What other types of grounding electrodes are there apart from ground rods?

What other types of grounding electrodes are there apart from ground rods?

Grounding Plates

Grounding plates are typically thin copper plates buried in direct contact with the earth. The National Electrical Code requires that ground plates have at least 2 ft2 of surface area exposed to the surrounding soil.  Ferrous materials must be at least 0.20 inches thick, while non-ferrous materials (copper) need only be 0.060 inches thick. Grounding plates are typically placed under poles or supplementing buried ground rings. 

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 as shown in “B”.  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 the same mass. Similar environmental conditions, which lead to the failure of the driven rod, also plague the grounding plate, such as corrosion, aging, temperature, and moisture.

Concrete Encased Electrodes

Originally, Ufer grounds were copper electrodes encased in the concrete surrounding ammunition bunkers. In today’s terminology, Ufer grounds consist of any concrete-encased electrode, such as the rebar in a building foundation, when used for grounding, or a wire or wire mesh encased in concrete.

The National Electrical Code requires that 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.  The advantages of concrete encased electrodes are that 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. 

Concrete encased electrodes also have some significant disadvantages. When an electrical fault occurs, the electric current must flow out of the conductor and through the concrete to get to the earth. Concrete, by nature retains a lot of water, which rises in temperature as the electricity flows through the concrete. If the concrete encased electrode is not sufficient to handle the total current, the boiling point of the water may be reached, resulting in an explosive conversion of water into steam. Many concrete encased electrodes have been destroyed, after receiving relatively small electrical faults. Once the concrete cracks apart and falls away from the conductor, the concrete pieces act as a shield preventing the copper wire from contacting the surrounding soil, resulting in a dramatic increase in the resistance-to-ground of the electrode.

There are many new products available on the market designed to improve concrete encased electrodes. The most common are modified concrete products that incorporate conductive materials into the cement mix, usually carbon. The advantage of these products is that they are fairly effective in reducing the resistivity of the concrete, thus lowering the resistance-to-ground of the electrode encased. The most significant improvement of these new products is in reducing heat buildup in the concrete during fault conditions, which can lower the chances that steam will destroy the concrete encased electrode. However, some disadvantages are still evident.

Again, these products do not increase the zone-of-influence and as such, the resistance-to-ground of the concrete encased electrode, is only slightly better than what a bare copper wire or driven rod would be in the ground. Also a primary concern regarding enhanced grounding concretes is the use of carbon in the mix. Carbon and copper are of different nobilities and will sacrificially corrode each other over time. Many of these products claim to have buffer materials, designed to reduce the accelerated corrosion of the copper, caused by the addition of carbon into the mix. However, few independent long-term studies are being conducted to test these claims.

Ufer Ground or Building Foundations

Ufer Grounds or building foundations may be used provided that the concrete is in direct contact with the earth (no plastic moisture barriers), that rebar is at least 0.500 inches in diameter and that there is a direct metallic connection from the service ground to the rebar buried inside the concrete.

This concept is based on the conductivity of the concrete and the large surface area, which will usually provide a grounding system, which can handle very high current loads. The primary drawback occurs during fault conditions, if the fault current is too great compared with the area of the rebar system, when moisture in the concrete superheats and rapidly expands, cracking the surrounding concrete, threatening the integrity of the building foundation. Another important drawback to the Ufer ground is that they are not testable under normal circumstances, as isolating the concrete slab in order to properly perform resistance-to-ground testing, is nearly impossible.

The metal frame of a building may also be used as a grounding point, provided that the building foundation meets the above requirements, and is commonly used in high-rise buildings. It should be noted that many owners of these high-rise buildings are banning this practice and insisting that tenants run ground wires all the way back to the secondary service locations on each floor. The owners will already have run ground wires from the secondary services back to the primary service locations and installed dedicated grounding systems at these service locations. The goal is to avoid the flow of stray currents, which can interfere with the operation of sensitive electronic equipment.

