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Electrical Symbols and Diagrams Questions Answers

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Language of Control

  1. Identify the differences between pictorial drawings, wiring diagrams, schematic diagrams, line diagrams, block diagrams, and flow charts.
  • A pictorial drawing is a drawing that shows the length, height, and depth of an object in one view. Pictorial drawings show physical details of an object as seen by the eye.
  • A symbol is a graphic element that represents a quantity or unit. Symbols are used to represent electrical components of electrical and electronic diagrams. An abbreviation is a letter or combination of letters that represents a word.
  • A wiring diagram is a diagram that shows the connection of all components in a piece of equipment. Wiring diagrams show, as closely as possible, the actual location of each component in a circuit. Wiring diagrams often include details of the type of wire and the kind of hardware by which wires are fastened to terminals.
  • A schematic diagram is a diagram that shows the electrical connections and functions of a specific circuit arrangement with graphic symbols. Schematic diagrams do not show the physical relationship of the components in a circuit. Schematic diagrams are intended to show the circuitry that is necessary for the basic operation of a device. Schematic diagrams are not intended to show the physical size or appearance of the device.
  • A line (ladder) diagram is a diagram that shows the logic of an electrical circuit or system using standard symbols. A line diagram is used to show the relationship between circuits and their components but not the actual location of the components. Line diagrams provide a fast, easy understanding of the connections and use of components.
  • Line diagrams are often incorrectly referred to as one-line diagrams. A one-line diagram is a diagram that uses single lines and graphic symbols to indicate the path and components of an electrical circuit. One-line diagrams have only one line between individual components. A line diagram, on the other hand, often shows multiple lines leading to or from a component (parallel connections).
  • A block diagram is a diagram that shows the relationship between individual sections, or blocks, of a circuit or system. The primary function of a block diagram is to show how the distinct parts of a system relate to each other. Block diagrams are used with schematic diagrams to help troubleshoot a system, such as a regulated power supply.
  • A flow chart is a diagram that shows a logical sequence of steps for a given set of conditions. Flow charts help a troubleshooter follow a logical path when trying to solve a problem. Flowcharts use symbols and interconnecting lines to provide direction.

