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Requirements for Electrical Installations: BS 7671:2008 Incorporating Amendment No 1: 2011: IET Wiring Regulations

Guidance Note 3: Inspection and Testing (Guidance Notes for Bs 7671)

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1. Ampere
2. Volts
3. Ohm
4. Siemens
5. Henry
6. Watt
7. Farad

Voltage

• Voltage, electromotive force (emf), or potential difference, is the pressure or force that causes electrons to move in a conductor. In electrical equations and formulas, voltage is symbolized with a capital E, while on laboratory equipment, schematic diagrams and testing equipment voltage is represented with a capital V.

Current

• Electron current, or amperage, is described as the movement of free electrons through a conductor. In electrical formulas, current is symbolized with a capital I, while in the laboratory
  or on schematic diagrams and testing equipment a capital A is used to indicate amps or amperage (amps).

Resisitance

• Resistance is the third key concept directly related to both voltage and current. Resistance is defined as the opposition to current flow. The amount of opposition to current flow produced by a material depends upon the amount of
  available free electrons it contains and the types of obstacles the electrons encounter as they attempt to move through the material. Resistance is measured in ohms and is shown by the electrical symbol (R) in equations. 1 ohm is defined as that amount of resistance that will limit the current in a conductor to 1 ampere when the potential difference (voltage) applied to the conductor is 1 volt. The shorthand notation for ohm is the Greek letter capital Omega (Ω).
  
  

  Omega

  
  If a voltage is applied to a conductor, current will flow. The amount of current flow depends upon the resistance of the conductor. The higher the resistance, the lower the current flow. The lower the resistance, the higher the current flow for a given amount of voltage.

Ohm’s Law

• George Simon Ohm discovered in 1827 that there was a definite relationship between voltage, current, and resistance in an electrical circuit. Ohm’s Law defines this relationship and can be stated in three ways.
  1. Applied voltage equals circuit current times the circuit resistance. This equation is a
     mathematical respresentation of this concept.
     E = I x R or E = IR
  2. Current is equal to the applied voltage divided by the circuit resistance. This equation is a mathematical representation of this concept.
     I = E/R
  3. Resistance of a circuit is equal to the applied voltage divided by the circuit current.
     This equation is a mathematical representation of this concept.
     R = E/I

  where

  I = current (A)
  E = Voltage (V)
  R = Resistance (ω)

  If any two of the component values are known, the third can be calculated.

Conductance

• The word “reciprocal” is sometimes used to mean “the opposite of.” The reciprocal(opposite) of resistance is called conductance. As described above, resistance is the opposition to current flow. Since resistance and conductance are opposites, conductance can be defined as the ability to conduct current. For example, if a wire has a high conductance, it will have low resistance, and vice-versa. Conductance is found by taking the reciprocal of the resistance. The unit used to specify conductance is called “mho,” which is ohm spelled backwards. The symbol for “mho” is the Greek letter omega inverted. The symbol for conductance when used in a formula is G. Equation (1-5) is the mathematical representation of conductance obtained by relating the definition of conductance (1/R) to Ohm’s Law, Equations R = E/I.

  G = 1/RESISTANCE = I/E

  Example: If a resistor (R) has five ohms, what will its conductance (G) be in mhos?

  Solution: G = 1/R = 1/5 = 0.2mhos

Power

• Electricity is in the general used to do some kind of work, such as generating heat or turning a motor. Specifically, power is the rate at which work is done, or the rate at which heat is generated. Watt is the unit commonly used to specify electric power. In equations, you will find power abbreviated with the capital letter P, and watts, the units of measure for power, are abbreviated with the capital letter W. Power is also described as the voltage (E) in a circuit times the current (I) across the circuit. This equation below illustrates the mathematical representation of this concept.

  P = E x I or P = E.I

  Using Ohm’s Law for the value of voltage (E),

  E = I x R

  and using substitution laws,

  P = I x ( I x R)

  power can be described as the current (I) in a circuit squared times the resistance (R) of the
  circuit. The equation is the mathematical representation of this concept:

  P = I² x R

Inductance

• Inductance is how the ability of a coil to store energy is defined, induce a voltage in itself, and oppose changes in current flowing through it. The symbol used to indicate inductance in electrical formulas and equations is a capital L. The units of measurement are called henries. The unit henry is abbreviated by using the capital letter H. One henry is the amount of inductance (L) that permits one volt to be induced (V∠) when the current through the coil changes at a rate of one ampere per second. The equation below is the mathematical representation of the rate of change in current through a coil per unit time.
  
