Chapter 10 Troubleshooting Principles and Test Instruments

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Chapter 10Troubleshooting Principles and Test
Instruments

• Troubleshooting Troubleshooting Methods
  Measurement Precautions Meter Abbreviations,
  Symbols, and Ratings Troubleshooting with Test
  Instruments

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• PLCs provide a centralized location for input
  device and output component wiring. PLCs are the
  logical place to start the troubleshooting
  process.

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• Troubleshooting by knowledge and experience is
  improved when standard maintenance practices are
  followed.

4

• Facility (plant) procedures are specific to the
  system or process used by an individual company.

5

• Manufacturer procedures vary from facility
  procedures in that manufacturer procedures are
  shorter and generally refer to a specific piece
  of equipment or part.

6

• PLC status indicator lights (LEDs) provide a
  visual display of operating conditions.

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• Flowcharts use symbols and interconnecting lines
  to provide a troubleshooter with a logical path
  to problem solving.

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• 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.

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• The users manual for a test instrument details
  specifications and features, proper operating
  procedures, safety precautions, warnings, and
  allowed applications.

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• Several precautions must be taken when using test
  instruments on a PLC-controlled system.

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• Abbreviations are used individually or in
  combination with prefixes.

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• Symbols provide quick recognition and
  interpretation regardless of the language spoken.

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• 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.

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• The IEC 1010 standard classifies the
  applications in which test instruments and meters
  can be used into four overvoltage installation
  categories.

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• Test lights provide a visual indication when
  voltage is present in non PLC circuits but do not
  indicate the amount of voltage.

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• Voltage testers indicate the approximate voltage
  amount and type of voltage (AC or DC) in a
  circuit.

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• A specific procedure is followed when using a
  voltage tester to take measurements.

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• DMMs are portable test instruments that measure
  two or more electrical properties and display the
  measured properties as numerical values.

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• 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.

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• 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.

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• A true-rms DMM must be used when taking AC
  voltage measurements in a PLC circuit or system.

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• PLC input devices are typically powered by DC
  voltages such as 24 VDC.

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• Continuity testers are simple test instruments
  that test de-energized circuits or components for
  a complete path for current.

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• A continuity test can be used to check the
  operation of a photoelectric switch.

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• Ohmmeters measure the amount of resistance (in
  ohms) in de-energized circuits, devices, or
  components.

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• Technicians must always verify that circuits,
  devices, or components do not have voltage before
  taking any resistance measurements.

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• 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.

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• Current measurements are taken using standard
  procedures.

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• Current measurements for both AC and DC can be
  taken with in-line ammeters.

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• Noncontact temperature instruments measure heat
  by measuring the infrared energy emitted by a
  material.

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• Infrared temperature measurements prevent
  problems by locating unwanted heat in electrical
  equipment enclosures before the heat can cause
  PLC or equipment failure.