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1. 1. 103 Measurement of Electrical QuantitiesUNIT 3 MEASUREMENT OF ELECTRICAL QUANTITIES Structure 3.1 Introduction Objectives 3.2 Moving Coil Galvanometer 3.2.1 Ammeter 3.2.2 Voltmeter 3.3 Navigational Lights 3.3.1 Intensity of Lights 3.3.2 Horizontal Sectors 3.3.3 Vertical Sectors 3.4 Colour Specification of Lights 3.4.1 Shapes 3.4.2 Visibility of Lights 3.5 Switches 3.6 Relays 3.7 Fuses 3.8 Electrical Safety 3.9 Summary 3.10 Key Words 3.11 Answers to SAQs 3.1 INTRODUCTION In Unit 2, you have studied simple concepts of electricity and magnetic effect. You were introduced to electrical quantities like voltage, current, elements like resistance, inductance and capacitance, and their units like voltage, amp, ohm, henry and farad. In this unit, we will learn some basic instruments used for measuring electric voltage and current through use of voltmeter and ammeter. You will also be introduced to safety norms, and safety control using different types of accessories such as switch, fuse, relay and navigational lights. Objectives After studying this unit, you should be able to • understand the basic galvanometer, • understand how a galvanometer is used as voltmeter or ammeter, • understand the importance of navigational light, • explain different types of light and colour specification, • appreciate the importance of shapes and visibility features of light, • describe the different types of switches in electrical control, • understand the operations of relay and fuse, and • appreciate the relevance and importance of safety standards in electrical utility.
2. 2. 104 Electricity 3.2 MOVING COIL GALVANOMETER Galvanometer is an instrument used to determine the presence, direction, and strength of an electric current in a conductor. All galvanometers are based upon the discovery by Hans C. Oersted that a magnetic needle is deflected by the presence of an electric current in a nearby conductor. When an electric current is passing through the conductor, the magnetic needle tends to turn at right angles to the conductor so that its direction is parallel to the lines of induction around the conductor and its north pole points in the direction in which these lines of induction flow. In general, the extent to which the needle turns is dependent upon the strength of the current. In the earlier galvanometers, a freely turning magnetic needle was hung in a coil of wire; in later versions the magnet was fixed and the coil made movable. Modern galvanometers are of this movable-coil type and are called d'Arsonval galvanometers (after Arsène d'Arsonval, a French physicist). If a pointer is attached to the moving coil so that it passes over a suitably calibrated scale, the galvanometer can be used to measure quantitatively the current passing through it. Such calibrated galvanometers are used in many electrical measuring devices. The DC ammeter, an instrument for measuring direct current, often consists of a calibrated galvanometer through which the current to be measured is made to pass. Since heavy currents would damage the galvanometer, a bypass, or shunt, is provided so that only a certain known percentage of the current passes through the galvanometer. By measuring the known percentage of the current, one arrives at the total current. The DC voltmeter, which can measure direct voltage, consists of a calibrated galvanometer connected in series with a high resistance. To measure the voltage between two points, one connects the voltmeter between them. The current through the galvanometer (and hence the pointer reading) is then proportional to the voltage (Ohm’s law). Ammeter is an instrument used to measure the magnitude of an electric current of several amperes or more. An ammeter is usually combined with a voltmeter and an ohmmeter in a multipurpose instrument. Most ammeters are based on the d′Arsonval galvanometer and are of the analog type, i.e. they give current values that can vary over a continuous range as indicated by a scale and pointer or digital readout. Voltmeter, instrument used to measure differences of electric potential, commonly called voltage, in volts or units that are multiples or fractions of volts. A voltmeter is usually combined with an ammeter and an ohmmeter in a multipurpose instrument. Most voltmeters are based on the d′Arsonval galvanometer and are of the analog type, i.e. they give voltage readings that can vary over a continuous range as indicated by a scale and pointer. However, digital voltmeters, which provide voltage readings that are composed of a group of digits, are becoming increasingly common. Since an oscilloscope is capable of giving a calibrated visual indication of voltage, it can be called a voltmeter. The constructional feature of a typical galvanometer is shown in Figure 3.1. Between the curved cylindrical pole pieces of a horseshoe magnet is placed a coaxial cylindrical soft iron core to strengthen the field and render it absolutely radial in the air gap between the core and the pole faces. A rectangular coil of several turns is also located between the pole faces so that its opposite arms lie in the air gap between the core and the pole faces. The axis of the radial magnetic field lies along the straight line joining the midpoints of its other pair of opposite arms. When this coil is connected to the closed circuit in which the current is to be detected by the galvanometer, it experiences a constant deflecting torque given by τ = B I A N where B = Flux density, I = Current, A = Area of cross-section, and N = Number of turns. In the simplest form of the instrument, this coil rests on an agate knife edge and is attached to one end of a spring whose other end is fixed to a rigid support. As the
