Construction Electrician Apprenticeship Program E-2. Learning guide E-2

Transcription

1 Construction Electrician Apprenticeship Program Level 1 Line E: Use Test Equipment E-2 Learning guide E-2 Use Digital Meters

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3 Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

4 Acknowledgements and Copyright Copyright 2011 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program. Harjit Dosanjh, Writer Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: Toll Free: Fax: crownpub@gov.bc.ca Website: Version 1 Revised, March 2012

5 LEvel 1, Learning guide E-2: use Digital meters Learning Objectives Learning Task 1: Describe digital multimeter functions Self-Test Learning Task 2: Describe power measurement Self-Test Learning Task 3: Describe insulation resistance measurement Self-Test Learning Task 4: Use digital meters Self-Test Learning Task 5: Maintain digital meters Self-Test Learning Task 6: Explain meter readings Self-Test Answer Key Construction Electrician Apprenticeship Program: LEvel 1 5

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7 Learning Objectives E-2 Learning Objectives The learner will describe digital multimeter functions. Activities Read and study the topics of Learning Guide E-2: Use Digital Meters Complete Self-Tests 1-4. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources All resources are provided in this Learning Guide. BC Trades Modules We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January 2011 Construction Electrician Apprenticeship Program: Level 1 7

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9 Learning Task 1: Describe digital multimeter functions Digital multimeters A digital multimeter (DMM) combines the features of an ammeter, a voltmeter, and an ohmmeter. Digital multimeters are available with advanced features for measuring frequency, temperature and other electrical quantities using accessories available from the manufacturer. These advanced features vary among DMM models from different manufacturers. Manual range DMMs have a range selector switch to select a specific range value for the quantity being measured, such as voltage, current or resistance. When the value of the measurement exceeds the selected range, the display on the DMM will indicate OL (overload). A DMM with the autoranging feature (Figure 1) will automatically select the range with the best accuracy and resolution for the measurement. The autorange feature can be changed to manual range using the range button. The digital multimeter will have a function control switch to select the electrical quantity to be measured, such as AC volts, DC volts and ohms. When the meter leads are connected to the device to be tested, the meter automatically selects the proper range and displays the values. Digital multimeters measure the waveform under test with either of two methods, using true rms (root mean square) meters or average responding meters. True rms meters use an integrated circuit to compute the rms value of the waveform under test, as with the meter in Figure 2. Average responding meters use a simple circuit to measure the average value of the waveform after the waveform is rectified into DC. The meter then calculates the rms value. Lighting circuits with electronic ballasts, circuits with variable-speed drives and computers have non-sinusoidal waveforms. To accurately measure these circuits, a true rms DMM is required. The manufacturer s specifications will indicate if the DMM is a true rms meter. If there is no indication in the manufacturer s specifications, the DMM is usually average responding. Figure 1 Autoranging DMM Figure 2 True rms DMM Construction Electrician Apprenticeship Program: LEvel 1 9

10 Learning Task 1 E-2 DMM voltmeter functions Voltage is the electrical force that drives current through an electrical circuit. Voltage is defined as the potential difference between two points in an electrical circuit. There are two types of voltage sources, AC voltage and DC voltage. The current flow caused by an AC voltage source changes in both magnitude and direction at regular intervals. The current flow caused by a DC voltage source does not change direction. Voltage measurements are taken using the DMM voltmeter functions. Digital voltmeters are more commonly used for voltage measurements and are more accurate than analogue type. Regardless of the range selected most digital meters have an input impedance of about 10 megohms, which is the meter s internal resistance. There are meters available with higher impedance values and also dual impedance digital multimeters. The input impedance is the ohmic value that limits the amount of current drawn by the meter when taking measurements. If a voltmeter has a resistance that is too low in comparison to the circuit, it will give a false reading. The false reading is due to the meter changing the circuit by lowering the total resistance of the circuit. Because the meter is placed in parallel with the device being measured, the current flow in the circuit increases. The effect a meter has on the circuit is called meter loading. This effect is more common with low-quality analogue meters. AC voltage function AC voltage is usually created by an alternator at a hydroelectric plant. There are also steampowered and wind-powered alternators. An AC voltage changes in both magnitude and polarity at regular intervals. The AC voltmeter function (Figure 3) is used to accurately measure this type of changing voltage. The AC voltage function is commonly used to test and troubleshoot receptacles, lights and other AC circuits in industrial, commercial and residential settings. Figure 3 DMM set to AC voltage function 10 Construction Electrician Apprenticeship Program: Level 1

