ground contact point at the main panel will ride up and down on the voltage of the collapsing electric field and soil current. There is no current flowing into the house. Unfortunately, a lightning strike is a huge pulse which will couple – inductively and capacitive – to circuits in the house beyond those exclusively formed by the simple red-black-while-green wires. This coupling is what often does the most and unexpected damage to home equipment.
If one ignores the NEC requirement of a single grounding point at the main disconnect and attaches another ground rod to somewhere else in the house circuit, there is now an additional circuit for lightning energy to flow. For example, a ground rod outside the radio room, run it into the radio room and attached to the various equipment ground. In the event of a lightning strike, the two ground rods – the main panel one and the radio one – are separated and therefore at different voltage potentials. The house wiring then makes a circuit connected these two rods and a huge lightning current may now flow into the house and on every item in the house.
One could connect the radio ground rod to the panel ground rod with a heavy wire so as to assure that the two rods are at the same voltage. Essentially, the single-point-ground at the disconnect panel is extended to the second rod. However, if the radio room ground rod has a hefty wire extending into the house, this provides a parallel circuit running into the house.
Note that NEC requires that antennas, telephone, cable TV, etc. all have grounds at and only at the disconnect ground. Again, this isolates the circuits in the house from currents running in the soil and differential voltages in the soil.
Ham Radio Grounding – highlights:
To keep it simple:
- An antenna has two conductors where a feedline presents a differential voltage between the two conductors. How do you kill an antenna and stop RF transmission? Short out the two conductors, i.e. connect them so that they are at the same voltage potential. If you have several pieces of equipment where there could be the possibility of having a different RF voltage on each of their cases, you stop them from becoming an antenna and producing RF-in-the-shack by simply making a low impedance connection between the cases. This is goal of the braid-and-copper-pipe configuration. You are simply making sure there is no RF voltage difference between cases and so they cannot become an antenna.
- What about current loops? Ground loops (or current loops) are frequency (wavelength) dependent. At HF, the wavelength is so much longer than the loop, there is not a problem. Unless, of course, you configure things such that the connection is highly resistive and there is a voltage drop along the interconnecting wire. This is not really a ground loop issue but rather simply poor implementation.
- For safety, the equipment cases are connected back to the main electrical panel’s ground. The copper pipe is probably already well referenced back via the several power cord ground-pin-to-case connections. But it never hurts to run a wire back to the ground of the wall power receptacle.
- What about lightning? Lightning issues are never fixed inside the shack. Lightning is addressed outside the shack (outside the house). Don’t connect a big wire from outside the house to the copper pipe inside the house (i.e. a heavy wire going into the house from an extra ground rod at the antenna entry point). Adding this wire so essentially provides a nice conduit for inviting lightning energy right into you shack.
- No such thing as RF ground. You don’t need it in the shack. Your handheld radio in the shack doesn’t need one and your HF radio doesn’t either.
What is Grounding?
Too often “grounding” is used as a catch-all term for the generalist magical cure of radio system problems and for protection from the weather gods. Indeed grounding affects all these issues but unfortunately the techniques are lumped together as a cure-all and are implemented haphazardly without understanding and applying the problem-cause-solution methodology underlying the cures.
It is important to understand that “grounding” addresses three distinct problem groups: lightning, safety, and RF signal.[1] The first two are closely intertwined and addressed by the various unpretentious, thought-out standards – NEC, NFPA 780, IEEE 142, 81, Motorola R56, MIL-STD-149a, etc. In general, if the standard calls for heavy gauge wire or straps, they are probably addressing lightning and safety grounding. Furthermore, much of what these standards address are problems associated with lightning and safety. RF grounding rarely, if ever, requires heavy gauge, high current capacity wire (solutions usually focus on conductor surface area). Blind extrapolation of the lightning and safety grounding techniques to RF signal matters is unwise, usually unproductive, and sometimes simply dangerous.
In all cases, the grounding system starts with an “earth electrode system” consisting of “a network of earth electrode rods, plates, maps, or grids and their interconnecting conductors.”[2] This system is external to the building and provides the system’s ground reference. Connection of this external system provides the principal ground point for grounding subsystems in the building.
