Electrical Switches PDF - 2022 - JAD-LC-A2-YP-EMT-LM-02

Classification: Internal Use Control page ASSIGNMENT DISCIPLINES SECTION TOPIC LEARNING OBJECT TITLE Electrical Plant Electrical Electrical Switches Switch Types Technicians...

Classification: Internal Use Control page ASSIGNMENT DISCIPLINES SECTION TOPIC LEARNING OBJECT TITLE Electrical Plant Electrical Electrical Switches Switch Types Technicians Installation Level-1 Process Switches Pole and Throw Contact "normal" state Generic Symbology DOCUMENT CONTROLS JADEER REFERENCE # JAD-LC-A2-YP-EMT-LM-02 REVISION # ISSUE DATE: NEXT REVIEW DATE: 2022.0 17-Aug-2022 16-Aug-2025 DEVELOPED BY: REVIEWED BY: APPROVED BY: Marwan Owaidhah, Ali A. Fallatah, Specialist Abdullah Saleh Al-Ghamdi, I&E Electrical Trainer/12943 Electrical, Maintenance /12610 Specialty Manager, Maintenance/21528 Ahmed Haresi, Specialist Electrical, Maintenance /33423 1 Classification: Internal Use Table of content SN SUBJECT PAGE NUMBER 1 Introduction 4 2 Switch Types 5 3 Process Switches 7 4 Pole and Throw 12 5 Contact "normal" state 16 6 Generic Symbology 17 7 Review 20 All Rights Reserved 2 Classification: Internal Use Objectives SN Objectives 1 To understand type, process and contacts of switches All Rights Reserved 3 Classification: Internal Use Introduction Switches An electrical switch is any device used to interrupt the flow of electrons in a circuit. Switches are essentially binary devices: they are either completely on "closed" or completely off "open". There are many different types of switches, and we will explore some of these types in this chapter. Though it may seem strange to cover this elementary electrical topic at such a late stage in this book series, I do so because the chapters that follow explore an older realm of digital technology based on mechanical switch contacts rather than solid- state gate circuits, and a thorough understanding of switch types is necessary for the undertaking. Learning the function of switch-based circuits at the same time that you learn about solid-state logic gates makes both topics easier to grasp, and sets the stage for an enhanced learning experience in Boolean algebra, the mathematics behind digital logic circuits. The simplest type of switch is one where two electrical conductors are brought in contact with each other by the motion of an actuating mechanism. Other switches are more complex, containing electronic circuits able to turn on or off depending on some physical stimulus (such as light or magnetic field) sensed. In any case, the final output of any switch will be (at least) a pair of wire-connection terminals that either will be connected together by the switch's internal contact mechanism ("closed"), or not connected together ("open"). All Rights Reserved 4 Classification: Internal Use Switch Types Hand Switches Any switch designed to be operated by a person is generally called a hand switch, and they are manufactured in several varieties: Toggle switch Toggle switches are actuated by a lever angled in one of two or more positions. The common light switch used in household wiring is an example of a toggle switch. Most toggle switches will come to rest in any of their lever positions, while others have an internal spring mechanism returning the lever to a certain normal position, allowing for what is called "momentary" operation. Pushbutton Switch Pushbutton switches are two-position devices actuated with a button that is pressed and released. Most pushbutton switches have an internal spring mechanism returning the button to its "out," or "unpressed," position, for momentary operation. Some pushbutton switches will latch alternately on or off with every push of the button. Other pushbutton switches will stay in their "in," or "pressed," position until the button is pulled back out. This last type of pushbutton switches usually have a mushroom-shaped button for easy push-pull action. Selector switch Selector switches are actuated with a rotary knob or lever of some sort to select one of two or more positions. Like the toggle switch, selector switches can either rest in any of their positions or contain spring-return mechanisms for momentary operation. All Rights Reserved 5 Classification: Internal Use Joystick switch A joystick switch is actuated by a lever free to move in more than one axis of motion. One or more of several switch contact mechanisms are actuated depending on which way the lever is pushed, and sometimes by how far it is pushed. The circle and-dot notation on the switch symbol represents the direction of joystick lever motion required to actuate the contact. Joystick hand switches are commonly used for crane and robot control. All Rights Reserved 6 Classification: Internal Use Process Switches Limit Switch Some switches are specifically designed to be operated by the motion of a machine rather than by the hand of a human operator. These motion-operated switches are commonly called limit