GROUND
We walk on it, play on it, and
live on it, but what is it in
reality when it comes to
communications systems, and
how do we obtain a “TRUE”
and SAFE ground?
Types Of Ground
• RF Ground
• DC Ground
• Lightning Ground
Not necessarily the same as they
have different characteristics!!
EXAMPLE
Ground Terminology
•Anode/Electrode
Factors to determine in selecting electrodes include, ampacity (the ability to handle
current or amps), corrosion resistance, life expectancy, resistance-to-temperature
change, and of course resistance over time..
•Bonding
Making sure that all the electrical “devices” and/or circuits are tied together to
form a common ground system or grid. THIS INCLUDES TOWERS!!
•Earthing
The process of electrically connecting to the earth itself is often called “earthing”,
particularly in Europe where the term “grounding” is used to describe the above
ground wiring. The term “Grounding” is used in America to discuss both earthing
and grounding.
Ground Factors
• Soil Resistivity
• Surface Area of Anode
• Zone or Sphere of Influence
ANODE/ELECTRODE
• An electrode is anything placed into the ground that is used to
provide an electrical connection to the earth. The most common
electrode is the copper-clad driven grounding rod. This grounding
rod is essentially an 8 or 10-foot long shaft of mild-steel, thinly
coated with copper and driven into the earth. The process of
installing an electrode would be called “earthing”.
• Each electrode has its own unique advantages and disadvantages.
In the case of the copper-clad driven rod, it is very inexpensive to
purchase, but can be overly labor-intensive and time-consuming to
install. It also has some poor electrical properties. On the other
hand, electrolytic rods while cost prohibitive, out perform
any other grounding electrode on the market today.
A common misconception is that the copper coating on a standard driven rod
has been applied for electrical reasons. While copper is certainly a conductive
material, its real purpose on the rod is to provide corrosion protection for the
Steel underneath. Many corrosion problems can occur because copper is not
always the best choice in corrosion protection. It should be noted that
galvanized driven rods have been developed to address the corrosion concerns
that copper presents, and in many cases are a better choice for prolonging the
life of the grounding rod and grounding systems. Generally speaking,
galvanized rods are a better choice in all but high salt environments.
Advanced Driven Rods can easily be installed to depths of 20 ft or
more, depending upon soil conditions.
Advanced Driven Rods are typically driven into the ground with a standard drill
hammer. This automation dramatically reduces the time required for installation. The
tip of an Advanced Driven Rod is typically made of carbide and works in a similar
manner to a masonry drill bit, allowing the rod to bore through rock with relative
ease. Advanced Driven Rods are modular in nature and are designed in five foot
lengths.
Grounding plates should be buried at least 30 inches below grade
level. While the surface area of grounding plates is greatly increased
over that of a driven rod, the zone of influence is relatively small. The
zone of influence of a grounding plate can be as small as 17 inches.
This ultra-small zone of influence typically causes grounding plates to
have a higher resistance reading than other electrodes of similar mass.
Concrete Encased Electrodes use a minimum No. 4 AWG copper wire
at least 20 feet in length and encased in at least 2 inches of concrete.
They dramatically increase the surface area and degree of contact with
the surrounding soil. However, the zone of influence is not increased,
therefore the resistance to ground is typically only slightly lower than
the wire would be without the concrete.
Eliminates the drawbacks of other grounding electrodes. This active
grounding electrode consists of a hollow copper shaft filled with
natural earth salts and desiccants whose hygroscopic nature draws
moisture from the air. The moisture mixes with the salts to form an
electrolytic solution that continuously seeps into the surrounding
backfill material, keeping it moist and high in ionic content.
BONDING
•
Coax Cable Entrances
• Tower(s) and Antenna(s)
• Electrical Entrance
• Lightning Rods
Zone/Sphere Of Influence
Determines how efficiently grounding electrodes
discharge electrons into the earth. The “Sphere Of
Influence” is the volume of soil throughout which
the electrical potential from a lightning strike is
dissipated. The greater the volume, the more
efficient the electrode. The greater the surface area
of the electrode, the greater the contact with the soil
and the more electrical energy that can be
discharged per unit of time.
A simpler version is used when the above formula is modified by
rounding Π (pi) down to to 3 and cross canceling to get the formula:
V=5L3
In this example, a single 10-foot driven rod would utilize 5,000
cubic feet of soil, where as a single 8 foot rod would utilize about
half the soil at 2,560 cubic feet.
MEASUREMENTS
In order to achieve reliable electrical grounding,
it is absolutely essential that you verify:
• Soil resistivity.
• Anode/Ground resistance
• Proper sized ground cable and
anode
Wenner 4-Point Soil
Resistivity Test
• Uses 4 probes spaced at equal distances
across the surface of the earth in a straight
line
• The distance that the probes are spaced
determines the testing depth into the soil
• Multiple tests are typically conducted at a
variety of probe spacing
3 Point Method Fall Of
Potential And Induced
Frequency Ground
Resistance Test
• Requires complete isolation from the power utility
• A short probe is driven into the earth at a distance
of ten times the length of the grounding system.
• Often referred to as “Megger Test”.
Induced Frequency Test
aka Clamp-On Testing
• Newest test method for measuring the
ground resistance of a grounding system or
electrode
• Induced Frequency test can be used on live
or ‘hot” systems
• This ground resistance test is rapid and
accurate
ADVANTAGES
• Measures Soil Resistivity
• Measures Electrode/Soil Contact
• Measures Ground System Current
• Measures Ground System Noise
Summary
• How good is YOUR ground?
• Grounding systems change over time.
• Regular testing a must to insure
reliability
• Lightning “Season” fast approaching!!