11 Learning Task 1 E-2 DC voltage function A DC voltage source causes a current to flow that that is unidirectional, and does not change direction. A DC voltmeter is used to measure this type of voltage. DC voltage is usually supplied from a battery or a power supply and can be supplied from an AC source using a rectifier. A rectifier is a device used to convert AC to DC. DC voltage can also be produced by electromagnetism, chemicals, heat and solar panels. A DMM set for the DC voltage function (Figure 4) can be used to test battery banks and to troubleshoot DC motors, DC generators and other DC circuits. Figure 4 DMM set to DC voltage function DMM ammeter function A digital ammeter is used to measure current in an electrical circuit. Current flows through a circuit when a power source is connected to a device, or load. Current is the flow of electrons in a circuit when voltage is applied to the circuit. This current flow is inversely proportional to the resistance of the load. Current is measured in amperes, or amps. Current measurements are taken to ensure that the electrical circuit or components are not overloaded. There are digital ammeters for measuring DC current and AC current. These meters are available as panel mount, bench type and clamp types, as well as DMMs. The DMM has a selector switch for AC and DC current. Most DMMs can measure DC or AC currents from the micro-amp range to a maximum of 10 amps. There are clamp-on current probe accessories available for DMMs to measure higher current values and safely take current readings without opening a circuit. The clamp-on accessories are available for AC currents only or for both AC and DC currents. Construction Electrician Apprenticeship Program: Level 1 11

12 E-2 Learning Task 1 Figure 5 DMM set to AC (left) and DC (right) amps functions DMM ohmmeter function A DMM has an ohmmeter function that measures the amount of resistance in a component or circuit. A DMM set for ohms must not be used on an energized circuit. If the circuit has a capacitor, the capacitor must be discharged before taking any resistance readings. A DMM has an internal battery to supply the current used to test the resistance of a component or circuit. A DMM set for ohms can be used to test the continuity of a fuse or a switch. The resistance of an open switch or faulty fuse would indicate OL (infinity) on the display, and a closed switch or good fuse would indicate a very small resistance value on the display (about 0.4 ohms). Figure 6 DMM set to the ohms function 12 Construction Electrician Apprenticeship Program: Level 1

13 Learning Task 1 E-2 DMM diode function A diode is an electronic device that allows current to flow in one direction only and blocks it in the other direction. A digital multimeter can be used to test if a diode is functioning properly. When the function switch of the DMM is set to diode test mode, the meter produces a small voltage of about 2.5 volts between the test leads. With the black test lead of the DMM connected to the cathode and the red test lead connected to the anode of the diode, the meter will display a forward bias voltage drop of the diode. For most silicon diodes this will be approximately 0.5 volts, ±0.2 volts. DMM continuity function The continuity function of a DMM provides a quick and easy method to verify that a circuit is complete. It can also be used for troubleshooting to test components such as fuses and switches. A fuse or switch that is good will have continuity and the DMM will emit an audible signal, usually a steady tone whenever there is continuity. This allows testing without removing your eyes from the work. The circuit must be de-energized when using this function. Digital clamp-on ammeter To measure the current being drawn by a circuit with a standard DMM, the circuit must be interrupted and the DMM must be directly connected into the circuit in series. The maximum current measurement of most DMMs is normally restricted to a value of 10 amperes. The digital clamp-on ammeter eliminates the need to break open the circuit to connect the ammeter directly. The jaws of the meter are simply clamped around one of the conductors supplying power to the load, and the current readings can be taken safely without opening or interrupting the circuit. Digital clamp-on meters with larger jaws are available to measure current in the kiloampere range. Clamp-on ammeters are available to measure alternating current. This meter uses a current transformer to operate. The core material of the transformer is located in the jaw of the meter. When the jaws of the meter are clamped around the current-carrying conductor, the changing magnetic field produced by the AC induces a voltage in the current transformer. The amount of induced voltage is proportional to the strength of the magnetic field. Clamp-on ammeters are also available to measure AC and DC current. These meters use the Hall effect generator principle as in the basic principle of operation. The main advantage of these meters is that they allow current measurement without having to shut down equipment to connect an ammeter into the circuit. The clamp-on meter commonly comes with test leads and also has the ability to measure AC and DC voltages as well as resistance. These meters are available with various jaw openings, and some have open jaws that can be slid over a conductor where space is limited. They are available with a Hold button to take reading in tight areas. Some also have a peak hold function that allows you to measure the inrush current drawn by a motor at start-up. Construction Electrician Apprenticeship Program: Level 1 13

14 Learning Task 1 E-2 Figure 7 Digital clamp-on ammeter Now do Self-Test 1 and check your answers. 14 Construction Electrician Apprenticeship Program: Level 1