switches, because they are often used to limit the motion of a machine by turning off the actuating power to a component if it moves too far. As with hand switches, limit switches come in several varieties: Lever actuator limit Switch These limit switches closely resemble rugged toggle or selector hand switches fitted with a lever pushed by the machine part. Often, the levers are tipped with a small roller bearing, preventing the lever from being worn off by repeated contact with the machine part. Proximity switch Proximity switches sense the approach of a metallic machine part either by a magnetic or high-frequency electromagnetic field. Simple proximity switches use a permanent magnet to actuate a sealed switch mechanism whenever the machine part gets close (typically 1 inch or less). More complex proximity switches work like a metal detector, energizing a coil of wire with a high-frequency current, and electronically monitoring the magnitude of that current. If a metallic part (not necessarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring circuit. The symbol shown here for the proximity switch is of the electronic variety, as indicated by the diamond-shaped box surrounding the switch. A non-electronic proximity switch would use the same symbol as the lever-actuated limit switch. Another form of proximity switch is the optical switch, comprised of a light source and photocell. Machine position is detected by either the interruption or reflection of a light beam. Optical switches are also useful in safety applications, where beams of light can be used to detect personnel entry into a dangerous area. All Rights Reserved 7 Classification: Internal Use Photoelectric proximity sensor In many industrial processes, it is necessary to monitor various physical quantities with switches. Such switches can be used to sound alarms, indicating that a process variable has exceeded normal parameters, or they can be used to shut down processes or equipment if those variables have reached dangerous or destructive levels. There are many different types of process switches: Speed Switch These switches sense the rotary speed of a shaft either by a centrifugal weight mechanism mounted on the shaft, or by some kind of non-contact detection of shaft motion such as optical or magnetic. Pressure switch Gas or liquid pressure can be used to actuate a switch mechanism if that pressure is applied to a piston, diaphragm, or bellows, which converts pressure to mechanical force. Temperature switch An inexpensive temperature-sensing mechanism is the "bimetallic strip:" a thin strip of two metals, joined back-to-back, each metal having a different rate of thermal All Rights Reserved 8 Classification: Internal Use expansion. When the strip heats or cools, differing rates of thermal expansion between the two metals causes it to bend. The bending of the strip can then be used to actuate a switch contact mechanism. Other temperature switches use a brass bulb filled with either a liquid or gas, with a tiny tube connecting the bulb to a pressure sensing switch. As the bulb is heated, the gas or liquid expands, generating a pressure increase which then actuates the switch mechanism. Liquid level (Float) Switch A floating object can be used to actuate a switch mechanism when the liquid level in an tank rises past a certain point. If the liquid is electrically conductive, the liquid itself can be used as a conductor to bridge between two metal probes inserted into the tank at the required depth. The conductivity technique is usually implemented with a special design of relay triggered by a small amount of current through the conductive liquid. In most cases it is impractical and dangerous to switch the full load current of the circuit through a liquid. Level switches can also be designed to detect the level of solid materials such as wood chips, grain, coal, or animal feed in a storage silo, bin, or hopper. A common design for this application is a small paddle wheel, inserted into the bin at the desired height, which is slowly turned by a small electric motor. When the solid material fills the bin to that height, the material prevents the paddle wheel from turning. The torque response of the small motor than trips the switch mechanism. Another design uses a "tuning fork" shaped metal prong, inserted into the bin from the outside at the desired height. The fork is vibrated at its resonant frequency by an electronic circuit and magnet/electromagnet coil assembly. When the bin fills to that height, the solid material dampens the vibration of the fork, the change in vibration amplitude and/or frequency detected by the electronic circuit. Liquid flow switch Inserted into a pipe, a flow switch will detect any gas or liquid flow rate in excess of a certain threshold, usually with a small paddle or vane which is pushed by the flow. Other flow switches are constructed as differential pressure switches, measuring the pressure