In all cases, for a home, building, or external structures, the standards call for ONE and only one heavy-gauge, low resistance entry point of the exterior earth electrode system into a building. The point of this is that with only one low resistance entry point there is not a differential voltage created in the building by a lightning strike – everything in the building is at the one ground reference voltage. If you have two rods and/or grounding points at different ends of the building, currents in the ground induced by a lightning strike produce two different voltages on the two rods. The enormous lightning current, taking the lowest resistance path, will then run up any high-current-capacity wires into the building as the primary travel path between the two rods. Everything in the building will thus become a conductor for this current and, most likely, this huge current will fry the wires and probably source a fire in the building.
Put in as many ground rods as you like to form the earth electrode system. You MUST connect them all together OUTSDIE the building with heavy gauge wiring so as to keep the lightning currents outside the building and on these low resistance external pathways. There is only one heavy gauge wire from this earth electrode grounding system attached to your house’s electrical web – and it is at your main power panel.[3] From there ground branches out to all the green/bare-copper Romex grounds in your house. But these branches all end within the house and do not connect to external voltages on any exterior lightning circuits.
Where your antenna cables come into the house, connect the exterior box to the exterior earth electrode grounding system and the shields of all the coaxes to the ground buss inside this box. For ham radio, there will often be a copper plate onto which gas-filled high-voltage/lightning arresting coax connections are made. The exterior earth electrode grounding system is heavy conductor, hard-wired to your main power panel thus this antenna exterior box becomes part of that system. Do NOT run a heavy gauge wire (or ribbon) from the antenna box into the house. Again, the low resistance path is on the exterior earth electrode grounding system and not up into your house. Don’t provide a low resistance path for lightning currents to enter and flow through your house. In your shack, the coax shield is connected to house ground and then to your main power panel ground. But this interior pathway needs to be a less favorable path for lightning currents to travel. This is accomplished by making a very, very low resistance high-current-capable interconnection web among your antenna entry box, the electrodes of the exterior earth grounding system, and the ground terminal at the mains power panel. You are essentially trying to make your antenna entry box and your mains panel electrically one and thus minimize the path for lightning currents to travel into your house via your antenna coax.
Who do some elmer-authors advise placing a heavy gauge, high current conducting wire from the radio station’s interior common ground to the exterior copper plate where the antenna wires enter the shack? Are they providing a way for lightning voltages to exit? But the previous discussion already shows why that is undesirable. Are they providing a way for lightning voltage to enter the shack? Surely not. Is if for “RF ground” or “common mode suppression”? If so, knowing that RF voltages travel only on the surface of the wire, why a heavy gauge, large cross section wire? Frankly, this effort is unproductive and borderline unsafe. They are simply shot-gunning grounding without understanding what they are doing. Placement of chokes on the antenna coax coming into the shack has much more value. Spending the time and treasure to heavy-gauge connect that exterior copper plate to the rest of the exterior grounding system is wisest and standard-following thing to do.
Note that in the industrial applications of the standards, they go to a lot of trouble to specify that the power and antenna entry point are very hard connected together (i.e. adjacent entry point with separate panels but physically heavy-gauge, hard connected, and all exterior to the building). From that single point they run a single heavy ground line into the building. This provides a path for stray currents out through that one entry point. If one insists, for some reason, to run a heavy gauge ground to the shack, it should run from the connection at the main power panel and only from there.
“If at all possible the equipment fault protection conductors should be physically separate from signal reference grounds except at the earth electrode subsystem.”[4] The “green” or bare grounding wires of the Romex power system should isolated from the signal grounds (antenna) except at the connection to the earth electrode system (i.e. the main power panel).
With respect to the mystical “RF ground” — there is no such thing. There is an “RF equipotential” (i.e. the “signal reference system”[5]) to which all the equipment in the shack should be connected/referenced (i.e. the ubiquitous and good copper grounding pipe). It should be connected to the house ground but with regard to RF signals there is NO VALID SCIENCE for tying it to the ground rod outside the shack. Doing so will not do anything for reduction of common mode or RF static. The differential input of your radio doesn’t care. Airplane antennas and radios all work just dandy without earth grounding. There is no magic in dirt. If dirt was good for RF conduction you wouldn’t need a copper wire counterpoise for your antenna. Because of ground rod impedance, skin effect, dirt resistance, etc. you might just as well stick a ground rod in the potted plant over in the corner and attach that to your coax shield.