Water Pipes

Water pipes have been used extensively over time as a grounding electrode. Water pipe connections are not testable and are unreliable due to the use of tar coatings and plastic fittings. City water departments have begun to specifically install plastic insulators in the pipelines, to prevent the flow of current and reduce the corrosive effects of electrolysis. The National Electrical Code requires that at least one additional electrode be installed, when using water pipes as an electrode. There are several additional requirements including:

10 feet of the water pipe is in direct contact with the earth,

Joints must be electrically continuous,

Water meters may not be relied upon for the grounding path,

Bonding jumpers must be used around any insulating joints, pipe or meters,

Primary connection to the water pipe must be on the street side of the water meter,

Primary connection to the water pipe shall be within five feet of the point of entrance to the building.

The National Electrical Code requires that water pipes be bonded to ground, even if water pipes are not used as a grounding electrode.

Electrolytic Electrode

The electrolytic electrode was specifically engineered to eliminate many of the drawbacks found in other types of grounding electrodes. The electrolytic electrode consists of a hollow copper shaft, filled with salts and desiccants whose hygroscopic nature draws moisture from the air. The moisture mixes with the salts to form an electrolytic solution, which continuously seeps into the surrounding backfill material, keeping it moist and high in ionic content. The electrolytic electrode is installed into an augured hole and is typically backfilled with a conductive material, such as bentonite clay. The electrolytic solution and the backfill material work together to provide a solid connection between the electrode and the surrounding soil, that is free from the effects of temperature, environment, and corrosion. The electrolytic electrode is the only grounding electrode that improves with age. All other electrode types will have a rapidly increasing resistance-to-ground as the season’s change and the years pass. The drawbacks to these electrodes are the cost of installation and the cost of the electrode itself.

Various backfill products are available in the market place; the primary concern should be if the product protects the electrode from corrosion and improves its conductivity. Carbon-based products should be avoided as they will corrode the copper over time. 

There are generally two (2) types of electrolytic electrodes that one can install, ones that use sodium chloride (table or rock salt), and those that use magnesium sulfate (Epsom salt).  There are advantages and disadvantages for each type.  The electrolytic electrodes that use sodium chloride have very long life-spans (30 to 50 years) and as such are often sealed closed as there is no need to access the tube.  The disadvantage is that very little salt actually enters the surrounding soil, so the time it takes to lower the resistance-to-ground can be very long (years if not decades).  The electrolytic electrodes that use magnesium sulfate come with an access cap at the top of the electrode, as the magnesium sulfate will rapidly dissolve away and out of the tube entering the surrounding soil, thus quickly lowering the resistance-to-ground.  The disadvantages of magnesium sulfate electrodes is that they require annual maintenance to refill the salts in the tube, and if the magnesium sulfate is exposed to high-heat, such as from a lightning strike, chemical reactions can occur resulting in some toxic substances.  The MSDS sheet for magnesium sulfate should be consulted prior to use.  Some grounding engineers have installed electrolytic electrodes with magnesium sulfate for the first few years of operation so as to rapidly lower the resistance-to-ground, and then switched over to a sodium chloride and desiccant mix for the long life and low-maintenance once the surrounding soil has been saturated with conductive materials. 

250.52 Electrodes Permitted for Grounding.

According to 250.53(A)(3), if multiple rod, pipe, or plate electrodes are installed to meet the requirements of 250.53(A)(2), they must be at least 6 feet apart.

Section 250.52 provides a list of what items can be used as a grounding electrode. This includes the following:

Metal Underground Water Pipes under certain conditions

Metal In-ground Support Structures under certain conditions

Concrete-Encased Electrodes

Ground Rings

Rod and Pipe Electrodes

Other Listed Electrodes

Ground Plates

Other Local Metal Underground Systems or Structures

According to 250.53(A)(2), a single rod, pipe, or plate electrodes needs to be supplemented with an additional electrode unless it can be proven that a single rod, pipe, or plate grounding electrode has a resistance to earth of 25 ohms or less.

