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Chapter 10 Troubleshooting Principles and Test Instruments

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1 Chapter 10 Troubleshooting Principles and Test Instruments
Troubleshooting • Troubleshooting Methods • Measurement Precautions • Meter Abbreviations, Symbols, and Ratings • Troubleshooting with Test Instruments

2 PLCs provide a centralized location for input device and output component wiring. PLCs are the logical place to start the troubleshooting process. Since PLCs are a major part of an electrical system and control the work performed by the electrical system of a machine or process, PLCs are the logical place to start troubleshooting. See Figure 10-1.

3 Troubleshooting by knowledge and experience is improved when standard maintenance practices are followed. Troubleshooting using knowledge and experience is a troubleshooting method used for finding a malfunction in a machine or process by applying information acquired from past malfunctions. In certain circumstances, troubleshooting using knowledge and experience is only partially effective because the primary malfunction is not corrected. For example, a fuse can blow or a circuit breaker can de-energize a part or all of a PLC-controlled system. Records may indicate that changing the fuse or resetting the breaker allows the system to continue operation, but the reason for the blown fuse or tripped circuit breaker may not be known. See Figure 10-2.

4 Facility (plant) procedures are specific to the system or process used by an individual company.
Troubleshooting using facility procedures is a method of finding malfunctioning equipment using the procedures recommended by company personnel. Most facilities have procedures for troubleshooting machinery or processes. Facility procedures are typically developed by supervisors or operators and are used to ensure safe and efficient troubleshooting of equipment by plant maintenance personnel. System troubleshooting procedures are specific to the machine or process in use by the company. See Figure 10-3.

5 Manufacturer procedures vary from facility procedures in that manufacturer procedures are shorter and generally refer to a specific piece of equipment or part. Troubleshooting using manufacturer procedures is a method of finding malfunctioning equipment by using the procedures recommended by the machine or process manufacturer. Manufacturer procedures differ from facility procedures in that manufacturer procedures are typically specific to an individual piece of equipment or section of equipment. See Figure 10-4.

6 PLC status indicator lights (LEDs) provide a visual display of operating conditions.
PLCs have status indictor lights that provide a visual display of the operating conditions of the PLC. The troubleshooting section of a PLC’s operations manual lists the meanings of the various status light conditions along with possible problems and suggested corrective actions. See Figure 10-5.

7 Flowcharts use symbols and interconnecting lines to provide a troubleshooter with a logical path to problem solving. Some PLC manufacturers include flowcharts with the PLCs to aid in troubleshooting. A flowchart is a diagram that shows a logical sequence of steps for a given set of conditions. Flowcharts help a troubleshooter follow a logical path when trying to solve a problem. Flowcharts use symbols and interconnecting lines to provide analytical direction to the troubleshooting process. See Figure 10-6.

8 PLC Error codes indicate problems such as memory errors, processor and software incompatibility, power failure, an empty address slot in a rack, or a module being inserted while the power is ON. Before calling a manufacturer help line, the technician should have specific manufacturer information ready, including the condition of LED status lights (ON or OFF)—For example, are any of the POWER, RUN, CPU FAULT, and BATTERY LOW lights on? Checking all the fault lights can help locate problems that might typically be overlooked. Also, checking the status of input and output LEDs is helpful. Knowing which output terminals are on (or not on) helps isolate the problem to specific circuits. When PLC processor error codes are indicated, the operations manual should be consulted for the meaning of the code. See Figure 10-7.

9 The user’s manual for a test instrument details specifications and features, proper operating procedures, safety precautions, warnings, and allowed applications. The user’s manual of a test instrument details specifications and features, proper operating procedures, safety precautions, warnings, and allowed applications. See Figure The user’s manual should always be consulted before using any test instrument and the test instrument used only after the information provided is completely understood and can be properly applied. See Appendix.

10 Several precautions must be taken when using test instruments on a PLC-controlled system.
Conditions can change quickly as voltage and current levels change in individual PLC-controlled circuits. See Figure 10-9.

