Grounding: study, design, calculation and measurements for safe electrical systems

What is a grounding system

A grounding system is the set of electrodes, conductors, joints, bars, meshes, equipotential links and connection points that allow parts of an electrical installation to be linked to the ground and to each other.

Your goal is not just to get low resistance. Its main function is to conduct fault, leakage, atmospheric discharge or transient currents in a controlled manner, maintaining dangerous voltages within acceptable limits for people and equipment.

In simple terms: a ground connection is not good just because it measures few ohms. It is good when it fulfills its safety and operation function in the real conditions of the electrical system.

What is grounding for?

A grounding system helps protect people from indirect contact. If a metal casing, panel, motor or structure becomes energized by a fault, grounding helps conduct the fault current and allows protections to operate.

It also allows controlling step and contact voltages, especially in substations, electrical rooms, towers, medium voltage systems and places where relevant fault currents may circulate.

In addition, it provides a common reference for control equipment, automation, instrumentation, communications, medical equipment, electronic systems and sensitive loads. When integrated with lightning protection and surge devices, it also helps dissipate transients and currents of atmospheric origin.

Key concepts

Soil resistivity

Soil resistivity indicates how easy or difficult it is for the soil to conduct electrical current. It is normally expressed in ohm meter, or Ω·m. It depends on humidity, temperature, soil type, compaction, dissolved salts, strata, rocks, fills and seasonal conditions.

Therefore, to design a grounding mesh it is not enough to assume a generic value. The correct thing to do is to measure the resistivity in the field and use that data to calculate the system.

Installed grounding resistor

Grounding resistance represents the opposition that the ground system presents to the passage of current to remote ground. It is an important measurement, but it is not the only safety criterion.

At low voltage it can be very relevant data to verify protection conditions. In medium and high voltage it must be complemented with fault current, clearance time, potential distribution, step and contact voltages, mesh geometry and ground conditions.

Step voltage and contact voltage

Step voltage is the potential difference that can appear between the feet of a person approximately one meter apart. Contact voltage is the potential difference that a person can experience when touching a grounded metal structure, equipment, or part while standing on the ground.

Both are critical in substations, panels, metal structures, fences, transformers, cells and medium or high voltage equipment.

equipotential bond

Equipotential bonding consists of electrically connecting metal parts, structures, masses, ground bars and related systems to reduce potential differences between them. A separate ground for each system may seem neat, but without proper equipotential bonding it can create electrical noise, unwanted currents, or risks to people and equipment.

Measurement methods in grounding systems

Ground resistivity measurement

Resistivity measurement is carried out before design, especially when designing ground grids, substations, industrial systems, towers, electrical rooms, solar plants, BESS or critical installations.

The most used methods are four-electrode Wenner and Schlumberger. In both cases, the measurements must be interpreted to estimate a soil model that then allows calculating mesh resistance, step stresses, contact stresses and system performance.

Installed resistance measurement

Once the system is built or exists, its resistance can be measured to verify condition and traceability. The potential drop method is the most classic when there is sufficient space to install auxiliary electrodes and adequately separate the system under test from the current and potential electrodes.

The clamp method may be practical for inspections of existing systems, but should not be treated as a universal method. Its reliability depends on the existence of parallel return paths and that the configuration allows a representative reading.

Measurement of step and contact voltages

In medium and high voltage systems, substations, electrical yards, transmission towers, industrial plants and generation or storage systems, measuring or calculating step and contact voltages can be more important than obtaining just a resistance value.

Physical inspection and continuity

An isolated electrical measurement is not a substitute for physical inspection. A grounding system can become ineffective due to corrosion, theft of conductors, loose joints, excavation, mechanical damage, changes in terrain, or undocumented modifications.

• Condition of registration chambers, ground bars, joints and welds.
• Continuity of protective conductors and connections to panels, transformers, cells and equipment.
• Interconnection of meshes, metallic structures and equipotential links.
• Update of plans, photographic evidence and instrument calibration certificates.

