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...