3. 3. 105 Measurement of Electrical Quantities deflecting torque turns the coil, the spring gets twisted and sets up a restoring torque which is proportional to the angle of twist. The restoring torque, therefore, grows until it is equal to the forward torque. About this position the coil oscillates under its own angular inertia until the dissipation of energy finally brings it to rest. At the rest position, B I A N = θ = k θ where k is the torsional constant of the suspension (i.e. the restoring torque per unit twist). Thus, k I BAN = θ Since the flux density B, the area A of the coil and its number of turns N are also constants, the deflection θ is proportional to the current, i.e. the instrument has a linear scale. This is due to the radial field. The deflection of the coil for each unit current flowing in it is called the current sensitivity of the instrument and is given by BAN I k θ = Pointer and Scale Wire Completing CircuitSoft Iron Core Agate Knife Edge Spring Strong Magnet with Cylindrical Pole Pieces Coil Wire Completing Circuit Figure 3.1 : Constructional Features of a Galvanometer The strong field magnet serves the dual purpose of rendering the earth’s magnetic field negligibly small, and increasing the current sensitivity of the galvanometer. The current sensitivity of the instrument is also enhanced by a coil of large number of turns of large area and a spring of low torsional constant (k). The cylindrical pole pieces create a radial field in which the deflecting couple is of constant torque (BIAN) which gives rise to a linear scale (I ∝ θ) for the instrument as shown in Figure 3.2. Current Deflection θ Figure 3.2 : Linear Scale between Current and Deflection The soft iron core coaxially placed with the coil and the field magnet strengthens the magnetic field and further ensures that it is indeed radial in the airspace between the core
4. 4. 106 and the pole faces. Its weight is supported by a brass pin fixed to the body of the instrument as shown in Figure 3.3. Electricity Coaxial Soft Iron Core Rigid Support Brass Pin Cylindrical Pole Pieces Figure 3.3 : Flux Distribution The rigid light frame on which the coil is wound prevents the coil from being distorted by balanced forces acting on the other pair of arms. If the frame is made of metal then as the coil swings in the field, the frame cutting through flux induces an alternating current in the frame itself. These currents, called eddy currents, dissipate the mechanical energy of oscillation and bring the coil rapidly to rest. The spring sets up the restoring torque which opposes the deflecting torque. It carries the pointer which moves over a scale to indicate the deflection of the coil. It further serves to complete the electrical circuit. However, in the spring type of instrument the torsional constant is rather high reducing current sensitivity at the expense of making the instrument robust. Alternative to the spring type is the suspension type of moving coil galvanometer in which the restoring torque is set up by the suspension wire made of phosphor bronze. This wire completes the circuit, supports the weight of the coil and carries a small mirror just above the coil. A beam of light reflects off this mirror and is made incident on a long metre scale as shown in Figure 3.4. The mirror lamp and scale together constitute an optical lever arrangement which greatly enhance the sensitivity of the instrument due to the low torsional constant (k) of the wire. However, this type of instrument is easily damaged by excess current. Mirror 2θ Scale Lamp Figure 3.4 : Optical Lever Arrangement A single range “multi-meter” might be designed as illustrated in Figure 3.5. Voltmeter Ohmmeter Ammeter Galvanometer Common Figure 3.5 : Circuit of a Multi-meter
5. 5. 107 Measurement of Electrical Quantities 3.2.1 Ammeter An ammeter is an instrument for measuring the electric current in amperes in a branch of an electric circuit. It must be placed in series with the measured branch, and must have very low resistance to avoid significant alteration of the current it is to measure. This is show in Figure 3.6. By contrast, voltmeter must be connected in parallel. The analogy with an in-line flowmeter in a water circuit can help visualize why an ammeter must have a low resistance, and why connecting an ammeter in parallel can damage the meter. This is shown in Figure 3.7. Modern solid-state meters have digital readouts, but the principles of operation can be better appreciated by examining the older moving coil meters based on galvanometer sensors. A V R Ammeter in series with resistor measures current through the resistor Figure 3.6 : Ammeter Connection to Measure Current F Pump Flowmeter A meter for volume flowrate