Massimo Mitolo

Effects of High Fault Currents on Ground Grid Design

Due to increased load demands, and reduced incentives to build new transmission lines, energy companies are increasing power flows on the existing transmission assets, which increases the fault current levels, both three-phase and phase-to-ground, throughout the power system. New generation sources to be added at the transmission and distribution network increase fault current intensities. It is crucial for the user of industrial facility to be aware of increased ground-fault current magnitude at the service entrance as well as of the actual condition of the grid. The protection that ground grids provide against step and touch potentials is only good up to the expected level and duration of ground fault currents, as originally communicated by the electric utility in the design phase. In addition, thermal and mechanical stresses to customers ground grid and ground grid connections can increase the grids resistance to ground and, at the same time, fault potentials. In order to prevent these problems from occurring, a ground grid assessment, utilizing field and utility updated data, should be carried out on a regular basis. This paper illustrates a CENELEC approach to ground grid design, aimed to maximize the electrical safety under ground fault. In addition, case studies were included, showing how high fault currents have damaged ground grids and what repairs are possible

Of Electrical Distribution Systems with Multiple Grounded Neutrals

Multiple grounded neutral systems require that, on most land-based AC current service installations, the bonding conductor used to connect the non-current-carrying metal parts of equipment, the system grounded conductor (neutral), and the grounding electrodes be bonded together at the service entrance box. Users, thus, share their ground with the serving utilitys neutral ground. This interconnection, although functional, poses some distinctive problems to utility companies as well as to users, which this paper seeks to clarify

An effective semi-analytical method for simulating grounding grids

The analytical study of grounding systems is only possible for basic electrodes, i.e. hemispherical and spherical electrodes, rods and horizontal wires. However, it is normal practice to employ more complex earthing systems, such as grounding grids integrated with rods, in order to obtain lower resistances-to-ground and improve the electrical safety of substations. This paper introduces a semi-analytical (or semi-numerical) method, consisting of an analytical approach integrated with a numerical solution, to study grounding grids of complex geometry and their effects on non-stratified soils. The algorithm that was created, and realized with MATLAB, allows the determination of all the quantities of interest for the design and the analysis of such grounding systems: ground-resistance, ground potentials and the distribution of the ground-fault current along the grids components (i.e. horizontal wires and rods). The model is based on the assumption that conductors forming grids have radii very small if compared to their lengths and that the wires can be considered equipotential cylindrical elements. A verification of the proposed algorithm through a finite element method (FEM) has also been carried out to confirm the validity of the results. Exemplary calculations of the ground resistance of grids are included in the paper.

Of International Terminology and Wiring Methods Used in the Matter of Bonding and Earthing of Low-Voltage Power Systems

The worldwide global market requires electrical engineers to have a deep understanding of the bonding and earthing practices adopted in different countries around the world. This knowledge is essential to obtain effective designs and high safety standards and can promote the elimination of technical obstacles that can still create market barriers. The full comprehension of the “grounding” theory requires the command of key technical concepts regarding the earthing methods, which may cause confusion when used in the North American technical realm rather than in the International Electrotechnical Commission (IEC) world. This issue is further worsened by the lack of literature in this matter, as well as of harmonization documents between national codes and international standards. This paper, by analyzing the protection against indirect contact in ac (50/60-Hz) low-voltage power systems by automatic disconnection of supply, seeks to clarify both the terminologies and each type of grounding system adopted in IEC standards, with the intent to create a common reference for practicing engineers in the matter of bonding and earthing of power systems. Major differences encountered between sizing procedures adopted in IEC standards and the North American National Electrical Code are also examined.

To Bond or Not to Bond: That is the Question

The bonding of electrical equipment plays a crucial role in maintaining the same potential between conductive parts likely to be energized and conductive parts liable to introduce a “zero” potential into the premises. Voltage rises between such parts are unsafe, as they may induce harmful currents through the human body, the magnitude of which may vary depending on a number of factors. This paper seeks to clarify the bonding requirements in low-voltage electrical systems, by using the concepts of exposed conductive parts and extraneous conductive parts, present in the International Electrotechnical Commission standards, applied to a proposed electric shock model of the human being. With the purpose of reducing the consequences of electric contacts, the authors propose objective criteria to decide whether conductive “dead” objects and enclosures of electrical equipment must be bonded or not.