According to 250.53(A)(3), if multiple rod, pipe, or plate electrodes are installed to meet the requirements of 250.53(A)(2), they shall not be less than 6 feet apart.

Below is a preview of Article 250. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC® edition of NFPA 70.

2020 Code Language:

250.52 Grounding Electrodes.

250.52(A) Electrodes Permitted for Grounding.

(1) Metal Underground Water Pipe. A metal underground water pipe in direct contact with the earth for 3.0 m (10 ft) or more (including any metal well casing bonded to the pipe) and electrically continuous (or made electrically continuous by bonding around insulating joints or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductor(s) or jumper(s), if installed.

(2) Metal In-ground Support Structure(s). One or more metal in-ground support structure(s) in direct contact with the earth vertically for 3.0 m (10 ft) or more, with or without concrete encasement. If multiple metal in-ground support structures are present at a building or a structure, it shall be permissible to bond only one into the grounding electrode system.

Informational Note: Metal in-ground support structures include, but are not limited to, pilings, casings, and other structural metal.

(3) Concrete-Encased Electrode. A concrete-encased electrode shall consist of at least 6.0 m (20 ft) of either (1) or (2):

(1) One or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (1∕ 2 in.) in diameter, installed in one continuous 6.0 m (20 ft) length, or if in multiple pieces connected together by the usual steel tie wires, exothermic welding, welding, or other effective means to create a 6.0 m (20 ft) or greater length; or

(2) Bare copper conductor not smaller than 4 AWG

Metallic components shall be encased by at least 50 mm (2 in.) of concrete and shall be located horizontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth. If multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.

Informational Note: Concrete installed with insulation, vapor barriers, films or similar items separating the concrete from the earth is not considered to be in “direct contact” with the earth.

(4) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 2 AWG.

(5) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 2.44 m (8 ft) in length and shall consist of the following materials.

(a) Grounding electrodes of pipe or conduit shall not be smaller than metric designator 21 (trade size 3∕ 4) and, where of steel, shall have the outer surface galvanized or otherwise metal-coated for corrosion protection.

(b) Rod-type grounding electrodes of stainless steel and copper or zinc coated steel shall be at least 15.87 mm (5∕ 8 in.) in diameter, unless listed.

(6) Other Listed Electrodes. Other listed grounding electrodes shall be permitted.

(7) Plate Electrodes. Each plate electrode shall expose not less than 0.186 m2 (2 ft2) of surface to exterior soil. Electrodes of bare or electrically conductive coated iron or steel plates shall be at least 6.4 mm (1∕ 4 in.) in thickness. Solid, uncoated electrodes of nonferrous metal shall be at least 1.5 mm (0.06 in.) in thickness.

(8) Other Local Metal Underground Systems or Structures. Other local metal underground systems or structures such as piping systems, underground tanks, and underground metal well casings that are not bonded to a metal water pipe.

Grounding Electrodes and Grounding Electrode Conductors

In today’s guide, we are getting into the integral components of an electrical system – the grounding electrode and its conductors. This discussion will encompass the correct sizing of grounding electrode conductors. We will explore the variety of grounding electrodes permitted according to Article 250 of the 2023 National Electrical Code.

Defining Grounding Electrode and Its Conductors

Before delving deeper, let’s understand the terminology as defined in NEC Article 100:

Grounding Electrode: A conducting object establishes a direct connection to earth.

Grounding Electrode Conductor: A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.

In simpler terms, the grounding electrode is a conductive object that forms a bridge between the electrical system and the earth. The grounding electrode conductor serves to link this electrode to the electrical apparatus and/or the grounded conductor (neutral).