11 Abbreviations are used individually or in combination with prefixes.
Electrical test instruments use abbreviations to indicate what can be measured, such as VAC, Hz, mA, or °F, and to indicate measurement features, such as MIN MAX. An abbreviation is a letter or combination of letters that represent a word. Abbreviations can be used individually, such as V for volts and A for amps, or in combination with prefixes, such as mV for millivolt or kV for kilovolt. See Figure

12 Symbols provide quick recognition and interpretation regardless of the language spoken.
Electrical test instruments use symbols to indicate information. A symbol is a graphic element that represents a quantity, unit, or component. Symbols provide quick recognition and are independent of language because a symbol can be interpreted regardless of the language a person speaks. Symbols can be used individually, such as the sine wave used to represent AC and the two straight lines (one dashed) used to represent DC, or in combination, such as the sine wave with two straight lines to represent AC or DC. See Figure

13 Test instruments are used to measure electrical quantities
Test instruments are used to measure electrical quantities. A technician should be able to recognize both the unit of measurement and the abbreviation used to represent the quantity. For each electrical quantity being measured by a test instrument, it is important to know the unit of measurement and the abbreviations used to represent the electrical quantity on both the test instrument and in the electrical formula. See Figure

14 The IEC 1010 standard classifies the applications in which test instruments and meters can be used into four overvoltage installation categories. Applications require that the CAT rating of test instruments be the same or higher than the application category. A test instrument with a rating of CAT III or higher must be used when taking measurements on electrical systems that include a PLC so that the test instrument can be used anywhere within the indoor electrical circuits. When a test instrument is to be used outdoors for taking measurements on PLC-controlled input devices and output components, a test instrument with a CAT IV rating must be used. See Figure

15 Test lights provide a visual indication when voltage is present in non PLC circuits but do not indicate the amount of voltage. A test light is a test instrument with a bulb, typically neon, that is connected to two test leads to provide a visual indication of when voltage is present. See Figure The test light bulb lights up when voltage is present in the circuit being tested. Test lights can also include several different bulbs used to indicate approximate voltage level (115 VAC or 230 VAC).

16 Voltage testers indicate the approximate voltage amount and type of voltage (AC or DC) in a circuit.
A voltage tester is a test instrument that indicates when voltage is present at a test point. Voltage testers indicate the approximate voltage amount and type of voltage (AC or DC) in a circuit by the movement of a pointer (and by vibration on some models). When a voltage tester includes a solenoid, the solenoid vibrates when the tester is connected to AC voltage. Some voltage testers include a colored plunger or other indicator, such as a light that indicates the polarity of the test leads as positive or negative when measuring a DC circuit. See Figure

17 A specific procedure is followed when using a voltage tester to take measurements.
Before taking any measurements using a voltage tester, verify that the voltage tester has a voltage rating higher than the highest potential voltage in the circuit being tested. See Figure Refer to the operating manual of the test instrument for all measuring precautions, limitations, and procedures.

18 DMMs are portable test instruments that measure two or more electrical properties and display the measured properties as numerical values. An analog multimeter is a portable test instrument that uses electromechanical components to display measured values. A digital multimeter (DMM) is a portable test instrument that uses electrical components to display measured values. DMMs are the most common multimeter used when troubleshooting PLC systems. See Figure

19 When the voltage is DC at the test point, the two measured values will be the same, but one will have a negative (–) reading and the other a positive (+) reading. When the type of voltage (AC or DC) is unknown, a measurement should be taken with the meter set to measure DC voltage. The meter test leads should be reversed and the measurements taken again. When the voltage is DC at the test point, the two measured values will be the same, but one will have a negative (–) reading and the other a positive (+) reading. See Figure

20 When the two measured voltage values do not indicate a DC voltage, the meter should be set to measure AC and the measurements should be retaken. When the two measured voltage values do not indicate a DC voltage, the meter should be set to measure AC and the measurements should be retaken. When the voltage at the test point is AC, both readings will be the same (for example, VAC and VAC and neither one will be negative). See Figure

21 A true-rms DMM must be used when taking AC voltage measurements in a PLC circuit or system.
When checking or troubleshooting a PLC system, loads, circuits, or individual components, voltage measurements must be taken. See Figure A DMM is typically used to take voltage measurements. Many PLCs include AC voltages for powering internal circuitry and for operating AC output components such as motor starters.