15 Learning Task 1 E-2 Self-Test 1 1. Which DMM function is used to measure resistance? 2. Which DMM function is used to measure current flow in an electrical circuit? 3. What is the impedance value of most digital voltmeters? 4. Which meters are more accurate: analogue or digital? 5. Which meters are more commonly used in the electrical trade: analogue or digital? 6. What is the abbreviation DMM short for? 7. Which meter is used to take current measurement without opening the circuit? 8. Which of the two meter types, true rms or average responding would be used by an electrician testing an electronic ballast in a fluorescent lighting fixture? 9. When measuring the voltage across a component, will meter loading cause the meter reading to be higher or lower than the voltage across the component without the meter in the circuit? Construction Electrician Apprenticeship Program: Level 1 15

16 Learning Task 1 E Which DMM setting would be use to measure the voltage output of an: Alternator Rectifier 11. The source voltage of the circuit is 120 volts AC, and the resistance of the circuit is 10 ohms. What type of ammeter would you use to measure the current in the circuit? 12. List two advantages of a clamp-on style of ammeter over a DMM. 13. Which meter function would be selected to provide quick and easy testing of whether a fuse is open or has a complete path through it? 14. When using the diode test function to a silicon diode, if the diode is good a DMM will display the 15. Which measurement function of a DMM requires an internal battery to provide current flow through the circuit? Go to the Answer Key at the end of the Learning Guide to check your answers. 16 Construction Electrician Apprenticeship Program: Level 1

17 Learning Task 2: Describe power measurement Digital wattmeter A digital wattmeter measures the power consumed in an electrical circuit. Wattage is the amount of power that is being used in an electrical circuit. Wattage is proportional to the amount of voltage and current in an electrical circuit. A digital wattmeter (Figure 1) takes current and voltage measurements electronically thousands of times per second. A computer chip in the digital wattmeter then calculates the wattage of the circuit being measured. Using a wattmeter Figure 1 Digital wattmeter Always refer to the manufacturer s instructions before using a digital wattmeter. Never exceed the individual current or voltage rating of the wattmeter. Follow these steps when using a digital watt meter: 1. Always de-energize the circuit before connecting or disconnecting the wattmeter from the circuit. 2. Connect the current jacks of the wattmeter in series with the circuit or the load to be measured. Some wattmeters may have clamp-on leads for current measurement. 3. Connect the voltage jacks in parallel with the circuit or load to be measured, just like a voltmeter. Construction Electrician Apprenticeship Program: LEvel 1 17

18 Learning Task 2 E-2 4. Energize the circuit and read the display on the wattmeter. 5. Once the reading has been taken, de-energize the circuit before removing the wattmeter. W ACA/DCA ACV/DCV Watt 1 Watt 2 Power source Load A COM V Watt AC Figure 2 Connecting a wattmeter to a circuit Now do Self-Test 2 and check your answers. 18 Construction Electrician Apprenticeship Program: Level 1

19 Learning Task 2 E-2 Self-Test 2 1. Connect the circuit below to measure the total power dissipated when the resistors are connected in parallel. W ACA/DCA ACV/DCV Watt 1 Watt 2 Power source Load A COM V Watt AC Figure 1 Go to the Answer Key at the end of the Learning Guide to check your answers. Construction Electrician Apprenticeship Program: Level 1 19

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21 Learning Task 3: Describe insulation resistance measurement Digital megger Digital insulation testers are used to measure the insulation resistance of motors, transformers, cables, circuit wiring and other electrical devices. These testers are commonly called meggers (Figure 1). Megger is the brand name of the company that first made these insulation testers. The early tester was a hand dynamo that generated voltages high enough to test resistance in the megohm range. The megger is often used at planned intervals as part of routine maintenance programs to test the insulation of electrical equipment for resistance changes. If there is a major change in the insulation s resistance value, this is an indication that the electrical equipment could fail. Some of the causes of insulation deterioration are mechanical damage, fault currents, excessive heat, oil, moisture and voltage stresses. Meggers are also used to check conductors installed in conduits or raceways for grounds, shorts and opens. The megger provides a high enough test voltage to ensure that the insulation will not break down when rated line voltage is applied to the conductor. Most ohmmeters can only supply up to about 9 volts, which is not sufficient to test the insulation properly. The megger can supply voltages from 250 volts DC to 5000 volts DC, depending on the model and the range selected. Before using a megger, always check the maximum voltage rating of the conductor or component to be tested. When long runs of conductors are installed in a conduit and tested with a megger, a potential hazard can be created by charging the conductors. If the opposite end of the conductors connected to the megger are open, the conductors can act like a capacitor and store a charge. If not discharged properly, this energy can be a potential shock hazard. Most digital meggers will have a discharge feature to drain this capacitive charge. If the digital megger does not have the discharge feature, it is good practice to ground the conductor before and after testing. Always refer to the manufacturer s instructions for testing procedures and features of the test equipment. Figure 1 Digital megger Construction Electrician Apprenticeship Program: LEvel 1 21