drop across a restriction built into the pipe. Another type of level switch, suitable for liquid or solid material detection, is the nuclear switch. Composed of a radioactive source material and a radiation detector, All Rights Reserved 9 Classification: Internal Use the two are mounted across the diameter of a storage vessel for either solid or liquid material. Any height of material beyond the level of the source/detector arrangement will attenuate the strength of radiation reaching the detector. This decrease in radiation at the detector can be used to trigger a relay mechanism to provide a switch contact for measurement, alarm point, or even control of the vessel level. Nuclear level Switch (for solid or liquid material) Both source and detector are outside of the vessel, with no intrusion at all except the radiation flux itself. The radioactive sources used are weak and pose no immediate health threat to operations or maintenance personnel. As usual, there is usually more than one way to implement a switch to monitor a physical process or serve as an operator control. There is usually no single "perfect" switch for any application, although some obviously exhibit certain advantages over All Rights Reserved 10 Classification: Internal Use others. Switches must be intelligently matched to the task for efficient and reliable operation. All Rights Reserved 11 Classification: Internal Use Pole and Throw Circuit symbols of switches, the common terminal in SPDT and DPDT types. The diagram on the package of a DPDT AC coil relay Since relays are switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways: Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called a Form A contact or "make" contact. Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called a Form B contact or "break" contact. All Rights Reserved 12 Classification: Internal Use Change-over (CO), or double-throw (DT), contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact ("break before make"). If this type of contact utilizes a "make before break" functionality, then it is called a Form D contact. The following designations are commonly encountered: SPST - Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity. SPDT - Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total. DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. The poles may be Form A or Form B (or one of each). DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil. The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single actuator. For example 4PDT indicates a four pole double throw switch. All Rights Reserved 13 Classification: Internal Use Here are a few common switch configurations and their abbreviated designations: All Rights Reserved 14 Classification: Internal Use Four-pole, double-throw (4 PDT) All Rights Reserved 15 Classification: Internal Use Contact "normal" state and make/ break sequence Any kind of switch contact can be designed so that the contacts "close" (establish continuity) when actuated, or "open" (interrupt continuity) when actuated. For switches that have a spring-return mechanism in them, the direction that the spring returns it to with no applied force is called the normal position. Therefore, contacts that are open in this position are called normally open and contacts that are closed in this position are called normally closed. For process switches, the normal position, or state, is that which the switch is in when there is no process influence on it. An easy way to figure out the normal condition of a process switch is to consider the state of the switch as it sits on a storage shelf, uninstalled. Here are some examples of "normal" process switch conditions: Speed switch: Shaft not turning Pressure switch: Zero applied pressure Temperature switch: Ambient (room) temperature Level switch: Empty tank or bin Flow switch: Zero liquid flow It is important to differentiate between a switch's "normal" condition and its "normal" use in an operating process. Consider the example of a liquid flow switch that serves as a low-flow alarm in a cooling water system. The normal, or properly operating, condition of the cooling water system is to have fairly constant coolant flow going through this pipe. If we want the flow switch's contact to close in the event of a loss of coolant flow (to complete an electric circuit which activates an alarm siren, for example), we would want to use a flow switch with normally-closed rather than normally-open contacts. When there's adequate flow through the pipe, the switch's contacts are forced open; when the flow rate drops to an abnormally low level, the contacts return to their normal (closed) state. This is confusing if you think of "normal" as being the regular state of the process, so be sure to always think of a switch's "normal" state as that which its in as it sits on a shelf. The schematic symbology for switches vary according to the switch's purpose and actuation. A normally-open