Grounding Plate Sizing And Installation

December 11, 2023 by Anju Thangam Joy

Grounding Plates- How They Work

Earthing electrode with copper plate

Figure 1: Earthing electrode with copper plate

Grounding plates are a crucial component of an earthing system. They are widely used in residential buildings, industrial installations, and power distribution systems to ensure electrical safety and prevent damage to electrical equipment. This article explores grounding plate sizing, installation, and maintenance considerations. Read our grounding overview article for more details on how electrical earthing works.

Table of contents

What is a grounding plate

Grounding

Grounding creates a low-resistance path for fault currents to flow to the ground. This path consists of several components:

  1. Grounding electrode: A metal rod or plate buried in the ground, providing a connection to the earth.
  2. Grounding conductor: A conductor, such as a grounding wire, connects the grounding electrode to the electrical equipment.
  3. Grounding busbar: A metal bar in an electrical panel that serves as a central point for connecting grounding conductors.

Ground plate electrode

When a fault occurs in an electrical system, a high current can flow through the equipment, potentially causing damage and posing a risk of electric shock. The grounding plate provides a safe and direct path for this fault current to dissipate into the earth, minimizing the risk of harm. It works by utilizing the conductive nature of the earth. When a metal plate is buried in the ground, it connects with the surrounding soil and rock. Despite being non-conductors in their pure form, these materials contain moisture and dissolved salts that impart conductivity.

To employ a grounding plate, one typically attaches a conductor - commonly a wire made of copper or aluminum - to the plate, which is then integrated into the grounding system of an electrical setup. Should an electrical fault occur, the current will pass through the conductor to the grounding plate, where it is safely conducted into the ground.

Choosing ground rod or plate as electrode

The choice between a ground rod and a ground plate for an application depends on several factors.

  • Soil conditions: Generally, a ground rod is a long, slender rod driven vertically into the earth and is effective in deep, moist soils where the resistivity is lower. It can reach lower strata that might have better conductivity. A ground plate is a flat piece of metal buried horizontally in the ground and is often used in areas with high soil resistivity, such as rocky terrain or sandy soil, where driving a rod deep enough to reach low-resistivity soil is impractical. Read our article on ground plate vs rods for different soil types to know more about how various soil types affect the selection of ground electrodes.
  • Available space: Ground rods require less space than ground plates, making them a good choice for applications where space is limited. Ground plates, however, can be more effective in areas with limited soil depth or underground obstructions.
  • Ease of installation: Ground rods may be more labor-intensive, particularly in hard or rocky soil. Specialized equipment may be needed, such as a rotary hammer or a ground rod driver. Ground plate installation is generally easier and less dependent on specialized equipment. However, it requires a larger trench for horizontal placement.

Read our grounding rod overview article for more details on the design and installation of grounding rods.

Materials

Grounding plates are usually constructed from metals like copper, aluminum, or steel, selected for their conductive properties, robustness, and ability to resist corrosion. Frequently, grounding plates are made from steel coated with electrolytic copper. The choice of material for a grounding plate is influenced by its intended use, the soil, and the environmental conditions where it will be installed, among other considerations.

Sizing

Selecting the appropriate size of a grounding plate is crucial for ensuring effective electrical grounding and safety. Several factors influence the selection process, including:

  1. Soil resistivity: Soil resistivity, measured in ohms per meter (Ω-m), indicates the soil's ability to conduct electricity. Higher resistivity values imply poorer conductivity, necessitating a larger grounding plate.
  2. Grounding current: The grounding current, measured in amperes (A), represents the maximum current expected to flow through the grounding system during a fault condition. A larger grounding current requires a larger grounding plate to dissipate the current effectively.
  3. Application: The grounding plate's application significantly affects its size. For instance, larger grounding plates are typically used for commercial and industrial applications with higher fault currents than residential ones.
  4. Number of grounding electrodes: If multiple grounding electrodes, such as grounding rods, are used with a grounding plate, the plate size can be reduced. The combined surface area of all grounding electrodes determines the overall grounding system's effectiveness.
  5. Local electrical codes: These codes often specify minimum grounding plate sizes based on regional factors and safety standards. It's essential to adhere to these guidelines to ensure compliance and safety.