must be in series to measure the flow, but must not appreciably affect the flow. Figure 3.7 : Flowmeter Analogy of an Ammeter Ammeter Design Ammeters, as well as voltmeters and ohmmeters, are designed with the use of a sensitive current detector such as a galvanometer. A V R RG Design current I amperes Voltage across galvanometer and thus across the shunt resistor RP G IG Current which must be diverted to the shunt to limit current in galvanometer. Design task: Given the galvanometer resistance and the current value which will produce a full-scale reading on the galvanometer, find the value of shunt resistance which will permit the flow of the design current and produce a full-scale reading. R =P I RG G I - Idesign G G IG Figure 3.8 : Ammeter Design An ammeter is placed in series with a circuit element to measure the electric current flow through it as shown in Figure 3.8. The meter must be designed to offer very little resistance to the current so that it does not appreciably change the circuit it is measuring. To accomplish this, a small resistor is placed in parallel with the galvanometer to shunt most of the current around the galvanometer. Its value is chosen so that when the design current flows through the meter it will deflect to its full-scale reading. A galvanometer full-scale current is very small : of the order of milliamperes.
6. 6. 108 Electricity If a galvanometer with GR = Ω, GI = mA is used to design an ammeter for a full scale current I = amperes, the required shunt resistor is given by design G G P G I R R I I = = − Ω Example 3.1 A 1 mA meter movement with an internal resistance of 100 Ω is to be converted into a (0 – 100) mA ammeter. Calculate the value of shunt resistance required. What particulars should be specified on the shunt. Solution Shut resistance design G G P G I R R I I = − (1 mA) (100 ) (100 mA 1 mA) PR Ω = − 1.01PR = Ω Voltage drop across the shunt = (1.0) (100) = 100 mV Equivalent resistance of shunt in parallel with meter 1.01 100 1.0 1.01 100 × = = Ω + ∴ Shunt should be specified as 1.0 Ω, 100 mA or, 1.0 Ω, 100 mV 3.2.2 Voltmeter A voltmeter measures the change in voltage between two points in an electric circuit and therefore must be connected in parallel with the portion of the circuit on which the measurement is made as shown in Figure 3.9. In analogy with a water circuit, a voltmeter is like a meter designed to measure pressure difference. It is necessary for the voltmeter to have a very high resistance so that it does not have an appreciable affect on the current or voltage associated with the measured circuit. Modern solid-state meters have digital readouts, but the principles of operation can be better appreciated by examining the older moving coil meters based on galvanometer sensors. V A voltmeter is connected in parallel to measure the voltage change across a circuit element Figure 3.9 : Voltmeter Connection to Measure Voltage Across Circuit Element Pump A pressure gauge is connected in parallel to measure the pressure drop across the resistance. Figure 3.10 : Pressure Gauge Analogy of Voltmeter Voltmeter Design
7. 7. 109 Measurement of Electrical Quantities Voltmeters, as well as ammeters and ohmmeters, are designed with the use of a sensitive current detector such as a galvanometer. Design Voltage V G RG IG I is the current to give full scale reading G Current limiting resistor in series with Galvanometer RS Design task: Given the resistance and the current which causes full scale reading on the galvanometer, find the value of the series current-limiting resistor which will give full scale reading with the design voltage of the voltmeter V Resistance of Galvanometer Galvanometer Figure 3.11 : Voltmeter Design A voltmeter is placed in parallel with a circuit element to measure the voltage drop across it and must be designed to draw very little current from the circuit so that it does not appreciably change the circuit it is measuring. To accomplish this, a large resistor is placed in series with the galvanometer as shown in Figure 3.11. Its value is chosen so that the design voltage placed across the meter will cause the meter to deflect to its full-scale reading. A galvanometer full-scale current is very small: of the order of milliamperes. If a galvanometer with Ω,GR = GI = mA is used to design voltmeter for a full scale voltage V = volts, the required series resistor is given by design S G = G V R R I = − Ω Example 3.2 A moving coil meter has a resistance of 2 Ω and gives full scale defection with 20 mA. Show how it can be used to measure voltage upto 250 V. Solution Meter resistance, RG = 2 Ω Meter current, IG = 20 mA = 0.02 A Design voltage Vdesign = 250 V ∴ External resistance required to be connected in series with the instrument to measure voltage upto 250 V design 250 2 0.02 S G G V R R I = − = − Ω 12,498SR = Ω SAQ 1 (a) Draw a circuit of a multimeter using galvanometer. (b) A moving coil meter has a resistance of 5 Ω and gives a full scale deflection with 10 mA. Show how it can be used to measure current up to 10A. (c) A moving coil instrument has a resistance of 10 Ω and gives a full scale deflection when carrying 50 mA current. Show, how it can be adopted to measure voltage upto 750 V and current upto 100A.