  The next equation is the mathematical representation for the voltage VL induced in a coil with inductance L. The negative sign indicates that voltage induced opposes the change in current through the coil per unit time (ΔI/Δt).
  

Capacitance

• Capacitance is defined as the ability to store an electric charge. It is symbolized by the capital letter C. Capacitance (C) is measured in farads and is equal to the amount of charge (Q) that can be stored in a device or capacitor divided by the voltage (E) applied across the device or capacitor plates when the charge was stored. The equation is the mathematical representation for capacitance:
  

Conductors

• The only way to describe Conductors are that they are materials with electrons that are loosely bound to their atom, or materials that allow free motion of a large number of their electrons. A good example of these conductors are Atoms with only one valence electron, such as copper, silver, and gold. Most metals are good conductors.

Insulators

• Insulators, or nonconductors, are materials with electrons that are tightly bound to their atoms and require a large amount of energy to free them from the influence of the nucleus. The atoms of good insulators have their valence shells filled with eight electrons, which means they are more than half filled. Any energy applied to such an atom will be distributed among a relatively
  large number of electrons. Rubber, plastics, glass, and dry wood are examples of good insulators.

Resistors

• Resistors are made of materials that conduct electricity, but restrict the flow of current. These types of materials are called semiconductors because they are not good insulators or good conductors. Semiconductors have more than one or two electrons in their valence shells, but less than seven or eight. Carbon, silicon, germanium, tin, and lead are examples of semiconductors or resistors. Valence electrons in these are four.

Voltage

• as before described in the blog, the basic unit of measure for potential difference is the volt (symbol V), and because the volt unit is used, potential difference is called voltage. An object’s electrical charge is determined by the amount of electrons that the object has gained or has lost. Because such a large number of electrons move, a unit called the “coulomb” is used to indicate the charge. One coulomb is equal to 6.28 x 1018 (billion, billion) electrons. For example, if an object gains one coulomb of negative charge, it has gained 280,000,000,000,000,000 extra electrons. A volt is defined as a difference of potential causing one coulomb of current to do one joule of work. A volt is also defined as that amount of force required to force one ampere of current through one ohm of resistance. This is also stated in “ohms law”.

Current

• The density of the atoms in copper wire is such that the valence orbits of the individual atoms overlap, causing the electrons to move easily from one atom to the next. Free electrons can drift from one orbit to another in a random direction. When a potential difference is applied, the direction of their movement is controlled. The strength of the potential difference applied at each
  end of the wire determines how many electrons change from a random motion to a more directional path through the wire. The movement or flow of these electrons is called electron current flow or just current. To produce current, the electrons must be moved by a potential difference. We use the symbol for current (I). The basic measurement for current is the ampere (A). One ampere of current is defined as the movement of one coulomb of charge past any given point of a conductor during one second of time. If a copper wire is placed between two charged objects that have a potential difference, all of the negatively-charged free electrons will feel a force pushing them from the negative charge to the positive charge. This force opposite to the conventional direction of the electrostatic lines of force is shown in the below image.
  
  The direction of electron flow, shown in the bottom image, is from the negative (-) side of the battery, through the wire, and back to the positive (+) side of the battery. The direction of electron flow is from a point of negative potential to a point of positive potential. The solid arrow shown in Figure 10 indicates the direction of electron flow. As electrons leave their atoms during electron
  current flow, positively charged atoms (holes) result. The flow of electrons in one direction causes a flow of positive charges. The direction of the positive charges is in the opposite direction of the electron flow. This flow of positive charges is known as conventional current flow and is shown in the bottom image as a dashed arrow. All of the electrical effects of electron flow from negative to positive, or from a higher potential to a lower potential, are the same as those that would be created by a flow of positive charges in the opposite direction. Fore that reason, it is important to realize that both conventions are in use and that they are essentially equivalent; that is, all effects predicted are the same. In this blog, we will be using electron flow in our discussions
  Generally, electric current flow can be classified as one of two general types: Alternating Current (AC) or Direct Current (DC). A direct current flows continuously in the same direction. An alternating current periodically reverses direction. We will be studying DC and AC current in more detail later in this blog. A Battery is a good example of DC current. AC current is what we commonly find in household supplies from the power company generators and substations.

