Fundamentals of Fault Current and Grounding in Electrical Systems

By Halden Morris, Norman Chambers

Trafford Publishing

Copyright © 2014 Halden Morris & Norman Chambers
All rights reserved.
ISBN: 978-1-4907-3561-0

Contents

Preface, xiii,
Acknowledgment, xv,
Abbreviations, xvii,
Chapter 1 The Fundamentals of Electrical Circuits, 1,
Chapter 2 Codes for Electrical Installation, 20,
Chapter 3 Circuit Protection, 42,
Chapter 4 Fault Current and Transformers, 64,
Chapter 5 Special Circuit Protection Devices, 82,
Chapter 6 Grounding Systems and Functions, 109,
Chapter 7 Improper Grounding Hazards and Special Grounding, 126,
Chapter 8 Earthing Configurations, 146,
Chapter 9 Fault Current Management, 167,
Chapter 10 Open Neutral Conditions, 197,
Chapter 11 Harmonics, 208,
Chapter 12 Lightning Protection, 229,
Chapter 13 First Aid Applications, 244,
Index, 267,

CHAPTER 1

The Fundamentals of Electrical Circuits

1.0 Introduction

In order to get an indepth understanding of earthing systems, it is important that a review of the fundamentals of electrical circuits be done. This is the foundation from which knowledge of advanced studies in electrical technology and engineering is built. In this chapter, the simple circuit, circuit protection, safety, and basic electrical concepts will be explored.

1.1 Simple Electric Circuit

An electrical circuit is a combination of four elements connected in a single loop. Figure 1.1 shows a simple circuit, which comprises a voltage source, a load, and a switch connected in a series configuration with conducting wires.

Complex devices are sometimes employed to enhance safety in electric circuits. Devices such as overcurrent protective devices (OCPD) are used to ensure that the current through the circuit elements does not exceed predetermined parameters. Overcurrent devices (OCDs) are designed to quickly isolate the circuit devices/appliances from their voltage sources if the current exceeds the required values. A circuit may exceed its predetermined parameters as a result of an overload or a short circuit.

1.2 Circuit Protection

Local electrical codes require that all electrical installations shall have adequate mechanical or insulation protection, grounding protection, and overcurrent protection. Circuit protection requires that unwanted currents and voltages be removed from electrical circuits as quickly as possible. Fuses, circuit breakers, and relays provide critical protection by disconnecting unwanted current and voltage within fraction of a second to avoid damage to the circuit components.

1.2.1 Mechanical Protection

Mechanical protection prevents direct access by unintended mechanical force to cables, circuit connections, and devices, and reduces the risk of physical damage to these elements. As shown in Figure 1.2, mechanical protection can be provided by using polyvinyl chloride (PVC) or metal enclosures, PVC or electrical metal tube (EMT)/rigid conduits, or any other provision which will provide adequate mechanical protection for cable and devices in an electrical installation.

In general, conduits used in an electrical installation may be exclusively EMT, exclusively PVC, or a combination of both EMT and PVC. In all cases where conduits are used, insulated conductors are pulled through the conduits and terminated in junction boxes.

1.2.2 Conduit Installation

Conduits play an extremely vital role in all electrical installations. They protect cables from environmental and mechanical damages. Electrical conduits can be overloaded by virtue of the number of cables being pulled through them. The term diversity factor refers to the degree to which a conduit is being loaded.

Overloading of conduits will cause overheating of conductors, which can lead to melting of cable insulation. Once the insulation of the cables inside an overloaded conduit is damaged, a short circuit is likely to occur, which will result in the loss of all cables inside that conduit. If all cables are damaged, it may require total rewiring and redesigning of the installation before returning it to service.

Figure 1.3 shows the layout of conduits, panel box, receptacle boxes, and outlet boxes for an installation. The method of conduiting shown in this figure provides guidance on basic conduiting works which separate circuits from each other and make wiring simple and easy. This method of conduiting reduces the risk of total electrical failure of an installation. If a fire occurs on any circuit, it will be limited to the circuits which are within that conduit.

1.2.2.1 Bending Conduits

Bending electrical conduits, according to specification, eliminates the waste of conduits and damage of cables during pulling. Table 1.1 provides guidance on bending specification in accordance with the following criteria:

1. Radius of conduit bends

2. Size of conduits

3. Tolerance for the bend

Bending of conduits requires using precise measurements to form exact radii and fit. Incorrect radii can lead to the kinking of the conduits. Once a conduit is deformed or kinked, the internal radius is decreased, and pulling of cables may result in damage to the cable insulation.