22 Learning Task 3 E-2 Using a megger to test insulation CAUTION: Never use a megger on a live circuit. To test the quality of insulation using a megger, always refer to the manufacturer s instructions and do not set the megger voltage at more than the rated voltage of the equipment under test. The megger produces a high voltage at the terminals. Most digital meggers have a discharge button to drain the capacitive charge produced during testing. If the megger does not have discharge capabilities, ground each conductor to discharge the capacitive charge once testing is complete. Before beginning a test, mark off the area with warning signs. To test the conductors feeding a motor or piece of equipment, follow these steps: 1. Connect one lead of the megger to the frame of the motor and connect the other lead to one of the motor leads. 2. Push the test button on the megger and read the display. 3. Repeat this for all the motor leads. The reading should be at least 1 megohm or higher. This reading indicates that the motor leads are not shorted to ground. The higher the voltage rating of the equipment being tested, the higher the insulation resistance should be. 4. Once the motor leads have been tested to ground, the quality of the insulation between each of the motor leads should also be tested with the megger. 5. Once the insulation quality of the motor leads has been tested, the conductors feeding the motor or equipment should be tested. To test the conductors feeding a motor or peice of equipment, follow these steps: 1. Disconnect the conductors feeding the motor or other equipment completely from both ends of the circuit. The bare ends of the conductors should be individually covered with electrical tape or a insulated wire connector at one end to protect against shock. 2. Connect one of the megger leads to the ground of the electrical system. 3. Connect the other test lead to the bare end of the conductor under test. Make sure the other end of the conductor is covered with electrical tape or an insulated wire connector to prevent shock. 4. Turn on the megger and note the readings in the display. This should be a very high megohm reading to indicate that the insulation is good and the conductor is not shorted to ground. 5. Once all the conductors have been tested to ground, test each individual conductor to the other conductors in the cable or conductors in the conduit to ensure the insulation quality is good between the conductors. This should also be a very high megohm reading. 6. Once the test is completed, drain the capacitive charge on the conductor using the discharge function on the megger, or discharge each conductor to system ground. Now do Self-Test 3 and check your answers. 22 Construction Electrician Apprenticeship Program: Level 1

23 Learning Task 3 E-2 Self-Test 3 1. List three causes of insulation deterioration. 2. List two applications where meggers are used. 3. List three types of electrical equipment that could have their insulation resistance tested? 4. What should always be done with a conductor before and after megger testing the conductor? 5. What is the minimum reading expected when meggering a motor conductor? Go to the Answer Key at the end of the Learning Guide to check your answers. Construction Electrician Apprenticeship Program: Level 1 23

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25 Learning Task 4: Use digital meters Describe the features of digital meters Digital meters are commonly used in the electrical trade for troubleshooting and for testing electrical circuits. The main difference between analogue meters and digital meters is the display (Figures 1 and 2). An analogue meter draws current from the circuit being measured to deflect the coil and pointer of the meter movement. The amount of current drawn depends on the ohms per volt rating of the meter. Digital meters use an analogue-to-digital converter to display a reading. An analogue-to-digital converter is an electronic device that converts an analogue voltage or current signal into a digital signal, which is displayed in numerical form. Digital meters display their reading in discrete digits instead of using a pointer, as in analogue meters. The digital display makes digital meters faster and easier to read than analogue meters. Figure 1 Digital multimeter Figure 2 Analogue multimeter Digital meters are available in various styles. Panel types which are usually mounted on electrical cabinet doors to display electrical measurements such as voltage, current or watts for fixed equipment such as motors and transformers. This makes it easier to monitor this equipment. There are also bench meters and portable hand-held meters. The bench type is commonly used in lab areas and for testing equipment on workbenches. The hand-held type, usually available in the form of a digital multimeter, is used for troubleshooting and field measurements in industrial, commercial and residential applications. Construction Electrician Apprenticeship Program: LEvel 1 25

26 Learning Task 4 E-2 Digital meter safety Most digital multimeters are protected by fuses on the current measuring scales. These fuses help prevent injury and protect the meter from damage. A common error occurs when a DMM that was used to measure current or resistance is then used to measure voltage without first placing the test lead properly and switching to the proper voltage scale. In most cases the fuses will protect the meter. If a fuse is damaged and requires replacement, it must be replaced with one specified by the manufacturer to provide proper protection. DMM category ratings Digital multimeters are classified into four measurement categories (Figure 3). These categories are set forth by an International Electrotechnical Commission (IEC) Standard for electrical measurement category ratings for test tools. The categories specify maximum transient voltages for selected measuring ranges in the meter. A transient voltage is a temporary voltage spike in an electrical circuit. Transient voltages can occur due to lightning strikes, unfiltered equipment, and power being switched on and off. The categories classify the application in which the DMM can be used and apply to test equipment used to test voltages of 1000 volts or less. Meters that are category-rated also have protection from overcurrent faults. Generally, the closer you are to the power source and the greater the short-circuit current available at a certain point, the higher the category number of the tester. Reproduced with permission from Fluke Figure 3 DMM category ratings Following is a summary of the categories: Category I: Protected electronic equipment Used where equipment is not directly connected to the mains Equipment connected to circuits in which transient over-voltage is limited to a low level Category II: Single-phase receptacle-connected loads such as appliances, portable tools and other household loads 26 Construction Electrician Apprenticeship Program: Level 1