8. 8. 110 Electricity 3.3 NAVIGATIONAL LIGHTS As these are vital circuits in a ship from the point of view of laws of navigation, they require special attention. They should be connected to a distribution board, which does not supply any other service, so that they cannot be put out of action by the inadvertent opening of a wrong switch. Also they should have a change over switch so that they can be transferred to another source of supply if the normal supply fails, for example by the blowing of a fuse. Warning devices are also required to indicate the failure of any individual navigation light. The indicating panel may also comprise the distribution board previously mentioned, i.e. it may incorporate the fuse protection. The warning device may be visual or audible or both. The requirements for navigation lights are prescribed by the International convention, and lamps of special construction and appropriate wattages and performance are necessary. A typical arrangement is shown in Figure 3.12 from which it will be seen that if an open circuit occurs, the relay will be de-energised and the contacts will close and operate the buzzer. If the indicating lamp fails the circuit is maintained through the parallel connected resistance. SwitchRelayIndicating Light Navigation Light Switch Buzzer Resistance Figure 3.12 : Typical Arrangement of Light Relay Switch Definitions (a) “Masthead light” means a white light placed over the fore and aft centerline of the vessel showing an unbroken light over an arc of the horizon of 225 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on either side of the vessel. (b) “Sidelights” means a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of the horizon of 112.5o and so fixed as to show the light from right ahead to 22.5o abaft the beam on its respective side. In a vessel of less than 20 meters in length the sidelights may be combined in one lantern carried on the fore and aft centerline of the vessel. (c) “Sternlight” means a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of 135o and so fixed as to show the light 67.5o from right aft on each side of the vessel. (d) “Towing light”, means a yellow light having the same characteristics as the “sternlight” defined in (c) of this rule. (e) “All-round light” means a light showing an unbroken light over an arc of the horizon of 360o . (f) “Flashing light”, means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.
9. 9. 111 Measurement of Electrical Quantities 3.3.1 Intensity of Lights The minimum luminous intensity of lights shall be calculated by using the formula : I = 3.43 × 106 × T × D2 × K −D where I is luminous intensity in candelas under service conditions, T is threshold factor 2 × 10− 7 lux, D is range of visibility (luminous range) of the light in nautical miles, and K is atmospheric transmissivity. For prescribed lights the value of K shall be 0.8, corresponding to a meteorological visibility of approximately 13 nautical miles. 3.3.2 Horizontal Sectors (a) In the forward direction, sidelights as fitted on the vessel shall show the minimum required intensities. The intensities shall decrease to reach practical cut off between 1o and 3o outside the prescribed sectors. (b) For sternlights and masthead lights at 22.5o abaft the beam for sidelights, the minimum required intensities shall be maintained over the arc of the horizon up to 5o within the limits of the sectors prescribed in rule 21. From 5o within the prescribed sectors the intensity may decrease by 50 per cent up to the prescribed limits; it shall decrease steadily to reach the practical cut-off at not more than 5o outside the prescribed sectors. (c) All round lights shall be so located as not to be obscured by masts, topmasts or structures within angular sectors of more than 6o , except anchor lights, prescribed in rule 30 which need not be placed at an impracticable height above the hull. 3.3.3 Vertical Sectors The vertical sectors of electric lights, as fitted with the exception of lights on sailing vessels underway shall ensure that : (a) At least the required minimum intensity is maintained at all angles from 5o above to 5o below the horizontal. (b) At least 60 per cent of the required minimum intensity is maintained from 7.5o above to 7.5o below the horizontal. (c) In the case of sailing vessels underway the vertical sectors of electric lights as fitted shall ensure that : (i) At least the required minimum intensity is maintained at all angles from 5o above to 5o below the horizontal; (ii) At least 50 per cent of the required minimum intensity is maintained from 25o above to 25o below the horizontal. (d) In the case of lights other than electric, these specifications shall be met as closely as possible. Manouevering Light The maneuvering light shall be placed in the same fore and aft vertical plane as the masthead light or lights and, where practicable, at a minimum height of 2 meters vertically above the forward masthead light, provided that it shall be carried not less than 2 meters vertically above the after masthead light. On a vessel where only one masthead light is carried, the manoeuvering light, if fitted, shall be carried where it can best be seen, not less than 2 meters vertically apart from the masthead light.