1.2.2.2 Bending of Rigid Conduits

Figures 1.4 shows the correct procedure for bending a ½ in rigid conduit, using the bending tolerance from Table 1.1. It is imperative that the bending tolerance for various size conduits be observed since this differ for each size. Failing to recognize this will result in wastage of material and improper bends.

1.2.2.3 Bending of PVC Conduits

Bending of PVC conduits can be achieved by using an electric heater blower, a portable propane torch, spring benders, or PVC heating blankets among many other methods. To avoid deformation of the conduit while bending, conduits and bending equipment must be used in accordance with the manufactures’ specifications. Figures 1.5: A and B, Figures 1.6 A and B, and Figures 1.7: A and B show various methods of bending PVC conduits.

1.2.3 Ground Protection

Grounding is an integral part of all electrical installation. It provides the correct path for fault current to return to earth through the circuit’s earth terminal. A fault is generated if a live conductor unintentionally comes in contact with an exposed grounded metal part of an installation.

Ground protection is simply connecting the metal frame of appliances or machineries to the main neutral bar and the general mass of earth to reduce the risk of electrocution. If an individual comes in contact with a live, exposed conductive part on an appliance, for example, refrigerator, washing machine, or dryer, the following effects can occur to the body in a matter of seconds if the current flowing exceeds 4 mA.

1. Negligible sensation or shock.

2. Finger muscles contract and fail to give up their grip almost immediately.

3. Restriction of breathing begins.

4. Disorientation of the control signals to the heart. When this occurs, the heart may be forced to stop working and may result in death.

It is, therefore, imperative that all electrical equipment are properly maintained to protect life and property by ensuring that a solid and effective grounding system exists at all times. Grounding and bonding connections reduce the hazards posed by fault current with the potential of causing electrocution. Figure 1.8 shows typical grounding and neutral connections inside a distribution panel.

1.2.4 Short-circuit Protection

Circuits are protected from damage due to short-circuit situations by the use of devices such as circuit breakers and fuses, which are rated below the rating of the cable and at the rating of the devices being protected.

If a short circuit is created inside a metal switch box as shown in Figure 1.9, the circuit breaker will activate and isolate the circuit immediately from the voltage source whenever the switch is turned on.

1.2.5 Overcurrent Protection

Circuits are protected from overcurrent situations by the use of circuit breakers, fuses, and other current protection devices. When a load exceeds the capacity of a circuit, the circuit protection device will isolate the load from the voltage supply. For example, when a circuit with a 3.5 kW halogen bulb is activated in a circuit rated at 20 A, as shown in Figure 1.10, the protection device will isolate that device momentarily since the current drawn by this device is above the capacity of the circuit protection device.

1.3 Practical Electrical Circuit

Most practical electrical circuits contain a combination of loads and multiple protection devices, which may include the following:

1. OCPDs

2. Main earthing conductor

3. Main neutral conductor

4. Equipment/circuit earthing conductor

5. Earth rod/electrode

6. Earthing clamp

7. Earthing bar in the supply panel

8. The general mass of earth

A practical electrical circuit which incorporates several protective elements and loads is shown in Figure 1.11.

1.4 Safe Work Practices

Safety considerations are critical when performing a task which involves interaction with electricity. Safety procedures are usually associated with these tasks, and deviation from such procedures can result in malfunctioning equipment, damage to property, serious injuries, or even loss of life.

Measures should be taken to mitigate against personal injuries, loss of life, or damage to property during electrical work activities or during the use of electrical equipment or appliances. These safety measures are not limited to but are guided by a set of electrical codes such as the JS31, the National Electrical Code (NEC), the IEE, or the Canadian Electricity Code, which were established by consultation and adhering to best practices.

Electrical injuries are caused by and include, but is not limited to, electric shock, electric burns, electrical explosions or arcing, as well as electrical fires. There are several procedures for working safely with electricity as depicted by Health and Safety Executive (HSE) in the Electricity at Work, Safety and You, 2003. In assessing whether safe work practices can be achieved in areas or on equipment on which work is to be carried out, the flowchart shown in Figure 1.12 can be employed.

The Electricity at Work Act outlined procedures in Figure 1.13 in the form of a flowchart for deciding whether to work on dead or live circuits. Figure 1.14 further outlines the procedures for working dead, and Figure 1.15 provides procedures for working live.

1.5 Troubleshooting Electrical Installations

Trouble shooting an electrical installation involves probing the circuitry to identify faults. This process can be made simple if the principles of operation regarding installations are understood and followed. The following are the three installation protection characteristics to be kept in mind:

1. Current protection (overcurrent device)

2. Touch protection (grounding protection)

3. Insulation protection (mechanical protection)

Most electrical protective devices are triggered by faults associated with the type of protective device protecting the circuit. It is for this reason that some electrical faults are easily identified.