Design and calculation of grounding

The design must start from real project data: site plan, type of installation, voltage level, ground fault current, protection clearance time, ground resistivity, available area, location of equipment, presence of people, type of surface, criticality, environmental conditions and regulatory requirements.

With these data, it is defined whether a simple bar, an electrode system, a buried mesh, perimeter rings, deep electrodes, interconnection with structures, soil treatment or additional measures are required.

A serious calculation usually considers mesh resistance, earth potential rise, maximum fault current, tolerable step and contact voltages, conductor section, thermal stress, burial depth, conductor separation, surface layer, interconnection with other earths and risk of transferred voltages.

Low, medium and high voltage

In low voltage, grounding is directly related to protection against indirect contacts, operation of protections, equipotentiality, panels, protective conductors, differentials and user safety.

In medium voltage, greater risks appear: higher fault currents, substations, transformers, electrical rooms, cells, fences, step and contact voltages, and the need for coordination with protections.

In high voltage, transmission, substations, towers, electrical yards, generation, storage and distribution, the analysis usually requires additional criteria. In Chile, when the supply of an installation is high or medium voltage, the application of RPTD No. 06 or the provisions that replace it must be reviewed.

Applicable standards and references

The applicable reference depends on the type of installation, voltage, scope of the project and current regulations. For real projects, the review must always be done from the official source and with the corresponding current edition.

• RIC N°06: grounding, lightning protection and equipotential bonding in consumer facilities.
• RPTD N°06: grounding for production, transportation, complementary services, storage and distribution facilities of electrical energy.
• IEEE Std 81-2025: methods for measuring resistivity, impedance of ground systems, surface potentials, step and contact voltages.
• IEEE 80: technical reference for safety in grounding systems of alternating current substations.
• IEC 60364-5-54: grounding arrangements, protective conductors and low voltage equipotential bonding.
• IEC 61936-1: electrical installations with power over 1 kV AC.
• IEC 62305-3: protection against lightning, physical damage to structures and risk from step and contact voltages near lightning protection systems.

Grounding and power quality

Grounding also influences power quality, especially in installations with electronic equipment, control systems, medical equipment, data centers, telecommunications, variable frequency drives, UPS, inverters, BESS and networks with disturbances or transients.

In these cases it is not enough to measure resistance. Equipotentiality, continuity of protective conductors, currents through protective conductor, electrical noise, inductive or capacitive couplings, protection against overvoltages, interconnection of functional and protective grounds, electromagnetic compatibility and low, high or very high frequency transients must also be reviewed.

Common mistakes

• Believing that a ground is just a buried rod.
• Evaluate only the value in ohms without checking step and contact voltages.
• Measure without checking if the method corresponds to the type of installation.
• Use clamp as a universal method.
• Do not consider corrosion, loose joints or mechanical damage.
• Separate lands without equipotentiality criteria.
• Do not update the design after extensions, new loads, BESS, generation or protection changes.

When is it appropriate to do a study

It is advisable to carry out a study, design, calculation or measurement of grounding when a new electrical installation is built, a substation or electrical room is planned, a transformer is installed, generation or BESS is incorporated, critical infrastructure is enabled, an installation is required to be declared or put into service, electronic failures or power quality problems appear, major maintenance is carried out or technical support is needed for audits and decision making.

A professional technical report should include objective, scope, applicable standards, description of the installation, instruments used, calibration certificates, measurement conditions, method applied, sketch, results, physical inspection, photographic record, technical analysis, compliance or deviations, recommendations and conclusions for operation, maintenance or design.

Conclusion

Grounding is an essential part of electrical safety. Its function is not only to lower resistance, but to control dangerous voltages, allow the operation of protections, dissipate fault currents or transients, maintain equipotentiality and provide a reliable reference for electrical and electronic equipment.