27 Learning Task 4 E-2 Outlets located more than 10 m from a CAT. III source Outlets located more than 20 m from a CAT. IV source Category III: Permanently installed loads such as three-phase motors, distribution panels and threephase outlets Single-phase commercial lighting Buses and feeders in industrial plants Category IV: Supply service connections, main panels and locations where fault current levels can be very high Overhead lines where high fault currents are present Service drop from utility pole to building, conductors between meter and electrical panel Meter leads Meter leads are a very important part of any test equipment. Proper care must be taken to ensure that proper test leads are used when using test equipment. When testing a component or a circuit, you must ensure that you are using the test leads that came with the tester or you could easily be compromising your safety. The test leads must have a category rating that is equal to or exceeds the rating of the tester. The category rating is usually stamped on the test probe. Test leads must be checked for wear and abuse that could damage the insulation. A test lead with insulation that is cracked, brittle, excessively dirty or damaged in any way should be replaced with a new test lead that has the same category rating as the tester. The test lead may also have an amperage rating stamped on it that is equivalent to or greater than the amperage rating of the tester V CAT III 600 V CAT IV 1000 V CAT III 600 V CAT IV 1000 V CAT III 600 V CAT IV Figure 4 DMM meter lead ratings Construction Electrician Apprenticeship Program: Level 1 27

28 Learning Task 4 E-2 Personal protective equipment Whenever working near or on energized exposed circuits, you must use the appropriate personal protective equipment (PPE). PPE is clothing and/or equipment worn by an electrician to reduce the possibility of injury when working near or on exposed electrical circuits. The recommendations for personal protective equipment are specified in CSA Standard Z462-08, Workplace Electrical Safety. PPE includes protective clothing, head, eye, ear, hand and foot, back and knee protection, and rubber matting. Protective clothing made of durable flame-resistant material provides protection from contact with hot and sharp objects. A hard hat and or protective helmet is required to protect from falling objects, impact and electrical shock. Safety glasses, gloves, safety shoes with steel toes and other protective equipment may be required, depending on the task being performed. The recommendations are specified in CSA Standard Z The standards are always being updated; always check the latest standards. Circuit placement If you are not sure how to use a particular meter function, always consult the manufacturer s manual for the appropriate directions prior to using the meter. Additionally, always: Ensure that your meter and leads have the appropriate Cat level for the energy source at the point being tested. Test a meter and leads on a known source before using it to test a circuit or component, to ensure that the meter is working properly. Assume a circuit is energized until it has been positively identified as de-energized by taking proper measurements: Test before touch. Wear appropriate personal protective equipment (PPE) as specified in CSA Z462, Standard for Electrical Safety in Workplace. Select the appropriate value and range for the circuit or component being tested. Observe correct polarity on DC measurements. Always consult the manufacturer s manual for the appropriate directions when using the meter. Always test a meter on a known circuit before using it to test a circuit or component, to ensure that the meter is working properly. Always assume a circuit is energized until it has been positively identified as de-energized by taking proper measurements. Always wear protective equipment appropriate for the testing and work area. DMM voltage measurement AC and DC digital voltmeters must be connected in parallel with the device or circuit being measured. If the voltmeter is connected in series, its high internal impedance will act as part of the series circuit and cause a false reading on the display. An AC voltage waveform changes polarity constantly with time, so it is not necessary to ensure correct polarity when connecting the test leads. A DC voltage should be measured with the 28 Construction Electrician Apprenticeship Program: Level 1

29 Learning Task 4 E-2 black lead in the Com jack and connected to the negative lead of the circuit or component. The red lead should be in the V/Ω jack and is connected to the positive lead of the circuit or component. It is recommended by the manufacturer that the black lead always be connected first when taking measurements. If a negative sign ( ) is displayed in the display window, the polarity is incorrect and the meter leads need to be reversed. AC voltage measurement with DMM To measure a AC voltage with a DMM, follow these steps: 1. Set the function switch to AC volts. If the voltage being measured is not known, set the range to the highest voltage. An autorange DMM will automatically select the range based on the voltage present. 2. Plug the black test lead into the jack labelled COM (common). 3. Plug the red test into the jack labelled V (voltage). 4. Connect the test leads in parallel to the source or the load (depending on what is being measured) and read the voltage measurement in the display window. Refer to Figure 5. MIN MAX PEAK RANGE HOLD Ω V ~ V ~ ma ma A ~ A A COM V Ω AC Figure 5 AC voltage measurement with a DMM Construction Electrician Apprenticeship Program: Level 1 29