Types of Grounding Electrodes

NEC 250.52(A) details several recognized grounding electrode types. When any of the following grounding electrodes are present within a building or structure, NEC 250.50 mandates their interconnection to create a unified grounding system:

 Grounding Electrode Types

Metal underground water pipes

In-ground support structures made of metal

Concrete-encased electrodes

Ground rings

Rods, pipe, and plate electrodes

Officially listed electrodes

Other local underground metal systems or structures

Examples of GEC’s – Source: https://www.electriciantalk.com/

Grounding Electrode Installation Requirements

NEC 250.53 outlines the installation criteria for different grounding electrodes. Some key stipulations include:

Rod, Pipe, and Plate Electrodes should be:

Situated beneath the moisture level, devoid of any coatings like paint or epoxy.

Accompanied by an additional electrode type as detailed in NEC 250.52(A). In most cases, achieving a resistance to earth below 25 ohms is necessary. You can connect this supplemental electrode in various ways, as the NEC lists.

Installation Depth and Protection: You must ensure that rod or pipe electrodes have a minimum of eight feet of contact with the soil and protect them against physical damage.

You may drive rod or pipe electrodes at a 45-degree angle when encountering rock bottom. You can also bury it in a trench at least 30 inches deep when encountering rock bottom at a 45-degree angle. The upper end of the electrode must remain flush with or below ground level, unless there is protection against physical damage for both the aboveground end and the attachment of the grounding electrode conductor.

The rod or pipe electrodes must be a minimum of six feet apart. Rods may only be installed at an angle if it is not possible to drive the rod eight feet vertically. Burying the rod horizontally 30 inches below grade is only permitted if driving the rods vertically or at angle is not possible due to rock bottom.

Plate Electrodes: These should be installed no less than 30 inches beneath the earth’s surface.

Examples of GEC depths with ground rods – Source: https://www.nfpa.org/

The key point to remember from this section is that, in many instances, you must install at least two ground rods, pipes, or plates unless you can measure or verify a resistance of 25 ohms or less. Measuring soil resistivity can pose challenges and tedium, making it advisable to install an additional ground electrode. Additionally, keep in mind that you can bond the additional grounding electrode in various ways, as mentioned. Most commonly, people bond the grounding electrodes together with a bonding jumper for ease of installation. 

Sizing Grounding Electrode Conductors

Determining the Correct Size for Grounding Electrode Conductors

To size grounding electrode conductors correctly, refer to NEC Table 250.66. This involves knowing the size of the largest ungrounded (hot) conductor which feeds the service. For example, if we have a service fed by 2/0 Copper conductors we would need to use a 4AWG Copper or 2AWG Aluminum GEC.

Source: NFPA Link – https://link.nfpa.org/

Note 1 mentions the proper way to size grounding electrode conductors for multiple sets. The way this is done is by taking the sum of the equivalent area of one phase/line conductor in each set. For example, if we had five sets of three 500kcmil copper conductors, we would add up the area of one conductor of each set. 5 x 500kcmil = 2000kcmil. We would use a 3/0 Copper GEC in this instance.

Raceways and Enclosures Used for Grounding Electrode Conductors

NEC 250.64(E) requires that all ferrous metal raceways and enclosures used for grounding electrode conductors shall be electrically continuous from the point of attachment and be bonded at each end of the raceway or enclosure to the grounding electrode or electrode conductor. This rule does not apply to nonferrous metal raceways and enclosures.

Closing Remarks

Understanding the grounding electrode and its conductors is pivotal in safeguarding electrical systems and ensuring their optimal functionality. Adhering to the regulations and guidelines set by the NEC not only facilitates the proper setup of these components but also guarantees a safer and more efficient electrical environment. It is our hope that this guide has shed light on the intricacies of grounding electrodes and their conductors, steering you toward a more informed application of these elements in your electrical systems.

All references to the National Electrical Code, NEC, or NFPA are copyrights of the NFPA, these citations are owned by the National Fire Protection Agency and are only here for educational reference.

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