Electrical grounding: types, standards and how to measure correctly

1. What is electrical grounding and what is it for

Electrical grounding is the deliberate connection between a part of an electrical installation (or equipment) and the earth (ground), creating a low-impedance path for the return of fault and protection currents.

This connection fulfills three fundamental and complementary functions:

• Personnel protection: in case of insulation failure, current flows to ground instead of passing through the human body. The protection system (circuit breaker or RCD) acts quickly to disconnect the circuit.
• Equipment protection: ensures a stable potential reference and drainage of transient overvoltages (caused by lightning or switching operations on the grid).
• Correct operation of protections: without adequate grounding, RCDs and differential circuit breakers cannot reliably detect fault currents.

2. Types of grounding electrodes

NBR 5410 and NBR 7117 recognize different types of electrodes, each with specific application characteristics:

2.1 Grounding rods (vertical electrodes)

They are the most common electrodes. They consist of copper-coated steel bars (copperweld) with a standard length of 2.4 m or 3 m and diameters of 3/4" or 5/8". They are driven vertically into the ground and connected to the grounding busbar by bare copper cable.

Advantages: easy installation, takes up little surface space, effective in low-resistivity soils. Limitation: in rocky or high-resistivity soils, it may not reach the required resistance with a single rod.

2.2 Horizontal mesh (buried plate or cable)

Bare copper cable (usually 35 mm² or 50 mm²) buried horizontally at a minimum depth of 0.5 m, forming a mesh or rings around the building. Very effective for large areas such as substations, industries, and hospitals.

2.3 Grounding plate

Metal plate (copper or galvanized steel) buried vertically. Minimum area of 0.5 m². Increasingly less used because it takes up too much space and is less efficient than rods combined with a mesh.

2.4 Foundation electrode (Ufer Electrode)

Copper cable embedded in the reinforced concrete foundation of the building. It takes advantage of the conductivity of damp concrete and the large contact area with the soil. Excellent solution for new constructions — low cost and very good performance over time.

2.5 Parallel electrodes

When a single rod does not reach the required resistance, multiple rods are installed in parallel. For the combination to be efficient, the spacing between rods must be at least twice the length of the rod (e.g., 3 m rods must have a minimum of 6 m between them). Rods that are too close together have a mutual shading effect in the soil.

3. Applicable standards

The main Brazilian standards that govern electrical grounding are:

4. Grounding resistance: what it is and acceptable values

Grounding resistance (Ra) is the opposition that the electrode-soil set offers to the passage of electric current towards the earth. It is measured in ohms (Ω) and is the main indicator of the quality of a grounding system.

Reference values by standard and application:

5. How to measure with earth tester (3-point method)

The most precise and widely accepted method for measuring grounding resistance in the field is the three-point potential drop method (Wenner method or Fall-of-Potential), described in NBR 7117.

Required equipment

Digital earth tester (grounding resistance meter), two auxiliary electrodes (iron or steel stakes), measurement cables, and tape measure.

Step-by-step procedure

1. Disconnect the electrode from the installation grounding busbar before measuring (to avoid interference from parallel grounds).
2. Position the current electrode (C2) at a distance D from the electrode under test, in the direction of least interference (away from buried cables and metallic structures).
3. Position the potential electrode (P2) at 62% of the distance D, on the same line (practical rule consecrated by the standard).
4. Connect the earth tester terminals: C1 and P1 to the measured electrode, C2 and P2 to the auxiliary electrodes.
5. Perform the measurement and record the value in Ω.
6. Repeat with D' = 0.5D and D'' = 1.5D. If the three values are close (variation < 10%), the measurement is reliable.

6. Factors affecting soil resistivity

Grounding resistance is not a constant — it varies with soil conditions:

• Soil resistivity (ρ): varies from 10 Ω·m (flooded soil) to more than 10,000 Ω·m (dry granite rock). Damp clay soils are the most favorable.
• Moisture: in dry periods, resistance can increase 5 to 10 times. Therefore, measurements should be made at different times of the year.
• Temperature: frozen soils have very high resistivity. In Brazil the impact is lower, but southern and southeastern regions with cold winters must consider this factor.
• Electrode depth: in deeper layers the soil generally has higher moisture and lower resistivity.
• Salinity: soils with a higher concentration of mineral salts are better conductors.

7. Grounding mesh: when to use

In large installations or with high-resistivity soils where isolated rods do not reach the required resistance, the solution is the grounding mesh: a grid of bare copper cables buried in depth, forming a network under the area of the building or substation.

The mesh serves two simultaneous objectives:

• Low overall resistance: the large contact area with the soil drastically reduces grounding resistance.
• Surface equipotentialization: reduces voltage gradients in the soil (step and touch voltage) in case of a fault, protecting people in the area.

Meshes are mandatory in medium and high voltage substations (NBR 14039) and strongly recommended in data centers, hospitals, and large industries.

8. Common grounding installation errors

Over years of inspection and consulting, these are the most frequently found problems:

1. Rod installed in an inadequate location: inside concrete sidewalks, close to pipes, or in rocky soil, with no possibility of effective contact with the ground.
2. Oxidized or loose connections: the connection between the cable and the rod must be made by certified compression connectors or exothermic welding (Cadweld). Simple wire tying is not acceptable.
3. Undersized grounding cable: the cross-section of the protective conductor must follow the NBR 5410 table — it cannot simply be "what was left over from the construction".
4. Grounding not measured after installation: installing the rod does not guarantee adequate resistance. It is mandatory to measure with an earth tester and document the result.
5. Multiple isolated grounding systems: electrical, LPS (SPDA), and telecommunications must be interconnected in a common busbar. Separate systems create dangerous potential differences during fault or lightning events.
6. Absence of SPD: good grounding without adequate SPDs does not protect electronic equipment against induced overvoltages.

9. Relationship with LPS (SPDA)

The grounding of the LPS (SPDA) and the electrical grounding of the installation must be integrated into a single grounding system, with the main equipotentialization busbar (MEB) interconnecting all systems. This is an explicit requirement of both NBR 5419 and NBR 5410.

The separation of the two systems was common practice in older installations, but is now considered inadequate and dangerous: during a lightning discharge, the current in the LPS (SPDA) creates a potential difference between the two systems that can cause dangerous sparks inside the building.

10. Conclusion

Electrical grounding is the silent foundation of any safe electrical installation. A well-designed system, with correctly sized electrodes, quality connections, and measured and documented resistance, is the difference between an installation that protects and one that poses a risk.

Always demand a technical report with the grounding measurement result (in ohms) and the ART (Technical Responsibility Note) of the responsible engineer. These documents prove that the system has been verified and is in compliance with current standards.