10. 10. 112 Electricity 3.4 COLOUR SPECIFICATION OF LIGHTS The chromaticity of all navigation lights conforms to the standards, specified for each colour by the International Commission on Illumination (CIE). Normally used colours are white, red, green and yellow. 3.4.1 Shapes Shapes shall be black and of the following sizes : (a) A ball shall have a diameter of not less than 0.6 meter; (b) A cone shall have a base diameter of not less than 0.6 meter and a height equal to its diameter; (c) A cylinder shall have a diameter of at least 0.6 meter and a height of twice its diameter; (d) A diamond shape shall consist of two cones as defined in (b) above having a common base; (e) The vertical distance between the shapes shall be at least 1.5 meter; (f) In a vessel of less than 20 meters in length, shapes of lesser dimensions but commensurate with the size of the vessel may be used and the distance apart may be correspondingly reduced. 3.4.2 Visibility of Lights (a) In vessels of 50 meter or more in length : (i) A masthead light, 6 miles; (ii) A sidelight, 3 miles; (iii) A sternlight, 3 miles; (iv) A towing light, 3 miles; (v) A white, green, red or yellow all-round light, 3 miles. (b) In vessels of 12 meters or more in length but less than 50 meters in length : (i) A masthead light, 5 miles; except that where the length of the vessel is less than 20 meters, 3 miles; (ii) A sidelight, 2 miles; (iii) A sternlight, 2 miles; (iv) A towing light, 2 miles; (v) a white, red, green, or yellow all-round light, 2 miles. (c) In vessels less than 20 meters in length : (i) A masthead light, 2 miles; (ii) A sidelight, 1 mile; (iii) A sternlight, 2 miles; (iv) A towing light, 2 miles; (v) A white, red, green or yellow all-round light, 2 miles. (d) In inconspicuous, partly submerged vessels or objects being towed : (i) A white all-round light, 3 miles. SAQ 2 (a) Discuss the importance of navigational lights. (b) What is the visibility specification of light in navigation?
11. 11. 113 Measurement of Electrical Quantities3.5 SWITCHES Switches are devices used to allow electric current to flow when closed, and when opened, they prevent current flow. Common switch types include pushbutton, rocker, toggle, rotary coded DIP, keylock, slide, snap action, and reed. Pushbutton Switch Pushbutton switch is a mechanical switch defined by the method used to activate the switch. The activation method is typically in the form of a plunger that in pushed down to open or close the switch. These switches are shown in Figure 3.13, and are classified as : (a) PUSH ‘ON’, RELEASE ‘OFF’ (b) PUSH ‘OFF’, RELEASE ‘ON’ (c) PUSH ‘ON’, PUSH ‘OFF’,‘PUSH TO CHANGE OVER’: Figure 3.13 : Type of Switches Rocker Switches Rocker actuators are familiar in many on-off switches; they rock or pivot about the centerline, and include both maintained and momentary contact types. Rocker switches, momentary contact, are defined by their motion and momentary contact function. Rocker switches move like rocking chairs when pushed on either side to open or close the circuit. In momentary contact, the switch is opened or closed only during actuation. Rocker switches are often differentiated on by the difference in their actuators. The momentary contact switch function can be momentary ON (normally open), momentary off (normally closed), three position momentary (center OFF), three position momentary (center ON), and three position momentary (center NEUTRAL). Momentary ON is a term used to describe contacts, which interrupt a circuit when in their normal position. When actuated, the circuit is established, but opened again when actuator is released. Momentary OFF is a term used to describe contacts, which establish a circuit when in their normal position. When actuated, the circuit is interrupted, but established again when actuator is released. An example of three position momentary center OFF is Momentary ON-OFF-Momentary ON. An example of three position momentary center ON is an ignition switch (OFF-ON-Momentary Start). An example of three position momentary center NEUTRAL is Momentary ON-NEUTRAL-Momentary OFF. Important electrical switch specifications to consider when searching for Rocker switches, momentary contact, include mechanical life, maximum current rating, maximum AC voltage rating, maximum DC voltage rating, and maximum power rating. Mechanical life is the maximum life expectancy of the switch. Often, electrical life expectancy is less than mechanical life. Toggle Switch The toggle moves or swings to make or break the circuit; includes maintained contact and momentary contact types.
