 |
| What
is grounding? And why ground? |
| What
is an electrode? |
| What
is Ground Resistance? |
| What
is Soil Resistivity? |
| What
is the difference between Soil Resistivity
and Resistance-to-Ground again? |
| What
are some other electrical performance criteria
for grounding electrodes? |
| Why
should I have an engineer design my site
grounding system? |
| What
is Ground Potential Rise? |
|
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|
| What
is grounding? And why ground? |
Electrical grounding or
“Grounding” originally began
as a safety measure used to help prevent
people from accidentally coming in contact
with electrical hazards. Think of your refrigerator.
It’s a metal box standing on rubber
feet with electricity running in and out
of it. You use magnets to hang your child’s
latest drawing on the metal exterior. The
electricity running from the outlet and
through the power cord to the electrical
components inside the refrigerator are electrically
isolated from the metal exterior or chassis
of the refrigerator.
If for some reason the electricity came
in contact with the chassis, the rubber
feet would prevent the electricity from
going anywhere and it would “sit”,
waiting for someone to walk up and touch
the refrigerator. Once someone touched the
refrigerator, the electricity would flow
from the chassis of the refrigerator and
through the unlucky person, possibly causing
injury.
Grounding is used to protect that person.
By connecting a wire to the metal frame
of the refrigerator, if the chassis inadvertently
becomes charged for any reason, the unwanted
electricity will travel down the wire and
out safely into the earth; and in the process,
trip the circuit-breaker stopping the flow
of electricity. Obviously, that wire has
to connect to something that is in turn
connected to the earth or ground outside.
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 aboveground wiring.
The term “Grounding” is used
in America to discuss both earthing and
grounding.
While grounding may have originally been
considered only as a safety measure, with
today’s advances in electronics and
technology, grounding has become an essential
part of everyday electricity. Computers,
televisions, microwave ovens, fluorescent
lights and many other electrical devices,
generate lots of “electrical noise”
that can damage equipment and cause it to
work less efficiently. Proper grounding
cannot only remove this unwanted “noise”,
but can even make surge protection devices
work better.
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| What
is an 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 rod. This 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”. Other
electrodes included concrete-encased electrodes,
ground plates, water pipes, building foundations,
and electrolytic rods, to name a few.
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.
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| What
is Ground Resistance? |
The most common performance
criteria or specification used today is
Resistance-to-Ground or commonly called
“ground resistance”. In the
electrical world, resistance is anything
that opposes the flow of electricity. Do
you remember all the hype about “super
conductors” that has been in the news
for the last decade or so? With super conductors,
Scientists are trying to develop a material
with zero resistance to electricity. They
have yet to succeed at any practical level.
It turns out, that all known materials have
an electrical resistance at some level,
even copper. So as you can imagine, dirt,
rock and sand have varying resistances,
and based on the particular composition
of the soil, the resistance to electricity
that your particular part of the earth provides
can be very different. In fact, the resistance
of the soil (per cubic meter) can vary from
location to location by thousands of ohms
(ohms is a unit of measurement used for
resistance) and that can make a big difference
in how effective your grounding will be.
Resistance-to-Ground (or ground resistance)
is a measurement of the actual resistance
of the electrodes in the grounding system.
The measurement is made in ohms with a target
level of 25-ohms or less being mandated
by the National Electric Code. * Technology
companies commonly require a target of 5-ohms
or less to maintain valid warranty requirements.
*The National Electric Code has a series
of rules and exceptions regarding this target,
and should be referenced directly for further
information.
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| What
is Soil Resistivity? |
We can measure the electrical
resistance of almost anything, including
soil. Understanding the resistance to electricity
that the soil provides enables engineers
to draft and design grounding systems to
meet engineering specifications. The difference
between resistance and resistivity is relatively
simple. Resistivity is resistance placed
in terms of weight or volume, such as, “the
resistance of a pound of copper” or
“the resistance of a gallon of water.”
In the case of soil, we want to measure
a particular volume, typically a cubic meter.
So, the resistivity of soil is given in
ohmmeters.
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| What
is the difference between Soil Resistivity
and Resistance-to-Ground again? |
Soil Resistivity is a measurement
of the earth itself. Resistance-to-Ground
is a measurement of the (metal) electrodes
placed in the ground.
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| What
are some other electrical performance criteria
for grounding electrodes? |
Besides Resistance-to-Ground,
other 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.
Seasonal changes in temperature can cause
very dramatic differences in the resistivity
of the soil, thus impacting the resistance-to-ground
of the grounding system. This is especially
true in areas where permafrost exists (permafrost
is a soil condition that exists in the extreme
northern and southern climates where the
top soil is always frozen).
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| Why
should I have an engineer design my site
grounding system? |
The short answer to
this question is simple: money! Why have
an engineer do blueprints and drawings
for your building? Because you need to
have a plan before you start construction.
Could you imagine a construction company
showing up at a site without blueprints
and not knowing if they were going to
build a skyscraper or an outhouse? In
order to achieve the 5-ohms resistance-to-ground
specification required for many sites,
the grounding system could be as simple
as a few driven rods to as complex as
40-foot deep electrolytic electrodes with
huge radials running hundreds of feet
away from the site.
Digging up the earth is one of the most
expensive things on a job site. Engineering
it in advance ensures efficient use of
construction time and ensures that you
will hit the desired resistance targets.
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| What
is Ground Potential Rise? |
I’ll try to answer
this in as basic terms as I can here in
the FAQ section, because this website
has an entire section dedicated to answering
this question and you will be able to
get more detailed information there.
A GPR is a phenomenon that happens when
a large amount of electricity enters the
earth. This happens with lightning strikes,
high-voltage line faults and at electrical
substations. This electricity must go
somewhere, and just like a pebble being
thrown into a pool of water, the electricity
moves away from the strike point just
like the ripples in a pool of water. You
can imagine that the closer you are to
the strike point, the more electricity
you could be exposed to. GPR events are
typically measured in Volts.
As the electricity travels across the
surface of the earth it will come into
contact with various objects, including
equipment and any personnel standing in
its way. There in lies the potential for
injury to occur to personnel and damage
to occur to equipment. GPR events are
very serious and should not be treated
lightly. Engineers can use special computer
programs to simulate GPR events in the
computer and design effective grounding
systems to provide protection from these
harmful voltages.
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