TN-Island Grounding System and the House of the Future

Low voltage, single-phase residential loads are usually supplied by the local utility, through a common distribution system (CDS) in a radial layout. In US and UE typical distribution systems, a faulted dwelling unit can affect a healthy one by transferring ground rise potentials, sometime dangerous. The authors propose a novel system defined as TN-island grounding system with the adoption for each customer of a local transformer (LT) with taps, grounded at the mid-point of the secondary winding, supplied by a common distribution system with the neutral grounded but not distributed. This solution allows to reach several goals: - to elevate the immunity of dwelling units, - to stop potential rises, due to ground faults, and of circulation of stray currents - to balance the distributed capacitance-to-ground - to limit the triplen current harmonics naturally caused by the LT - to supply more customers and locally high power common loads HPCL of buildings - to organize common emergency system (UPS and generator sets), common power system management, loads shedding, etc. Moreover, for special conditions (i.e. swimming pool, medical rooms, damp houses), the use of local transformers LTs allows to adopt proper reduced voltage system recommendable owing to the lower value of touch voltage and to adopt further local ungrounded system (IEC IT-system)

Electrical safety in the industrial workplace: An IEC point of view

Electrical professionals working on the installation, construction, repair, and maintenance of facilities are exposed to the risk of electric shock due to their possible interaction with live parts but also due to the contact with the enclosure of equipment, normally not energized, which has become live under fault conditions. This necessary interaction with equipment for its intended use can therefore not be limited. Thus, protective measures are put in place to reduce the risk of electric shock under both fault-free and single-fault conditions to levels considered acceptable by codes, technical standards, or authorities having jurisdiction. Protective measures must be compatible with the working environment, as well as with the skills and knowledge of workers, and may include strategies not just limited to the disconnection of the supply when faults occur. This paper discusses the protective measures against electric shock in the industrial workplace, where installations are controlled or supervised by skilled or instructed persons, as per the definitions of the International Electrotechnical Committee. Protection from other ways in which a worker can get injured from electrical equipment (e.g., arc flash and blasts), although always present, is not hereby considered.

Numerical Simulation of Heart-Current Factors and Electrical Models of the Human Body

Contact with energized parts at different potentials may cause the circulation of body current and the possible inception of the ventricular fibrillation, which is generally considered the most life-threatening effect imposed on the cardiac muscle. The heart-current factors currently present in international standard have been determined owing to measurements on cadavers as well as experiments with volunteers. Due to the difficulties in experimental verifications, the authors propose the use of numerical techniques to investigate the behavior of the human body when it is subjected to electric fields, with the fundamental purpose of increasing the electrical safety of installations. This work is based on a mathematical representation of the human anatomy, which takes into account the boundaries of the internal organs. The simulations carried out and documented in this paper show results that do not entirely match the published IEC heart-current factors; most noticeably, for the pathways hands-feet, right hand-feet, and left hand-right foot, the heart-current factors seem to be overestimated by the IEC.

An Analytical Evaluation of the Prospective

At the occurrence of three-phase or single-phase faults, abnormal levels of thermal energy are developed during the time taken by protective devices to clear them. By conservatively assuming an adiabatic process, all of the thermal let-through energy I2t, also referred to as Joule Integral, is accumulated within the components involved in short circuits; therefore, the temperature of their conductive materials is elevated. The thermal energy is proportional to the square of the short-circuit current. Evaluating the prospective I2t is, therefore, crucial in order to assess the short-circuit capability of cables and busways to withstand the thermal stress without failing or triggering fires in neighboring materials. In this paper, in the general case of resistive-inductive circuits, methods to evaluate the Joule Integral and to perform the assessment will be provided. The differences for power frequencies of 50 and 60 Hz are also shown.

{\rm I}^{2}{\rm t}

The bonding of electrical equipment plays a crucial role in maintaining the same potential between conductive parts likely to be energized and conductive parts liable to introduce a “zero” potential into the premises. Voltage rises between such parts are unsafe, as they may induce harmful currents through the human body, the magnitude of which may vary depending on a number of factors. This paper seeks to clarify the bonding requirements in low-voltage electrical systems, by using the concepts of exposed conductive parts and extraneous conductive parts, present in the International Electrotechnical Commission standards, applied to a proposed electric shock model of the human being. With the purpose of reducing the consequences of electric contacts, the authors propose objective criteria to decide whether conductive “dead” objects and enclosures of electrical equipment must be bonded or not.