12. 12. 114 This is often used on equipment as a power supply ‘ON-OFF’ switch, either in the Single Pole Single Throw (SPST) form or Single Pole Double Throw (SPDT) form or Double Pole Double Throw (DPDT) type as shown in Figure 3.14. Electricity (SPST) (a) SPST Switch (b) SPDT Switch Circuit 1 2 (c) DPDT Switch Figure 3.14 : Toggle Switches The rating for a particular switch depends on whether it is to be used in AC or DC circuits. Life of switch depends on its rating. If rated values exceed, then life of switch is shortened, when a circuit is switched off. Sparking occurs at the switch contacts and vaporizes the metal. In general, switches last longer with DC than AC. Physical appearance of different type of switches is shown in Figure 3.15. Figure 3.15 : Various Kinds of Switches Rotary Coded DIP Switches Rotary coded DIP switches are activated by means of a rotating shaft that can have several stop positions. For each position of the shaft (the input) the switch generates an output binary code. A DIP (Dual In-line Package) switch is an electronic package (circuit board) consisting of a series of tiny switchs. Typically used in the configuration of computers and computer peripherals (for example, circuit boards, modems etc.). Rotary switches move in a circle, and can stop in several positions along its range. Rotary Wafer Switches One or more discs (wafers) of paxolin (an insulator) are mounted on a twelve position spindle as shown in Figure 3.16. The wafers have metal contact strips on one or both sides and rotate between a similar number of fixed wafers with springy contact strips. The contacts on wafers can be arranged to give 1 pole – 12 way, 2 pole – 6 way, 3 pole – 4 way, or 6 pole – 2 way switching etc.
13. 13. 115 Measurement of Electrical Quantities 4 Poles 3 ways 4 Poles Figure 3.16 : Rotary Wafer Switches Keylock Switches Keylock switches have key as the means of activation. The key is turned in a circle, and can stop in several positions in its range. Key Board Switch Switches used on computer key board, they are of SPST push type to make momentary contact which can be mounted on a printed circuit board. Slide Switch A slider moves linearly (slides) from position to position. Change Over or Single Pole Double Throw (SPDT) The poles are the number of separate circuits the switch makes or breaks at the same time. The ‘Throws’ are the number of positions to which each pole can be switched. Figure 3.17 : SPDT Switch Double Pole Double Throw Switch (DPDT) Operates two circuits simultaneously. The circuits are those connected to X and Y and each one can go to either of two positions – to P or Q for X and to R and S for Y as shown in Figure 3.18. P Q R S X Y Figure 3.18 : DPDT Switch
14. 14. 116 Electricity Non-magnetic Contact Snap Action Switch Snap action switch is a mechanical switch that produces a very rapid transfer of contacts from one position to another. They are useful in situations that require a fast opening or closing of a circuit, such as a mouse button or appliance setting. Reed Switches Reed switches are magnetically activated switches. They are typically manufactured with two ferromagnetic reeds (contact blades), which are sealed in a glass capsule. In the presence of a magnet, the blades (contacts) close. Used for fast switching operations of a single circuit, e.g. line in a telephone exchange. The reeds are thin strips of easily magnetisable and demagnetisable material. They are sealed in a glass tube containing an inert gas such as nitrogen to reduce corrosion of the contacts. A typical reed switch schematic is shown in Figure 3.19. Un-magnetized Reeds Terminal Magnetized Reeds Terminals S N Magnetic Contact Reed Figure 3.19 : Reed Switches The switch is operated either by bringing a magnet near or by passing a current through a coil surrounding it. In both cases the reeds become magnetized, attract each other and on touching, they complete the circuit connected to the terminals. They separate when the magnet is removed or the current stops flowing in the coil. When the change over reed switch operates, the reed is attracted from the non- magnetic contact to the magnetic area. Pole and throw configurations for switches can be single pole single throw (SPST), single pole double throw (SPDT), double pole single throw (DPST), or double pole double throw (DPDT). SPST is a switch that makes or breaks the connection of a single conductor in a single branch circuit. This switch typically has two terminals. It is commonly referred to as a “Single-Pole” Switch. SPDT is a switch that makes or