switch contact is drawn in such a way as to signify an open connection, ready to close when actuated. Conversely, a normally-closed switch is drawn as a closed connection which will be opened when actuated. All Rights Reserved 16 Classification: Internal Use Generic Symbology Normally-open Normally-closed Pushbutton switch There is also a generic symbology for any switch contact, using a pair of vertical lines to represent the contact points in a switch. Normally-open contacts are designated by the lines not touching, while normally-closed contacts are designated with a diagonal line bridging between the two lines. Compare the two: Generic switch contact designation Normally-open Normally-closed The switch on the left will close when actuated, and will be open while in the "normal" (unactuated) position. The switch on the right will open when actuated, and is closed in the "normal" (unactuated) position. If switches are designated with these generic symbols, the type of switch usually will be noted in text immediately beside the symbol. Please note that the symbol on the left is not to be confused with that of a capacitor. If a capacitor needs to be represented in a control logic schematic, it will be shown like this: Capacitor In standard electronic symbology, the figure shown above is reserved for polarity sensitive capacitors. In control logic symbology, this capacitor symbol is used for any type of capacitor, even when the capacitor is not polarity sensitive, so as to clearly distinguish it from a normally-open switch contact. With multiple-position selector switches, another design factor must be considered: that is, the sequence of breaking old connections and making new connections as the switch is moved from position to position, the moving contact touching several stationary contacts in sequence, All Rights Reserved 17 Classification: Internal Use The selector switch shown above switches a common contact lever to one of five different positions, to contact wires numbered 1 through 5. The most common configuration of a multi-position switch like this is one where the contact with one position is broken before the contact with the next position is made. This configuration is called break-before-make. To give an example, if the switch were set at position number 3 and slowly turned clockwise, the contact lever would move off of the number 3 position, opening that circuit, move to a position between number 3 and number 4 (both circuit paths open), and then touch position number 4, closing that circuit. There are applications where it is unacceptable to completely open the circuit attached to the "common" wire at any point in time. For such an application, a make- before-break switch design can be built, in which the movable contact lever actually bridges between two positions of contact (between number 3 and number 4, in the above scenario) as it travels between positions. The compromise here is that the circuit must be able to tolerate switch closures between adjacent position contacts (1 and 2, 2 and 3, 3 and 4, 4 and 5) as the selector knob is turned from position to position. Such a switch is shown here: When movable contact(s) can be brought into one of several positions with stationary contacts, those positions are sometimes called throws. The number of movable contacts is sometimes called poles. Both selector switches shown above with one moving contact and five stationary contacts would be designated as "single pole, five- throw" switches. All Rights Reserved 18 Classification: Internal Use If two identical single-pole, five-throw switches were mechanically ganged together so that they were actuated by the same mechanism, the whole assembly would be called a "double-pole, five-throw" switch: All Rights Reserved 19 Classification: Internal Use REVIEW: A switch is an electrical device, used to control continuity between two points. Hand switches are actuated by human touch. Limit switches are actuated by machine motion. Process switches are actuated by changes in some physical process (temperature, level, flow, etc.). The normal state of a switch (shelf condition) is that where it is unactuated. For process switches, this is the condition its in when sitting on a shelf, uninstalled. A switch that is open when unactuated is called normally-open. A switch that is closed when unactuated is called normally-closed. Sometimes the terms "normally-open" and "normally-closed" are abbreviated N.O. and N.C., respectively. The generic symbology for N.O. and N.C. switch contacts is as follows: Generic switch contact designation Normally-open Normally-closed Multi position switches can be either break-before-make (most common) or make-before-break. The "poles" of a switch refers to the number of moving contacts, while the "throws" of a switch refers to the number of stationary contacts per moving contact. All Rights Reserved 20 Classification: Internal Use THANK YOU All Rights Reserved 21