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Electrical Safety of Low Voltage Systems by Massimo A. G. Mitolo
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Electrical Safety of Low Voltage Systems by Massimo A. G. Mitolo


Electrical safety may be perceived only as a list of prudent actions to or not to undertake in the presence of energized objects, constituting the defense against direct contact with live parts. However, the safety of persons also depends on their exposure to indirect contact, that is, contact with parts normally not in tension, but likely to become energized due to faults.

Thus, the attitude toward live parts is not the only key in preventing accidents. This book, prompted by this concept, is an attempt, from the academic point of view, to bridge the existing gap between life-safety electrical issues in low-voltage systems (i.e., not exceeding 1 kV) and their proper comprehension and design solution, in light of applicable IEC and IEEE standards.

We assume, in fact, that we can analytically quantify the hazards caused by indirect contact, thereby promoting a proper design for the electrical system and minimizing the related risk.

The book, based on my 20-year-long experience as a professor and as a professional engineer, provides an explanation of the fault-loops in different types of grounding systems (i.e., TT, TN, and IT) and of the faults occurring on both sides of the supply (i.e., the primary and secondary of substation transformers).

The crucial role played by the state of the neutral is deeply examined, thereby allowing the comprehension of the reasons behind the methodologies of protection against electric shock, which are required by current standards and codes.

The book’s audience consists of electrical engineering students who need to know the principles of electrical safety as well as professional engineers who are involved in the bonding and grounding of power systems.

Background requirements include a knowledge of a.c. electric circuits, algebra, complex numbers, and basic calculus. Each chapter is arranged in a format that is aimed at promoting the reader’s understanding by providing many figures and equivalent circuits to clarify, both visually and analytically, the concepts discussed, such as the determination of fault currents and touch voltages.

Several chapters also have a section of frequently asked questions at the end, with relative answers based on the actual inputs of students and professionals. The first three chapters explain the fundamental principles of electrical safety, providing the basic concepts of protection against direct contact and indirect contact as well as the mathematical interpretation of safety and risk of standard protective measures.

Chapter 4 discusses the role of the earth as an available return path to the supply source of fault currents, thus analyzing the theory of ground potentials and ground resistances of electrodes. Chapter 5 describes the effects of currents passing through the human body as interfering with the body’s own electricity as well as causing thermal stress to its tissues.

This chapter also explains the concepts of permissible body current and permissible touch voltage as used in IEC and IEEE technical standards. Chapters 6 through 9 explain the protection against indirect contact in different grounding systems, such as TT, TN, PME, and IT, and detail voltage exposures and protective issues in each of them.

Chapter 10 is devoted to the extra-low-voltage systems and describes the safety issues arising under fault conditions. Chapter 11 describes the fundamental components of earthing arrangements, explains their functions, and provides minimum acceptable sizes following applicable technical standards.

An analytical method to determine the minimum cross-sectional area of protective conductors, assuming an adiabatic thermal process during faults, is also offered. Chapter 12 discusses the effects of overvoltages, in particular the temporary ones, within different types of grounding systems as well as the stress voltages that may arise under fault conditions, possibly causing the breakdown of the basic insulation of equipment.

Chapter 13 examines the safety issues caused by static electricity and residual voltages, eventually present on de-energized items. The energy stored in charged objects is calculated and the mitigation strategies to reduce it are described.

Chapter 14 discusses the methodologies of measurement employed during the design phase (e.g., soil resistivity test) and after the installation of the electrical system as well as prior to putting it into service (e.g., earth resistance test).

The final chapter analyzes the safety requirements against indirect contact employed in special installations or locations, where environmental conditions may increase the risk of indirect contact (i.e., marinas, train stations, swimming pools, surgery rooms, etc.).

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