breaks the connection of a single conductor with either of two other single conductors. This switch typically has 3 terminals, and is commonly used in pairs and called a “Three-Way” switch. DPST is a switch that makes or breaks the connection of two circuit conductors in a single branch circuit. This switch typically has four terminals. DPDT is a switch that makes or breaks the connection of two conductors to two separate circuits. This switch typically has six terminals and is available in both momentary and maintained contact versions. The switch function can be maintained contact, momentary contact, or alternating contact. In a maintained contact switch, an actuator stays in thrown position. This includes on- off, but it also includes Three-Position – (Center-off) and Three Position – (No Center-off) switches where the switch remains in its actuated position. In a momentary contact switch, the switch must be held in position; it reverts to normal position when actuating force is removed. Alternating action such as push on, push off characterizes an alternating contact. A normally open (NO) switch has contacts that are open or disconnected in their unactuated (normal) position. A normally closed (NC) switch has contacts that are closed or connected in their unactuated (normal) position. Important electrical switch specifications to consider when searching for switches include maximum current rating, maximum AC voltage rating, maximum DC voltage rating, and maximum power rating. Other important parameters to consider when searching for switches include the terminal type, construction materials, common features, and environmental conditions.
15. 15. 117 Measurement of Electrical Quantities SAQ 3 What are the various types of switches used in electrical circuits? Briefly describe each type. 3.6 RELAYS A relay is a switch operated by an electromagnet. It is useful when a small current in one circuit is required to control another circuit containing a device such as a lamp or electric motor which requires a large current or several different switch contacts are to be operated simultaneously. The structure of a relay and its symbol are shown in Figure 3.20. When the controlling current flows through the coil, the soft iron core is magnetized and attracts the L shaped soft iron armature. This rocks on its pivot and opens, closes or changes over, the electrical contacts in the circuit being controlled. In the figure shown, it closes the contacts. Figure 3.20 : Relay Configuration The current needed to operate a relay is called the ‘pull in’ current and the ‘drop out’ current is the current in the coil when the relay just stops working. Protective relays and monitoring relays detect or monitor for abnormal power system conditions. Protective relays detect defective lines, defective apparatus, or other power system conditions of an abnormal or dangerous nature and initiate appropriate control circuit actions. Monitoring relays are used to verify conditions in the power system or in the protective system. Monitoring relay functions include fault detection, voltage checking, and direction-sensing that confirms power system conditions but does not directly sense the fault or problem. Protective relays and monitoring relays can be sensitive to voltages, power or phase, current, or frequency. Important specifications to consider for voltage sensitive relays include under voltage, over voltage, and differential voltage. An under voltage relay trips when the voltage drops below a set point. An over voltage relay trips when a voltage rises above a set point. A differential voltage relay responds to the difference between incoming and outgoing voltage associated with the protected apparatus. A power or phase sensitive relay can monitor phase sequence, phase reversal, ground or earth fault, power factor, phase failure or loss, and phase unbalance. A phase sequence relay monitors for correct phase sequence. A phase reversal relay monitors for a change of one-half cycle or 180o in phase. A ground earth fault relay monitors for any undesired current path from a point of differing potential to ground. Power factor is the cosine of the phase angle between the voltage and current in alternating-current power transmission and distribution. A phase failure relay monitors for voltage with the incorrect phase sequence or one or more phases open. A phase unbalance relay operates when the magnitude of one current exceeds the magnitude of another current by a predetermined degree. Voltage Balance operates similarly. Important specifications to