22 PLC input devices are typically powered by DC voltages such as 24 VDC.
Many PLC systems include DC voltages somewhere. Typically, PLC input devices are DC-powered (24 VDC) and PLC output components that require a DC voltage (DC motors or solenoids) can be supplied with DC voltage. See Figure As when measuring AC voltages, caution must be exercised when taking any circuit measurement that has low DC voltage (12 VDC, 24 VDC). Measurements of DC voltages exceeding 50 V and DC measurements near any battery require extra caution to prevent an electrical shock or spark, which can cause a fire or an explosion.

23 Continuity testers are simple test instruments that test de-energized circuits or components for a complete path for current. A continuity tester is a test instrument that tests for a complete path for current to flow. For example, a closed switch that is operating properly has continuity; however, an open switch does not have continuity. See Figure Continuity tests can be performed on any nonpowered mechanical switch, such as the dry contact terminals or alarm contacts on a PLC.

24 A continuity test can be used to check the operation of a photoelectric switch.
A continuity test is used to check input switches to ensure their proper operation before they are connected to the input section or module of a PLC. For example, a continuity test can be used to check the operation of a photoelectric switch. See Figure

25 Ohmmeters measure the amount of resistance (in ohms) in de-energized circuits, devices, or components. An ohmmeter is a test instrument that measures the resistance of a device or circuit. Ohmmeter resistance measurements are taken to determine the resistance of de-energized devices, components, or circuits. See Figure The significance of a resistance measurement depends on the component being tested.

26 Technicians must always verify that circuits, devices, or components do not have voltage before taking any resistance measurements. Before taking any resistance measurements using an ohmmeter, verify that the ohmmeter has an output voltage rating higher than the highest potential voltage output required for the circuit being tested. Refer to the operating manual of the test instrument for all measuring precautions, limitations, and procedures. See Figure

27 Current measurements are typically measured using clamp-on ammeters or multimeters with clamp-on current probe accessories. In low-current applications, in-line ammeters can be used. Current is typically measured using clamp-on ammeters or multimeters with clamp-on current probe accessories. Small amounts of current can be measured using a multimeter connected as an in-line ammeter. See Figure

28 Current measurements are taken using standard procedures.
Clamp-on ammeter current measurements can be taken on loads that are connected to PLC output sections or modules to ensure the loads do not exceed the current rating of the PLC output section or module. Before taking any current measurements using a clamp-on ammeter, verify that the clamp-on ammeter has a current rating higher than the highest potential current in the circuit being tested. Refer to the operating manual of the test instrument for all measuring precautions, limitations, and procedures. See Figure

29 Current measurements for both AC and DC can be taken with in-line ammeters.
In-line current measurements can be taken on DC or AC circuits. The only difference is the setting of the meter to DCA or ACA. Before taking any current measurements using an in-line ammeter, verify that the in-line ammeter has a current rating higher than the highest potential current in the circuit being tested. See Figure Refer to the operating manual of the test instrument for all measuring precautions, limitations, and procedures.

30 Noncontact temperature instruments measure heat by measuring the infrared energy emitted by a material. A thermal imaging camera is a meter that measures heat energy by measuring the infrared energy emitted by a material and displays the temperature as a color-coded thermal picture. See Figure Because heat can be a cause of PLC problems and failure, temperature measurements must be taken to ensure that the temperature in a PLC enclosure is within set limits.

31 Infrared temperature measurements prevent problems by locating unwanted heat in electrical equipment enclosures before the heat can cause PLC or equipment failure. Before taking any temperature measurements using an infrared temperature meter, verify that the meter has a temperature rating higher than the highest potential temperature in the enclosure being tested. See Figure Refer to the operating manual of the test instrument for all measuring precautions, limitations, and procedures.


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