Ground plate installation

The installation procedure for a grounding plate varies based on the plate type, soil, and environmental conditions. However, the following steps provide a general guide to installing a grounding plate:

  • Identify an installation location for the grounding plate, ideally a level and clear area without obstructions, with ample access for installation tools and equipment.
  • Remove any vegetation or debris from the selected area. This ensures optimal soil contact for the grounding plate and simplifies the digging and installation process.
  • Excavate a hole using a shovel or another digging tool. The hole should be deep enough to cover the grounding plate completely and slightly larger in width than the plate.
  • Insert the grounding plate into the hole, ensuring it is flat and aligned. As a general rule, grounding plates should be buried at a depth of at least 30 inches (76 cm) and below the frost line (the depth to which the ground freezes in the winter). The grounding plate should be buried deeper than in clay soils in sandy or gravelly soils. This is because sandy and gravelly soils are less conductive than clay soils.
  • Compact the soil surrounding the grounding plate with a tamper or similar tool to guarantee solid contact between the plate and the soil.
  • Connect a conductor, such as copper or copper-clad steel wire, to the grounding plate to tie it into the electrical circuit's grounding system.
  • Perform a test on the installed grounding plate to confirm correct placement and functionality, utilizing a grounding meter or other appropriate testing equipment.

Maintenance

As a standard practice, grounding plates require an annual inspection to identify any signs of wear or damage. These inspections focus on detecting corrosion, cracks, or other imperfections hindering the grounding plate's functionality. Prompt repairs or replacements are necessary if any grounding plates are damaged or defective.

Beyond routine checks, it's important to examine grounding plates following significant weather events or other incidents that could impact the earthing system. This includes assessing the plate for any direct damage and verifying the conductor's condition and other earthing system elements. Damaged components should be repaired or replaced as needed.

International standards for grounding plates

  • The IEC 60364 series outlines electrical installation standards, including earthing systems and grounding plate specifications.
  • IEC 62561-1, which falls within the IEC 62561 series focusing on earthing and lightning protection, defines the design, installation, testing, and maintenance standards for earthing systems, including grounding plates. The standard addresses various aspects such as earthing principles, different earthing system types, and specifications for earthing components such as grounding plates.
  • While UL does not offer a particular standard for grounding plates, it does provide standards pertinent to earthing systems and components within electrical installations. Standards such as UL 467 outline the requirements for ground rods, and UL 467A specifies criteria for ground plates and other grounding connections.
  • In the United States, NFPA 70 is the prevailing standard for the safe installation and maintenance of electrical systems. Within this standard, Article 250 pertains to grounding and bonding requirements, including those for ground plates. This article encompasses various topics like earthing principles, system types, and component specifications, including those for grounding plates.

Quality considerations

When selecting earthing plates, it is important to evaluate several quality attributes. Key quality features to consider when purchasing earthing plates encompass:

  • Material: Choose earthing plates made from materials that exhibit resistance to corrosion and have good conductivity to guarantee both effectiveness and longevity. Typical materials for earthing plates are copper, aluminum, and stainless steel.
  • Size and shape: The earthing plate's size and shape should suit its intended application and the specific soil and environmental conditions. The plate should be adequately sized to establish an effective ground connection while remaining manageable for installation and maintenance.
  • Connections: The connections on the earthing plate must be sturdy and dependable for connecting the conductor and integrating with the rest of the system. These connections should endure the forces exerted during installation and usage and resist corrosion and degradation.
  • Testing and certification: Ensure that the earthing plates have undergone testing and certification to comply with applicable standards and regulations, affirming their safety and efficacy. This could include manufacturer's certification or third-party testing and certification by an independent entity.

FAQs

What is a grounding plate?

A grounding plate is a flat, conductive metal plate buried in the ground and connected to an electrical system to provide a safe path for fault currents and dissipate them into the earth.

How deep does a grounding plate need to be?

A grounding plate should be buried at least 30 inches or below the frost line if it is lower than 30 inches.

What is an earthing rail?

An industrial earthing rail consolidates grounding connections from various electrical equipment, providing a common point for safe current dissipation into the earth.