Indeed all the equipment in the shack should be tied together and referenced to a single equipotential (i.e. the copper-pipe-ground acting as the “signal reference”). “The signal reference subsystem establishes a common reference for C-E equipments, thereby also minimizing voltage differences between equipments.”[6] The connection of each piece of equipment to the copper pipe reduces the current flow between station components thus minimizing noise voltages on signal paths or circuits. All this reduces RF voltage differences on the chassis of the various pieces of equipment so they don’t become radiating antennas.
Use copper ribbon, use braid, but heavy gauge copper wire is undesirable and simply wasteful. The RF currents are low and on the surface of the conductor (more surface area the better; heavy gauge wire is for lightning and safety, not RF). The copper pipe is tied to the equipment’s chassis with copper ribbon/braid, the chassis ground of each piece of equipment, and thus the wall socket ground via the power cord. Because one should not rely on unseen connections within the equipment to provide safety grounding, a direct connection from the pipe to the wall outlet ground with a piece of #12awg wire is suggested. To reduce common mode currents or RF-in-the-shack, use chokes, baluns and toroids. Running a copper ribbon or wire out to the ground rod at the antenna panel just invites lightning currents in and does not reduce any RF issues.
ALL equipment MUST be connected to the safety ground. Nothing in the station should have a “floating” chassis. The earth electrode subsystem must be connected to the power neutral at the source side of the first service disconnect or service entrance panel (i.e. at the power meter and primary circuit breaker and panel of the house). At this first power breaker box, Romex neutrals and grounds for the house are all connected to the same bus bar. Nowhere else are they connected together (i.e. at other breaker panels in the house/facility, the grounds and neutrals are on their own bus bar which is not connected together in that panel).
Simple Ham Shack
Addressing lightning strikes is ALL done outside the shack/house. Example, ground rods attached to antennas and entry point boxes/plates EXTERIOR to house. And, of course, lightning surge protectors on cables at the entry point. Do it all outside and minimize the entry of lightning strike current into the house (i.e. no additional heavy copper wire or ribbon alongside the coax going from outside to inside the house — don’t throw out the red carpet for lightning currents to come into the house!).
Inside the shack, the concern is with safety and stray RF. For stray RF, connect the chassis of each piece of equipment to a common baseline potential point (i.e. the copper pipe) using a low RF impedance connection (i.e. via braid or copper ribbon from each chassis to the pipe). Use ferrites to kill common mode RF.
What about ground loops? Some elmers get very excited about ground loops associated with connections in the shack. Again, shotgun application of technology without understanding the underlying application parameters will get you into trouble. Ground loops are associated with low-impedance multi-point connections. In order for a ground loop to radiate energy, the linear distance around the loop must be similar to the RF wavelength. Without this loop distance, there can be no significant voltage drop and radiation – it is simply not a functional antenna. For HF stations, it is unlikely that loops will be in the multi-meter dimension. You are best off connecting everything together to minimize voltage differences in the grounding system than worrying about the unseen bugaboos of ground loops. HOWEVER, if your system operates at sub-meter frequencies, then indeed worry about ground loops.
Because each equipment chassis is connected to the ground pin of the three-wire power plug, the copper pipe is likewise connected to house safety ground. This is good. The point is that it could make one “feel better” to connect that copper pipe directly to the ground of the power outlet. This would be an extra-satisfying visible safety ground (NOT an RF or lightning ground).
Notes on Earth Electrode Systems
For the fault protection subsystem, the NEC (2-2) states in Article 250 that a single electrode consisting of a rod, pipe or plate which does not have a resistance to ground of 25 ohms or- less shall be augmented by one additional made electrode.[7]
For military systems, 10 ohms is the standard for the earth electrode subsystem (EES) in MIL-STD-188-124A.
The current which flows in a direct lightning stroke may vary from several hundred amperes to as much as 300 thousand amperes… Experience has shown that a grounding resistance of ten ohms gives fairly reliable lightning protection to buildings, transformers, transmission lines, towers, and other exposed structures.[8]
For a typical ground rod (10ft long, 1in diameter), “… 85 percent of the total resistance to earth of a 10-foot long ground rod is established within 10 feet of the rod…[and the] effects of rod length do predominate over the effects of rod diameter”[9]
The resistance of a vertical rod to earth is R1 where rho is the resistivity of the soil (ohm-cm), l is the length of the rod (cm) and d is the diameter of the rod (cm).[10] 85 (92) percent of the total resistance to earth of a 10-foot long ground rod is established within 10 (20) feet of the rod. The resistivity to ground is simply proportional to the soil resistivity. 10ft rod (244cm), ½ in diameter (1.27 cm) is 0.0034 * rho. For a 10,000 ohm-cm soil resistivity, the resistivity to ground is 34ohms.
