Electrical Drafting

Chapter 1

INTRODUCTION TO ELECTRICITY

Electricity is a form of energy generated by friction, induction or chemical change having magnetic, chemical and radiant effect. In short, electricity is electrons in motion.

Electricity consists of the following:

  • Electron
  • Proton
  • Neutron

Electron is the negatively charged particle of an atom which is also referred to as the negative charge of electricity.

Proton is the positively charged particle of an atom which is also referred to as the positively charged particle of an atom which is also refereed to as the positive charge of electricity.

Neutron is the particle which is not electrically charged.

 

ELECTRICAL TERMS

VOLT or VOLTAGE is the electrical pressure that cause the electrons to move through a conductor. In short, voltage is the electromotive force. It is named after an Italian scientist, Alessandro Volta.

AMPERE the standard unit in measuring the strength of an electric current named after Andre M. Ampere. It is the amount of current flow sent by one volt through a resistance of one ohm ( 1 OHM ).

When there is too much flow of electricity in a small conductor or wire, heat is produced which may eventually burn the insulator and cause fire of blow-off the protective device called fuse.

WATT named after a Scottish inventor, James Watt, is the rate of power used or consumed. It represents the equivalent heat volts and ampere consumed by lights, appliances or motors. A term commonly labeled on light bulb or appliances which gives us the idea of the kind of circuit that would be installed.

RESISTANCE is the opposition to the flow of current. Resistance is the term for DC electricity and Impedence for AC electricity.

 

ELECTRIC CURRENT

By definition, electric current is the flow or rate of flow of electric force in a conductor. A current will only flow if a circuit is formed comprising a complete loop and contains all the required components such as:

  1. Source of voltage
  2. A closed loop of wiring
  3. An electric load
  4. A means of opening and closing the circuit

Classification of Circuits

  1. Direct Current (DC)
    • Direct Current electricity flows only in one direction. The flow is said to be from negative to positive. The common source of DC id the fry cell or storage battery.
  2. Alternating Current (AC)
    • Alternating Current electricity constantly reverses its direction of flow. This type of electricity id generated by machines called AC generators. It is the type of electricity that is used on buildings.

Relationship of Current, Voltage, and Resistance:

  1. The higher the voltage, the larger the current
  2. The higher the resistance, the lower the current

This relationship is expressed in the following equation known as the Ohm’s Law:

DC Electricity :        I = V/R

Where:

I = current

V = voltage

R = resistance

AC Electricity:       I = V/Z

Where:

I = current

V = voltage

Z = impedence

 

COMPARISON OF THE AC AND THE DC ELECTRICITY

Under the principle of DC, power is the product of voltage and current, thus:

Watts = Volts X Ampere

Under the principles of AC, the product of volts and ampere is equal to the quantity called volt-amperes, thus:

Volts-Amperes = Volts X Ampere

 

THE OHM’S LAW

In 1972, George Simon Ohm, a German Scientist discovered the relationship among the Current, Voltage, and Resistance now referred to as the Ohm’s Law which states that: “The higher the voltage, the larger the current and the higher the resistance, the lower the current”.

I = V / R

Thus, to find:

  • Voltage                  V = IR
  • Current                  I = V / R
  • Resistance            R = V / I

Illustration:

Determine the current flow in a circuit having a resistance of 5 ohms on a 120 volts and 240 volts supply.

Solution:

1. for 120 volts

I = V/R

= 120/5

I = 24 amperes

2. for 240 volts

I = V/R

= 240/5

I = 48 amperes

Try this:

A current has a resistance of 20 Ohms and the current flows at 12 amperes. Determine the voltage.

 

SERIES AND PARALLEL CIRCUIT

A circuit components can be arranged in several ways but with two fundamental types of connections, namely:

  1. Series circuit
  2. Parallel circuit

In a Series connection, a single path exist for current flow where elements are arranged is a series one after the other with no branches. Being a single path in a series arrangement, voltage and resistance simply adds thus:

Vt = V1+V2+V3+…

Rt = R1+R2+R3+…

Parallel circuit is sometimes referred to as the multiple connection where the loads are placed across the same voltage consisting a separate circuit. This is the type of connection the exists in building wirings.

 

Next: Chapter 2: Conductors and Wiring Accessories

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