An Introduction to Safety Grounding

Asser A. Zaky

• Publisher: CRC Press
• Year: 2021
• eBook ISBN: 9781000408294
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería eléctrica y telecomunicaciones

Safety or protective grounding is of vital importance for the protection of individuals from electric shock and structures and industrial concerns from potentially damaging lightning and electrostatic discharges. To many electrical engineers the notion of grounding is nebulous and safety grounding is quite often confused with neutral grounding of the power supply. The main objective of this book is to give the reader a better understanding of safety grounding, why it is needed, where it is needed, and what are the requirements which must be met in order to have an effective grounding system. The text as a whole serves to provide the reader with the necessary background for a better appreciation of the various National and International Standards concerned with safety grounding.

This book gives the reader a good understanding of the fundamentals of safety grounding. It is a practical guide that provides a comprehensive coverage of all types of grounding requirements and is intended for students and practicing electrical engineers alike.

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• Summarizes the physiological effects of current on the human body and the effect of current duration

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• Gives the various methods of measuring soil resistivity and measuring the resistance to ground of an electrode or grounding system

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• Reviews different types of ground electrodes and the effect of their geometry and numbers on the resistance to ground

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• Presents the components of a ground system, methods of improving soil resistivity, the types of welds and joints, the criteria for determining conductor cross-sections, galvanic corrosion, and a survey of the different grounding practices used at substations and the different types of grounding systems used for the protection of consumers

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• Deals with electrostatic and lightning hazards that can cause serious damage and the measures used to protect against such damage

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• Throughout the text frequent reference is made to various National and International Standards and their requirements as compliance with these standards is highly advised

Asser A. Zaky, Ph.D., FIET, F.Inst.P., FIEEE, is Emeritus Professor of Electrical Engineering at University of Alexandria, Egypt.

1 Effect of Current on the Human Body

1.1 Introduction

There are essentially two types of grounding:¹

1. System grounding
2. Protective or equipment grounding

In the first type of grounding the star points of the equipment may be solidly grounded or grounded through a resistance or an inductance (Petersen coil) according to the operating requirements of the network. These requirements depend on several factors such as the maximum permissible stress on the insulation, the magnitude of the short circuit current, and the overall protective characteristics of the network.

The second type of grounding, which is the subject of this book, has two objectives:

1. To protect people (and animals) in the event of the occurrence of a fault to ground.
2. To protect buildings and installations against fire and lightning.

In order to protect people (operators, maintenance and repair technicians, and the public at large) against electric shock if they come in contact with metal parts which normally are not live and do not carry any current, it must be ensured that under fault conditions the potential of such parts does not rise to a value which would be considered dangerous to persons or give rise to leakage currents which, even if very small, can with time raise the temperature of the material through which it flows to a value sufficient to initiate a fire if the material is readily inflammable. Protection is provided by deliberate grounding of all metal structures, motor, generator and transformer frames, metal enclosures of all tools and control equipment, connection boxes, cable trays, and all other metal bodies which contain or are adjacent to electric circuits and which are within reach of any person.

Figure 1.1 shows the equivalent circuit of an electrical equipment connected to a supply source of voltage V. The resistances shown are as follows:

FIGURE 1.1 Isolated equipment.

R₁: resistance of the insulation between live parts and equipment case,

R_g: resistance between casing and ground, and

R_b: resistance of a person’s body to ground.

¹ The terms grounding and earthing are used interchangeably.

The voltage V_t which appears on the case is

$$V_t=\frac{V{R}_g}{R_1+R_g}(1.1)$$

When a short circuit occurs between the live conductor and the case (R₁ = 0), V_t becomes equal to V and the current which flows through the body of a person who touches the frame is

$$I_b=V\text{/}{R}_b.$$

If the case were connected to a perfect ground such that R_g = 0 then the case would always be at zero potential. However, in practice the resistance to ground is never zero so that under fault conditions the case voltage is V and remains at that value until the protective device (fuse or circuit breaker) disconnects the supply (Figure 1.2). To make certain that the person who touches the case is not at risk the circuit must be disconnected within a specified time (see Section 1.3); since the operating time of the protective devices depends on the magnitude of the short circuit current, it is necessary to ensure that the resistance between the case and ground is sufficiently small to allow the passage to ground of a current whose value is sufficient to operate the protective device within the permitted time. To fulfill this requirement the resistance to ground must not exceed a certain value usually specified by national or international standards. This value varies between 1 and 25 Ω depending on the magnitude of the short circuit current.

FIGURE 1.2 Earthed equipment.

As for the protection of buildings and installations against lightning strikes, this is accomplished by earthing systems especially designed for this purpose and its implementation is determined by how critical such a protection is. Protection of structures against lightning is dealt with in Chapter 8.

In all of the national and international specifications protective grounding of all equipment is an obligatory safety requirement irrespective of other considerations such as electromagnetic compatibility, for example. It is therefore of primary importance that there be a close cooperation between the engineering consultant responsible for the design of supply and grounding system of any installation and the designer of the electronic equipment to be installed in the building in order to choose the optimum system which will meet both the safety requirements and the compatibility requirements within the electromagnetic environment at the premises.

1.2 Effect of Electric Shock on Human Beings

A person is subjected to an electric shock if he touches any live conductor or other metallic body while at the same time touching another grounded body or standing on a moist ground or is barefooted. Although the common belief is that the voltage is the cause of the shock, the consequences and severity of the shock depend on many factors, the most important of which is the magnitude of the current which flows through the body, the path of this current, and its duration.

Numerous studies and observations have shown that the effect of a low-frequency current (0–300 Hz) on the human body varies with the magnitude of the current which flows through the chest area. Table 1.1 shows a summary of the effects produced...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Chapter 1 Effect of Current on the Human Body
• Chapter 2 Resistance to Ground
• Chapter 3 Earthing Electrodes
• Chapter 4 Step, Touch, and Transfer Voltages
• Chapter 5 Grounding Systems
• Chapter 6 Substation Grounding Systems
• Chapter 7 Static Electrification
• Chapter 8 Protection against Lightning
• Appendix A: Wire Sizes
• Bibliography
• Index