Electrical Circuits

  1. State the five basic components of a basic electrical circuit.
  • Electrical circuits must include a load that converts electrical energy into some other usable form of energy such as light, heat, or motion; a source of electricity; conductors to connect the individual components; a method of controlling the flow of electricity (switch); and a protection device (fuse or circuit breaker) to ensure that the circuit operates safely and within electrical limits.
  • A power source is a device that converts various forms of energy into electricity. The power source in an electrical circuit is normally the point at which to start when reading or troubleshooting a diagram.
  • A conductor is a material that has very little resistance and permits electrons to move through it easily. Copper is the most commonly used conductor material.
  • A control switch is a switch that controls the flow of current in a circuit. Switches can be activated manually, mechanically, or automatically.
  • A load is any device that converts electrical energy into motion, heat, light, or sound. Common loads include lights, heating elements, speakers, and motors.
  • An overcurrent protection device (OCPD) is a disconnect switch with circuit breakers (CBs) or fuses added to provide overcurrent protection for the switched circuit. A fuse is an overcurrent protection device with a fusible link that melts and opens the circuit when an overload condition or short circuit occurs. A circuit breaker is an overcurrent protection device with a mechanism that may manually or automatically open the circuit when an overload condition or short circuit occurs.
  1. Understand how to draw a circuit using symbols that illustrate the five basic components of an electrical circuit.
  • Electrical circuit components may be shown using line diagrams, pictorial drawings, and/ or wiring diagrams.
  • For example, an automobile interior lighting circuit includes the five components of a typical electrical circuit.
  • The source of electricity is the battery, the conductors may be the chassis wires or the car frame, the control device is the plunger-type door switch, the load is the interior light, and the fuse is the protection device.
  1. Explain the difference between a manual control circuit and an automatic control circuit.
  • A manual control circuit is any circuit that requires a person to initiate an action for the circuit to operate.
  • A line diagram may be used to illustrate a manual control circuit of a pushbutton controlling a pilot light. In a line diagram, the lines labeled L1 and L2 represent the power circuit. In this circuit, the voltage is 115 VAC but maybe 12 VAC, 18 VAC, 24 VAC, or some other voltage. The dark black nodes on a circuit indicate an electrical connection. If a node is not present, the wires only cross each other and are not electrically connected. Line diagrams are read from left (L1) to right (L2). In this circuit, pressing pushbutton 1 (PB1) allows current to pass through the closed contacts of PB1, through pilot light 1 (PL1) and on to L2, forming a complete circuit that activates PL1. Releasing PB1 opens the PB1 contacts, stopping the current flow to the pilot light, and turning the pilot light (PL1) off.
  • Automatically controlled devices have replaced many functions that were once performed manually. As a part of automation, control circuits are intended to replace manual devices. Any manual control circuit may be converted to automatic operation.
  • For example, an electric motor on a sump pump can be turned on and off automatically by adding an automatic control device such as a float switch. This control circuit is used in basements to control a sump pump to prevent flooding. When water reaches a predetermined level, the float switch senses the change in water level and automatically starts the pump, which removes the water.
  1. Explain what a solenoid is used for.
  • A solenoid is an electric output device that converts electrical energy into a linear mechanical force.
  • A solenoid consists of a frame, plunger, and coil.
  • A magnetic field is set up in the frame when the coil is energized by an electric current passing through it. This magnetic field causes the plunger to move into the frame. The result is a straight-line force, normally a push or pull action.
  • A solenoid may be used to control a door lock that opens only when a pushbutton is pressed. In this circuit, pressing pushbutton 1 allows an electric current to flow through the solenoid, creating a magnetic field. The magnetic field, depending on the solenoid construction, causes the plunger to push or pull. In this circuit, the door may open as long as the pushbutton is pressed. The door is locked when the pushbutton is released. This circuit provides security access to a building or room.
  1. Define the contactor, give an example of its usage, and state the two types of electrical contactors.
  • A contactor is a control device that uses a small control current to energize or de-energize the load connected to it.
  • The electrical operation of a contactor can be shown using a line diagram, a pictorial drawing, and/or a wiring diagram. In this circuit, pressing pushbutton 1 (PB1) allows current to pass through the switch contacts and the contactor coil (C1) to L2. This energizes the contactor coil (C1). The activation of C1 closes the power contacts of the contactor. These contacts are not normally shown in the line diagram but are shown in the pictorial drawing and wiring diagram.
  • The two main types of contactors are heating contactors and lighting contactors. Heating contactors are used to control high-power electrical heating elements. Lighting contactors are used to control banks of lamps, such as those used in malls and sports stadiums.
  1. Define magnetic motor starter and give an example of its usage.
  • A magnetic motor starter is an electrically operated switch (contactor) that includes motor overload protection. Magnetic motor starters are used to start and stop motors. Magnetic motor starters are identical to contactors except that they have overloads attached to them.
  • The electrical operation of a magnetic motor starter may be shown using a line diagram, a pictorial drawing, and/or a wiring diagram. In this circuit, pressing the start pushbutton allows current to pass through coil M1 and the overload contacts. This energizes coil M1. With coil M1 energized, auxiliary contacts M1 close and the circuit remains energized even if the start pushbutton is released. This circuit is de-energized if the stop pushbutton is pressed, a power failure occurs, or any one of the overloads senses a problem in the power circuit. Coil M1 de-energizes, causing auxiliary contacts M1 to return to their NO condition if one of these situations occurs. When a motor stops because of an overload, the overload must be removed, the overload device reset, and the start pushbutton pressed to restart the motor.
  1. Explain how prints are read and how they are used during troubleshooting.
  • Electrical components are represented by prints as symbols and pictorial drawings. The symbols are used to simplify the understanding of the circuit operation and individual component function. However, because symbols represent a physical component and the physical component is installed and checked when troubleshooting, a clear understanding must exist between the component symbol and the physical shape and location of the component. A typical electrical print uses standard symbols and a pictorial drawing to show the location of the circuit components.
  • Printreading involves identifying an electrical symbol, understanding where the component is located within a system, and understanding how it is used.
  • Troubleshooting requires the testing of individual components within a system to determine a circuit or component problem. Troubleshooting requires reading a print to determine the location and use of individual components and an understanding of how to use test equipment.
  • In an electrical system, the print normally shows all of the circuit components and connections. Understanding all symbols and components of a print is required when installing, modifying, testing, and troubleshooting a component or part of a system. Locating the actual places to connect the digital multimeter test leads requires transferring the circuit symbol information to the actual location of the component in the control panel.
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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.