1.5.1 Electrical Faults

Electrical faults are abnormal electrical occurrences which will cause a monitoring or protective device to operate and isolate the voltage source. Faults such as those resulting from moisture, phase-to-ground short, phase to neutral short, phase to phase short, low voltage (LV) and high voltages (HV) are common.

Presented in Table 1.2 are additional types of electrical faults which may result in loss of power, intermittent tripping of circuit breaker, electrical shock, and frequent damage to circuit devices and loads.

1.6 Additional Safety Tips

1. Familiarity breeds contempt; treat all routine tasks as if it were a first approach.

2. Incompetence compromises safety; never attempt to carry out a task without being competent.

3. Be extraordinarily calm and meticulous when working live or dead.

4. Always use insulated tools.

5. Use the correct test instruments to carry out the type of test you are attempting to do.

6. Identify nearest emergency exits.

7. Never attempt to work in a wet area on live circuits.

8. Always wear electrical gloves, simple latex for dead work and testing and UL-rated (Underwriters Laboratories) HV gloves for live work.

9. Wear electrical insulated boots when working on electrical and electronic circuits.

10. Wear electrical insulated gloves when working on electrical and electronic circuits.

11. Wear a helmet when on the job.

12. Wear eye protection when required.

13. Always use electrical tools such as pliers and screwdrivers with their insulation rated at 1000 V.

14. Implement lockout and tag-out systems where possible.

1.7 Conclusion

Whether the installation is small or large, the principle of installation remains the same. A simple circuit comprises a voltage source, a load, and a switch connected in a series configuration with connecting conducting wires. Mechanical protection is essential for all electrical cables. Protection may take the form of PVC or EMT conduits, enclosures, and protective boxes.

It is imperative that safe work practices are exercised to prevent nicking of conductors, which causes short circuits, fire, and electrocution. Keen attention must be given to the loading and the sizing of conduit in an effort to provide free air space when cables are drawn through them. Free air space in conduits prevents cables from exceeding normal operating temperatures. Free space also allows the easy pulling of cables and reduces the risk of harmful rubbing of cable against the conduit. In electrical construction and maintenance, good practices will eliminate the fear of damages or injuries and will promote a safe working environment.

1.8 Test Your Knowledge:

1. What is an electrical circuit?

2. Draw and label a simple electrical circuit.

3. Name the basic components of an electrical circuit.

4. What is the purpose of an earth conductor in an installation?

5. What effect would a fault current have on the user of an appliance if a fault current is present and the grounding system is defective?

6. Explain the difference between a short circuit and an overload.

7. State three methods of providing mechanical protection for electrical cables.

8. State three types of protection that an electrical installation requires.

9. Why is it important to ground all metal boxes in an electrical installation?

10. What steps are to be taken before attempting to engage in an electrical task?

11. What is electrical safety?

12. List three methods used for monitoring potential fire hazards.

13. List four fire alarm initiating devices.

14. In a fire alarm system, what is the device used to signal persons with sight and hearing disabilities?

15. What is the key feature of an electrical tool?

16. What is the primary cause of an electric shock?

17. List four primary causes for loss of power.

18. List five primary causes of intermittent tripping of circuit protection devices.

CHAPTER 2

Codes for Electrical Installation

2.0 Introduction

Electrical codes are a set of standards established to guide safe installation of electrical accessories, cables, and equipment. These codes are prescribed by law, and all installations shall meet the minimum standards stipulated by the codes.

Electrical installations incorrectly done can pose a high risk of electrocution or serious injuries. Electrical work practices must, therefore, follow the established electrical codes. To assure safety, all electrical installations are inspected by government electrical inspectors or GEIs. This activity is established to eliminate poor work practices carried out by electricians and contractors. Electrical inspections are done prior to connections to the utility’s power supply or as stipulated by a manufacturer of the device(s) being installed.

2.1 The Mandate of an Electrical Code or Regulation

The electrical codes or regulations are guidelines documented to provide protection of persons, livestock and property, from damage which may occur because of unethical electrical work practices during the electrical installation process. Each code or regulation is guided by the auxiliary shall. The auxiliary shall in any electrical code indicates that the point of reference is mandated by law. This means that deviation from the code of reference may present serious consequences if the matter is taken to court for matters arising from breaches.

The auxiliary May is conscionable and is left to one’s discretion about the point of reference. Strict attention must be paid to these key words; these terms, along with their references, are your only protection in a court of law.

(Continues…)Excerpted from Fundamentals of Fault Current and Grounding in Electrical Systems by Halden Morris, Norman Chambers. Copyright © 2014 Halden Morris & Norman Chambers. Excerpted by permission of Trafford Publishing.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.