30 Learning Task 4 E-2 DC voltage measurement with DMM To measure a DC voltage with a DMM, follow these steps: 1. Set the function switch to DC volts. Select the highest range if the voltage being measured is not known. An autorange DMM will automatically select the range based on the voltage present. 2. Plug the black test lead into the jack labelled COM (common). 3. Plug the red test lead into the jack labelled V (voltage). 4. Maintaining proper polarity, connect the test leads in parallel to the source or the load (depending on what is being measured) and read the voltage measured in the display window. If a negative sign appears in the display window, the test leads need to be reversed across the source or load, depending on what is being measured. Refer to Figure 6. MIN MAX PEAK RANGE HOLD Ω V ~ V ~ ma ma A ~ A A COM V Ω DC + Figure 6 DC voltage measurement with a DMM 30 Construction Electrician Apprenticeship Program: Level 1

31 Learning Task 4 E-2 Connecting a DMM in-line to measure AC amps To measure AC amps with a DMM, follow these steps: 1. Set the function switch to ACA (AC amps). Different manufacturers have various symbols for the function switch, so refer to individual manuals for each DMM. Select the highest current range if the approximate current being measured is not known. Do not change the function switch position while the circuit is energized. 2. Plug the black test lead into the jack labelled COM (common). 3. Plug the red test lead into the highest current jack. If the current is higher than what the meter is rated for, use an AC clamp-on ammeter. 4. Turn off the power to the circuit under test and open the circuit at the point to be tested. The black lead is connected to one lead of the open circuit and the red lead is connected to the other lead of the open circuit. 5. The ammeter should be connected in series with the source or device to be measured. Never connect the ammeter in parallel with the source or load to be measured, as an ammeter has very little resistance and this will cause a short circuit and damage the meter. 6. Ensure that the ammeter is properly placed in the circuit, turn on the circuit and read the measurement in the display window. 7. Turn off the power to the circuit and remove the in-line ammeter. 8. Once the current measurements have been taken, to avoid accidently connecting the meter in parallel, which would damage the meter, turn the function selector switch to the OFF position, then place the test leads in the common jack and the voltage jack. Refer to Figure 7. Construction Electrician Apprenticeship Program: Level 1 31

32 Learning Task 4 E-2 MIN MAX PEAK RANGE HOLD Ω V ~ V ~ ma ma A ~ A A COM V Ω AC Figure 7 AC amperage measurement with a DMM Connecting a DMM in-line to measure DC amps To measure DC amps with a DMM, follow these steps: 1. Set the function switch to DCA (DC amps). Different manufacturers have various symbols for the function switch, so refer to individual manuals for each DMM. Select the highest current range if the approximate current being measured is not known. Do not change the function switch position while the circuit is energized. 2. Plug the black test lead into the jack labelled COM (common). 3. Plug the red test lead into the highest current jack. If the current is higher than what the meter is rated for, use a DC clamp-on ammeter. 4. Turn off the power to the circuit under test and open the circuit at the point to be tested. The black lead is connected to the negative lead of the open circuit and the red lead is connected to the positive lead of the open circuit. 5. The ammeter should be connected in series with the source or device to be measured. Never connect the ammeter in parallel with the source or load to be measured, as an ammeter has very little resistance and this will cause a short circuit and damage the meter. 32 Construction Electrician Apprenticeship Program: Level 1

33 Learning Task 4 E-2 6. Ensure that the ammeter is properly placed in the circuit, turn on the circuit and read the measurement in the display window. 7. Turn off the power to the circuit and remove the in-line ammeter. 8. Once the current measurements have been taken, to avoid accidently connecting the meter in parallel, which would damage the meter, turn the function switch to the OFF position, then place the test leads in the common jack and the voltage jack. Refer to Figure 8. MIN MAX PEAK RANGE HOLD Ω V ~ V ~ ma ma A ~ A A COM V Ω DC + Figure 8 DC amperage measurement with a DMM Using the DMM ohms function to measure resistance To measure resistance using the DMM ohms function, follow these steps: 1. Ensure that the power to the circuit is off. Never connect a DMM set for ohms to an energized circuit, as this will damage the meter. Always remove or isolate the component to be tested. 2. Set the function switch on the DMM to resistance (Ω). 3. Plug the black lead into the common jack. 4. Plug the red lead into the resistance jack (Ω). Construction Electrician Apprenticeship Program: Level 1 33

