Grounding Studies, Measurements and Reports
A properly designed and installed grounding system is one of the fundamental requirements for the proper operation of an electrical system, especially regarding reliability and safety.
The main functions of this system are:
– To Ensure that the earth resistance is as low as possible to safely conduct earth fault currents;
– To keep the potentials produced by the fault current within safety limits, preventing harm to people and animals;
– To make protective devices more sensitive, allowing earth leakage currents to be quickly isolated;
– To provide safe conduction of lightning discharge currents;
– To eliminate static charges generated on equipment enclosures.
Soil Resistivity
To begin the study and design of a grounding system, it is necessary to understand the soil resistivity as well as its main characteristics regarding soil type or types, its stratification in layers, moisture content, temperature, etc.
amount of water dissolves the salts present in the soil, creating an electrolytic medium favorable to the passage of ionic current.
From these considerations, we can conclude that grounding systems improve their characteristics during rainy periods and worsen during dry periods.
Temperature also influences soil resistivity variation. Regarding temperature alone, resistivity values do not vary much between 10°C and 60°C; however, they increase significantly as the temperature approaches 100°C. At this point, vaporization dries the soil, creating internal bubbles that hinder current conduction. Resistivity also rises sharply and significantly when temperatures drop below zero, since ice causes an abrupt change in the bonding state between the granules that form the electrolytic concentration, making the soil drier.
Regarding soil type and stratification, most soils are not homogeneous but consist of various layers with different resistivities and depths. These layers, due to geological formation, are typically horizontal and parallel to the soil surface. This variation in resistivity across different soil layers causes variation in the dispersion of fault or discharge currents and must be taken into account in the design of the grounding system.
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GROUNDING OF EQUIPMENT AND METALLIC PARTS
When a short circuit or current leakage to ground occurs, it is expected that the current will be high enough for the protection system to operate, eliminating the fault as quickly as possible. While the protection has not yet operated, the fault current flows into the soil, creating different potentials on metallic masses and soil surfaces.
To have a safe and efficient system, all metallic parts that may come into contact with energized parts must be grounded so that, in the event of a fault, the system establishes a phase-to-ground short circuit. This raises the current to values sufficient to trigger the protection, de-energizing the system. Therefore, electrical equipment must be connected to grounding as effectively as possible, given soil conditions, so that the protection can respond quickly and the touch and step voltages remain below the critical limits for ventricular fibrillation of the human heart.
The grounding system can basically be made using a simple rod, multiple interconnected rods (arranged in a line, triangle, circle, etc.), conductive plates, wires or extended cables, or other forms specified by standards. The choice of grounding system depends on the size, voltage, importance, and characteristics of the system. The most efficient system is when grounding is done through a grounding mesh.
The design of a grounding system must basically follow the steps below:
- a) Define the grounding location;
- b) Perform multiple soil resistivity measurements at the planned site;
- c) Based on these measurements, determine the soil stratification and its respective layers;
- d) Define the type of grounding system required;
- e) Calculate the apparent soil resistivity;
- f) Design the grounding system, taking into account the sensitivity of the relays and the personal safety limits, that is, the ventricular fibrillation threshold of the human heart.