Electrical Grounding Frequently Asked Questions
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 above ground 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.
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 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”. Other electrodes included concrete-encased electrodes, ground plates, water pipes, building foundations, and electrolytic grounding 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.
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 (NEC) has a series of rules and exceptions regarding this target, and should be referenced directly for further information.
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.
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.
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).
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.
What is Ground Potential Rise?
I’ll try to answer this in as basic terms as I can here in the FAQ section. View our detailed page for information on what is Ground Potential Rise.
A Ground Potential Rise or 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. Ground Potential Rise 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 Ground Potential Rise events are very serious and should not be treated lightly. Engineers can use special computer programs to simulate Ground Potential Rise GPR events in the computer and design effective grounding systems to provide protection from these harmful voltages.