34 Learning Task 4 E-2 5. Connect the leads together. If the battery symbol appears in the display, replace the battery. The meter should display a small amount of resistance (about 0.2 Ω 0.5 Ω).This is the test lead resistance. With the test leads held apart, the meter should display OL or 1, depending on the manufacturer. This indicates an infinite amount of resistance. 6. Connect the test leads across the component under test and read the display. Make sure there is a good connection between the test leads and the component under test to get an accurate reading. 7. After all the resistance readings have been completed, turn the DMM off to prevent the battery from draining. Refer to Figure 9. MIN MAX PEAK RANGE HOLD Ω V ~ V ~ ma ma A ~ A A COM V Ω Figure 9 Resistance measurement with a DMM Now do Self-Test 4 and check your answers. 34 Construction Electrician Apprenticeship Program: Level 1

35 Learning Task 4 E-2 Self-Test 4 1. What CSA standard covers workplace electrical safety? 2. What category of DMM is required for testing where fault currents are very high? 3. The category rating of test leads of a DMM must meet or exceed the category rating of the DMM. a. True b. False 4. What must be done before using a DMM on a circuit? 5. How is a voltmeter placed in a circuit? 6. How is an ammeter placed in a circuit? 7. Which feature of a DMM would best be used to detect a rapidly changing signal such as contact bounce on a microswitch? Construction Electrician Apprenticeship Program: Level 1 35

36 Learning Task 4 E-2 8. Connect the circuit in parallel and connect the voltmeter to measure the voltage across R3 MIN MAX PEAK RANGE HOLD Ω V ~ V ~ ma ma A ~ A A COM V Ω DC 3 Figure 1 36 Construction Electrician Apprenticeship Program: Level 1

37 Learning Task 4 E-2 9. Connect the circuit in parallel and measure the current through R2 MIN MAX PEAK RANGE HOLD Ω V ~ V ~ ma ma A ~ A A COM V Ω Figure 2 Go to the Answer Key at the end of the Learning Guide to check your answers. Construction Electrician Apprenticeship Program: Level 1 37

38 38 Construction Electrician Apprenticeship Program: Level 1

39 Learning Task 5: Maintain digital meters Digital meters are widely used in the electrical industry. Care should always be taken when using, handling and storing them. Always refer to the manufacturer s instructions when using a particular meter. A meter must only be used for taking measurements for which it is designed. Inspection Always inspect the meter for visible damage before using it. Inspect the test leads (Figure 1) to ensure they are safe to use, and confirm that the leads match or exceed the category rating of the meter. Do a visual inspection of the insulation, probe and connection of each lead. Make sure the insulation is not nicked or cracked. Once the test leads have been inspected to ensure there is no visible damage, do an ohms measurement to confirm they are electrically reliable. To electrically test the leads, place the DMM function selector switch to resistance (Ω) and touch the leads together. If the leads are in good condition the display should read 0.5 ohms or less. This will also test the battery of the DMM. If a battery symbol appears in the display, the battery needs to be replaced. With the leads connected, move and wiggle the leads to test for any intermittent problems. Figure 1 Meter leads Construction Electrician Apprenticeship Program: LEvel 1 39

40 Learning Task 5 E-2 Storage Care should be taken when storing a digital meter. Consult the manufacturer s instructions regarding individual meters. Store meters in an area where humidity, temperature and dust are controlled. The meter should be stored in a protective case or holster provided by the manufacturer (Figure 2). Figure 2 DMM protective cases Calibration To ensure that the meter performs according to its specifications, the meter must be calibrated according to the manufacturer s instructions. This is usually done once a year. If the meter s accuracy is in doubt, you should always compare readings with a similar, reliable meter. Service If the meter fails to operate, check the battery and fuses and replace according to the manufacturer s instructions. If the meter still does not operate, review the manufacturer s instructions to make sure it is being used correctly. Battery replacement Consult the manufacturer s manual for the meter for battery replacement. To replace a DMM s battery, follow these steps: 1. Turn the DMM off. 2. Disconnect the test leads and remove the screws from the back cover. 3. Disconnect the battery and replace it with one specified by the manufacturer. 4. Replace and secure the back cover. Refer to Figure Construction Electrician Apprenticeship Program: Level 1

41 Learning Task 5 E-2 Battery Figure 3 Location of DMM battery DMM fuse replacement Consult the manufacturer s manual for fuse replacement. To replace a fuse in a DMM, follow these steps: 1. Turn the DMM off. 2. Disconnect the test leads and remove the screws from the back cover. 3. Remove the faulty fuse from the PCB board and replace with a new fuse. The new fuse must meet the manufacturer s specifications for current and voltage ratings. 4. Replace and secure the back cover. Refer to Figure 4. Fuse Fuse Figure 4 Location of DMM fuse Now do Self-Test 5 and check your answers. Construction Electrician Apprenticeship Program: Level 1 41