If using multiple rods, “the separation between driven vertical ground rods in a group of rods should not be less than the length or greater than twice the length of an individual rod.”[11] If a number, N, of equal length vertical ground rods (with tops flush with the surface) are separated equally along a straight line and connected together by an insulated conductor at the tops of the rods, the resultant resistance will be somewhat greater than l/N times the resistance of single isolated rod. For N rods of length at spacing s, the total resistance RN where r is the radius of each rod:
For a pair of 10foot long ½-in diameter rods spaced 20ft apart in 10,000ohm-cm soil, the resistance to earth is about 16ohms (from 34 ohms with a single rod).
Resistivity of Dirt
Dirt is resistive. That is why we use counterpoise wires on a vertical dipole antenna.
The unified soil classification system and selected resistivity values
| Major Divisions | Group Symbol | Typical Names | Resistivity Range | |
| ReCoarse-grained soils
(more than half of the material is larger than #200) |
Gravel and Gravelly Soils | GW | Well-graded gravels or gravel-sand mixtures, little or no fines | 60,000-100,000 |
| GP | Poorly graded gravels or gravel-sand mixtures, little or no fines | 100,000–250,000 | ||
| GM | Silty gravels, gravel-sand-silt mixtures | |||
| GC | Clayey gravels, gravel-sand- clay mixtures | 20,000–40,000 | ||
| Sand and Sandy Soils | SW | Well-graded sands or gravelly sands, little or no fines | ||
| SP | Poorly graded sands or gravelly sands, little or no fines | |||
| SM | Silty sands, sand-silt mixtures | 10,000–50,000 | ||
| SC | Clayey sands, sand-silt mixtures | 5,000–20,000 | ||
| Fine-grained soils
(more than half of the material is smaller than #200) |
Silts and Clays (liquid limit < 50) |
ML | Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity | 3,000–8,000 |
| CL | Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays | 2,500–6,000 | ||
| OL | Organic silts and organic silt- clays of low plasticity | |||
| Silts and Clays (liquid limit > 50) |
MH | Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts | 8,000 – 30,000 | |
| CH | Inorganic clays of high plasticity, fat clays | 1,000 – 5,500 | ||
| OH | Organic clays of medium to high plasticity, organic silts | |||
| Highly organic soils | Pt | Peat and other highly organic soils | ||
Table 2 exhibits resistivity values for some types of soils and rocks, expressed in common terms and measured by different methods. Observe the full range of values for each type of soil. The actual resistivity can vary within the maximum and minimum values quoted there.
Table 2 Resistivity of different soils
| SOIL TYPE | Minimum (Ω∙cm) |
Average (Ω∙cm) |
Maximum (Ω∙cm) |
| Ashes, brine, waste | 590 | 2,370 | 7,000 |
| Clay | 340 | 4,060 | 16,300 |
| Same as above with varying proportions of sand and gravel |
1,020 | 15,800 | 135,000 |
| Gravel rock, sand, stones with little clay |
59,000 | 94,000 | 458,000 |
| Granite, basalt | 1,000,000 | ||
| Slate | 1,000 | 10,000 | |
| Limestone | 500 | 400,000 | |
| Marl | 100 | 5,000 | |
| Sandstone | 2,000 | 200,000 | |
| Shist, shale | 500 | 10,000 | |
| Soil, chalky | 10,000 | 1,000,000 | |
| Sea water | 20 | 100 | 200 |
| Lake water | 20,000 | 20,000,000 | |
| Tap water | 1,000 | 5,000 | |
| Fertile land hills | 3,000 | ||
| Coastal land, dry, flat, sandy |
30,000 | 50,000 | 500,000 |
Geological investigations give valuable information to have an idea of the range of resistivity values. Still, the reliable design of a grounding system requires measurements at the site.
In addition to the variation due to the soil type, the resistivity will change by several orders of magnitude with small fluctuations in moisture content, salt content, and temperature.