The electrons in an atom exist in different energy levels. The energy level of an electron is proportional to its distance from the nucleus. So the higher the energy level of the electron that exist in orbits, or shells, the farther away the electron is away from the nucleus. These shells nest inside one another and surround the nucleus. The nucleus is the center of all the shells. The shells are lettered beginning with the shell nearest the nucleus: K, L, M, N, O, P, and Q. A shell can only hold a maximum number of electrons. For example, the K shell will hold a maximum of two electrons and the L shell will hold a maximum of eight electrons. As shown in the Figure below, each shell has a specific number of electrons that it will hold for a specific atom.

To predict the electron distribution of any element we use two simple rules that concern electron shells that make it possible:

• The maximum number of electrons that can fit in the outermost shell of any atom
  is eight.
• The maximum number of electrons that can fit in the next-to-outermost shell of
  any atom is 18.

The atom becomes very stable, or very resistant to changes in its when the outer shell of the atom contains eight electrons. This also means that atoms with one or two electrons in their outer shell can lose electrons much more easily than atoms with full outer shells. We call the electrons in the outermost shell valence electrons. When external energy, such as heat, light, or electrical energy, is applied to certain materials, the electrons gain energy, become excited, and may move to a higher energy level. Some of the valence electrons will leave the atom when enough energy is applied to the atom. We call these electrons free electrons. It is the movement of free electrons that provides electric
current in a metal conductor. An atom that has lost or gained one or more electrons is said to be ionized or to have an ion change. If the atom loses one or more electrons, it becomes positively charged and is referred to as a positive ion. If an atom gains one or more electrons, it becomes negatively charged and is referred to as a negative ion.

Because of the force of its electrostatic field, these electrical charges have the ability to do work by moving another charged particle by attraction or repulsion. We call this ability to do work “potential”, that is why when we get a charge difference between objects, there is a potential difference between them. The calculated sum of the potential differences of all charged particles in the electrostatic field is what we refer to as electromotive force (EMF).
The basic unit of measure we use to measure a potential difference is the “volt”. The potential
difference symbol used is “V” The “V” indicates the ability to do the work of forcing electrons to move. Potential difference is also called “voltage.” We will cover the unit volt in greater detail in a soon to follow chapter.

The repulsion and attraction strength of the forces depends upon two factors: (1) the amount of charge on each object, and (2) the distance between the objects. Thus means that the greater the charge on the objects, the greater the electrostatic field will be and as can be imagined the greater the distance between the objects, the weaker the electrostatic field between them will be, and vice versa. This brings us to the law of electrostatic attraction, mostly referred to as Coulomb’s Law of electrostatic charges, it states that “the force of electrostatic attraction, or repulsion, is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them” as
shown in this Equation.

F = K * {(q1 q2)/d2}

Where:

• F = force of electrostatic attraction or prepulsion (Newtons)
• K = constant of proportionality (Coulomb 2/N-m2)
• q1 = charge of first particle (Coulombs)
• q2 = charge of second particle (Coulombs)
• d = distance between two particles (Meters)

If q1 and q2 are both positively or negatively charged, the force ill be repulsive. If q1 and q2 are opposite polarity or charge, the force will be attractive.

In the Atom and its forces section, the following will be explained;

• The Atom Structure
• Electrostatic force
• Electrostatic field
• Potential difference
• Electromotive force (EMF)
• Ion charge

Both the proton and electron carry equal charges in magnitude to each other. the neutron in the nucleus is slightly heavier than the proton and is electrically neutral, like the name implies. Depending upon the element involved, determines the various combinations of these two particles. The fundamentally negative charge (-) of electricity electron revolves around the nucleus of the atom in concentric rotations. The fundamental positive charge (+) of electricity proton, is located in the nucleus. The number of protons involved in the nucleus specifies the atomic number of that atom and of that element. For example, carbon atoms contain 6 protons in its nucleus thus gives the atomic number for carbon 6.

The atom in its natural state has equal amounts of electrons and protons. Also the negative charge of the electron is equal in magnitude to its protons positive charge in the nucleus. This two opposing charges cancels each other out giving the atom its electrically neutral balance.

Some atoms can lose and gain electrons, making it possible to transfer electrons from one object to another. The equal distribution of negative and positive charges no longer exists when this happens. An object that gained excess electrons is what we call negatively charged, and the object that lost electrons become deficient in electrons and become positively charged. These objects, which can contain billions of atoms, will then follow the same law of electrostatics as the electron and proton talked about above. The excess electrons can move around within an object and are called free electrons. We will be discussing free electrons in more detail in a later section. The greatness of the electric charge depends on the the number of these free electrons an object contains. So therefore the measure electrons is a direct measure of its electric charge.