42 Learning Task 5 E-2 Self-Test 5 Are the following statements True or False? 1. Once you are finished using a DMM, you can just throw it into your toolbox. a. True b. False 2. Meters test leads can be replaced with the cheapest leads available. a. True b. False 3. If a meter fuse is blown, it can be replaced with any fuse. a. True b. False 4. Ideally, meters must be stored in a dark room. a. True b. False 5. You should always consult the manufacturer s manual for battery and fuse replacement instructions. a. True b. False Go to the Answer Key at the end of the Learning Guide to check your answers. 42 Construction Electrician Apprenticeship Program: Level 1

43 Learning Task 6: Explain meter readings Digital display A digital meter has a window called a display. Numbers appear in this window to indicate the numeric value of the electrical quantity being measured. The meter may use either lightemitting diodes (LEDs) or a liquid crystal display (LCD) to display these digits. Each digit is shown by a 7-segment display that is uniquely energized by analogue-to-digital electronic circuitry. There may be an additional segment for the decimal point. In Figure 1, if segments a, b and c were lit, then the decimal number 7 would appear on the display. Most seven-segment digits also include a decimal point ( dp ). a f g b e c d dp Figure 1 Segment display Symbols Using the digital meter is fundamentally no different than using the equivalent analogue meter. The digital meter uses symbols to identify different functions and display features (Figure 2). Always refer to the user s manual before using the meter.! Important safety information + Low battery Dangerous voltage may be present Manual range or automatic touch hold Not applicable to identified model Continuity beeper AC Diode DC Fuse AC or DC Double insulation Ground Figure 2 Common digital meter symbols Construction Electrician Apprenticeship Program: LEvel 1 43

44 Learning Task 6 E-2 Figure 3 shows the application of some of these symbols in the display window. A low battery symbol lets the user know that the internal battery should be replaced. With the autopolarity feature, a negative or minus sign to the left of the reading indicates that the lead polarity is actually reversed for DC measurements. Autoranging will automatically select the proper measurement range, while manual ranging lets you lock into a specific range for repetitive measurements. In manual range, OL (overload) is displayed if the range is too low for the measurement. A bar graph is sometimes available to indicate the reading compared to the full scale of the measurement range. The units of measurement for the reading are displayed according to the range switch setting. Most digital multimeters also have special test positions, such as a beeper for continuity testing, and a diode testing feature. Refer to the symbols in Figure 2. Replace battery Autopolarity Manual range/ touch hold OL Bar graph Figure 3 Display features Bar graph readings A digital multimeter may include a bar graph in the display to indicate changes and trends in a circuit. The bar graph on a DMM reacts like the needle on an anologue meter. The number of segments displayed in the bar graph indicates the measured value compared to the full scale of the measurement range. The bar graph reading is updated much faster than the numerical reading on a DMM and is easier for the eye to detect. The quick response of a bar graph can easily reveal problems caused by loose connections or contact bounce on relays. An analogue meter indicates these problems by the movement of its needle. There is a polarity indicator to the left and an overload indicator to the right of the bar graph. 44 Construction Electrician Apprenticeship Program: Level 1

45 Learning Task 6 E-2 Figure 4 illustrates an example of a bar graph measuring a 1.5 volt battery source. Polarity indicator Bar graph Figure 4 Bar graph Counts The display count determines the maximum range and resolution of the DMM. A 31 2-digit meter is commonly used and can display three full digits ranging from 0 to 9, and one half digit at the left which displays only a 1 or is left blank. A 31 2-digit meter or 2000-count display has a maximum reading of 1999, one less than the display count. Resolution The resolution of a digital multimeter is the smallest measurement of the scale that can be shown on the display it determines how fine a reading a meter can make as well as the accuracy of the meter s measurements. Resolution of a DMM is determined by the display count (the number of digits the meter can display) and the level of voltage under test. Table 1 gives the ranges and resolution for a 31 2-digit or 2000-count meter. Table 1 shows that for a given value of voltage measurement, meter resolution can be improved by reducing the range setting of the DMM to that closest to (but higher than) that of the expected value of the voltage under test. Figure 5 illustrates an example of how resolution can be increased by selecting an appropriate range when testing a 1.5 volt battery. Table 1: 31/2-digit DMM range and resolution Meter range Maximum reading Resolution 600 V 600 V 1 V 200 V V 0.1 V 20 V V 0.01 V 2 V V V Construction Electrician Apprenticeship Program: Level 1 45