From MIL-HBK-419A Table 2-2
From MIL-HBK-419A Table 2-3
Lightning
The lightning process involves charge buildup in the cloud (mostly negative charge). Like a capacitor, an opposite (positive) charge builds up on the surface of the earth. There is now a growing electric field between the cloud and the earth’s surface.
The lightning process begins with an initial pilot streamer extending a hundred feet or so from the cloud. A stepped leader lowers additional negative charge around the pilot streamer and the pilot streamer than extends another hundred feet. This happens repeatedly and rapidly. Each pilot streamer takes a different ionization path – thus producing the zig-zag pattern.
Because of high ionization (i.e. effectively high conduction), the stepped leader is at the same voltage potential as the cloud. As the leader gets closer to the ground, the voltage gradient to the ground increases and air ionization increases. As the leader approaches the ground, a leader may extend several meters from the ground to meet the downward leader from the cloud. Once they meet, i.e. a lightning strike, an enormous positive charge current moves upward though the ionized column to neutralize the charge in the cloud. Additional pockets of charge in the cloud may discharge to earth via additional dart leaders. This may happen several times.
The energy per flash of lightning has been estimated to be 108 watt-seconds.
From MIL-HDBK-419a Table 3-1
What if there is a lightning strike?
Assume there is a lightning strike nearby. This strike produces currents running to it through the dirt. Two points in the dirt will therefore see a voltage difference because of the current running in the resistive dirt. Two rods in the dirt will have this voltage difference. A wire connecting the two rods will provide a path with lower resistance than the dirt. The current produced by the lightning will now see the wire between the two rods as a lower resistance path and so will run up one rod, along the wire, and out the other rod.
If you have ground rods stuck in the dirt at opposite ends of your house connected to your house ground wire, you are now inviting the lightning current to use your house ground wiring as its preferred path.
That is why ALL ground rods MUST be connected with heavy gauge wire EXTERNALLY to the house. The house ground wiring web should be only connected in one place – at the breaker box – and not providing a circuit for the lightning current. The ground wiring in the house is a dead end.
Draw your grounding situation on paper. Imagine a nearby lightning strike producing currents in the ground. Does that strike current see your house ground wiring as a better conductor than the dirt for the points between your power-panel ground and your station grounds? Yep, you’ve now got lighting current running in your house.
One last thing. One of Jim’s points is that there is no such thing as (or need for) RF ground. The station ground in the shack needs only use the house ground. The “grounding” that you want is for all the equipment chassis to be interconnected with a low RF impedance so that all the station chassis are at the same RF potential. If the chassis are at the same RF potential, then there is no RF current flowing, and they chassis won’t act as a transmitter. The chassis RF potential relative to the dirt potential is unimportant. Your handhelds don’t connect to the dirt. Airplanes don’t connect to the dirt. Why does your station need to connect to the dirt?
Skindepth
One “skindepth” reduces the electric field, and correspondingly the current, by 1/e. Three skindepths is usually considered the point at which the field has essentially dropped to a negligible amount (i.e. less than 3% of the maximum). As a result, when considering how “thick” a conductor is needed, 3 skindepths is considered acceptable and anything beyond that has minimal electrical contribution.
The figure below is lifted from Wikipedia. Note that aluminum and copper are very similar. For HF (3 to 50kHz), the skindepth runs from 1 to 0.3 mm and therefore a copper or aluminum shield or conductive conduction plate need only be no more than 3mm thick (about 1/8 inch).
[1] MIL-HDBK-419a refers to this as the “three primary functions” of the grounding system: personnel safety, equipment and facility protection, and electrical noise reduction
[2] MIL-HDBK-419a section 1.5.1
[3] The main power panel is the point where power from the power company is metered and run to a circuit break box. This panel usually is within a few feet of the meter and usually has a one or two ground rods very close by.
[4] MIL-HDBK-419a section 1.5.1.c
[5] MIL-HDBK-419a section 1.5.1.d
[6] MIL-HDBK-419a section 1.5.1.d
[7] MIL-HDBK-419a section 2.2.2.1
[8] MIL-HDBK-419a section 2.2.3
[9] MIL-HDBK-419a section 2.6.1.1
[10] MIL-HDBK-419a table 2-5
[11] MIL-HDBK-419a section 2.6.2