Electrical Instrumentation Questions and Answers

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Herein, we have covered several Electrical instruments like Digital multimeter, Analog Multimeter, Clamp Meter, Megohmmeter, Oscilloscope, and Phase Sequence Indicator in detail.


  1. Understand the stated rules for proper and safe use of hand and power tools.
  • Tools should be kept in good working condition. Periodic inspection of tools helps to keep them in good condition. A tool should always be inspected for worn, chipped, or damaged surfaces prior to its use. A tool that is in poor or faulty condition must not be used.
  • Damaged tools are dangerous and less productive than those in good working condition and should not be used. A tool should be repaired or replaced immediately when inspection shows a dangerous condition.
  • Cutting tools should be sharp and clean. Dull tools are dangerous. The extra force exerted while using dull tools often results in losing control of the tool. Dirt, oil, or debris on a tool may cause slippage while it is used and cause injury.
  • Power tools should only be used after gaining knowledge of their principles of operation, methods of use, and safety precautions.
  • All power tools should be grounded unless they are an approved double-insulated design. Power tools must have a grounded three-wire cord. A three-prong plug connects into a grounded electrical outlet (receptacle). Approved receptacles may be locking or non-locking.
  1. Understand the stated rules for proper and safe usage of test instruments.
  • Never assume a test instrument is operating correctly. Check the test instrument that will be measuring the voltage on a known (energized) voltage source before taking a measurement on an unknown voltage source. After taking a measurement on an unknown voltage source, retest the test instrument on a known source to verify the meter still operates properly.
  • Always assume that equipment and circuits are energized until positively identified as de-energized by taking proper measurements.
  • Never work alone when working on or near exposed energized circuits that may cause an electrical shock.
  • Always wear personal protective equipment (PPE) appropriate for the test area.
  • Never assume that a circuit is de-energized or equipment is fully discharged. Capacitors can hold a charge for a long time—several minutes or more. Always check for the presence of voltage before taking any other measurements.
  • Check test leads for frayed or broken insulation. Electrical shock can occur from accidental contact with live components.
  • Use meters that conform to the IEC 1010 category in which they will be used.
  • Use one hand when working on a live circuit to reduce the chance of an electrical shock passing through the heart and lungs.
  • The 5S organization system is a workplace organizational system that stands for sort, straighten, sweep or shine, standardize, and sustain. The system is used to reduce waste, optimize productivity, and promote safety through maintaining an orderly workplace. The main goals of the 5S organization system are improved workplace morale, safety, and efficiency.

General Use Test Instruments

  1. Explain how to troubleshoot fuses using a continuity tester.
  • A continuity tester is a test instrument that tests for a complete path for current to flow. A continuity tester is an economical tester that is used to test switches, fuses, and grounds and is also used for identifying individual conductors in a multi-wire cable.
  • Fuses can be checked using a continuity tester placed across a fuse that has been removed from a circuit.
  • When testing a good fuse, the continuity tester beeps because the circuit inside the fuse is closed, has a low resistance, and allows current flow through the fuse and continuity test circuit.
  • When testing a bad fuse, the continuity tester does not beep because the circuit inside the fuse is open, has an infinite resistance (OL), and does not allow current to flow through the fuse and continuity test circuit.
  1. Explain how to properly use a receptacle tester to test a receptacle.
  • A receptacle tester is a device that is plugged into a standard receptacle to determine if the receptacle is properly wired and energized.
  • When testing a receptacle, the indicator light code indicates whether the receptacle is wired correctly.
  • The situation of having the hot and neutral wires reversed is a safety hazard and must be corrected. The hot slot on a receptacle is the short slot, while the neutral slot is the wide slot. The round hole in the ground. Improper grounds are also a safety hazard and must be corrected.
  1. Explain how to properly use a voltage tester to test for voltage in a circuit.
  • A voltage tester is a device that indicates an approximate voltage level and type (AC or DC) by the movement and vibration of a pointer on a scale.
  • Before using a voltage tester or any voltage measuring instrument, it is important to always check the voltage tester on a known energized circuit that is within the voltage rating of the voltage tester to verify proper operation.
  • If a voltage tester does not indicate a voltage, a voltage that can cause an electrical shock may still be present.
  • Voltage testers are considered the best test instrument for testing GFCI receptacles because test lights, voltage indicators, and DMMs do not draw enough current to trip a GFCI receptacle.
  1. Explain how a measurement is displayed on an analog multimeter using linear and nonlinear scales.
  • An analog multimeter is a meter that can measure two or more electrical properties and displays the measured properties along calibrated scales using a pointer.
  • Most analog multimeters have several calibrated scales, which correspond to the different selector switch settings (AC, DC, and R) and placement of the test leads (mA jack and 10 A jack).
  • Analog scales may be linear or nonlinear. A linear scale is a scale that is divided into equally spaced segments. A nonlinear scale is a scale that is divided into unequally spaced segments.
  • Analog scales are divided into segments using primary divisions, secondary divisions, and subdivisions.
  • A primary division is a division with a listed value. A secondary division is a division that divides primary divisions into halves, thirds, fourths, fifths, etc. A subdivision is a division that divides secondary divisions in halves, thirds, fourths, fifths, etc.
  • When reading an analog scale, the primary, secondary, and subdivision readings are added.
  1. Explain how measurements are displayed on a digital multimeter (DMM).
  • A digital multimeter (DMM) is a meter that can measure two or more electrical properties and displays the measured properties as numerical values.
  • Digital multimeters display measurements as numerical numbers, not a scale position. They are not likely to be misread unless the prefixes and symbols accompanying numerical values are misapplied.
  • Digital displays display values using either a light-emitting diode (LED) display or a liquid crystal display (LCD).
  • The exact value on a digital display is determined from the numbers displayed and the position of the decimal point.
  • Accurate readings are obtained by using the range that gives the best resolution without overloading the meter.
  1. Describe bar graph and wraparound bar graph as displayed on a digital multimeter.
  • Most digital displays include a bar graph to show changes and trends in a circuit. A bar graph is a graph composed of segments that function as an analog pointer. The displayed bar graph segments increase as the measured value increases and decrease as the measured value decreases.
  • A wraparound bar graph is a bar graph that displays a fraction of the full range on the graph at one time. The pointer wraps around and starts over when the limit of the bar graph is reached.
  1. Describe what each CAT rating means on a test instrument.
  • A higher CAT rating indicates an electrical environment with higher power available, larger short-circuit current available, and higher energy transients.
  • A DMM designed to the CAT III standard is resistant to higher energy transients than a DMM designed to the CAT II standard.
  • Power distribution systems are divided into categories because a dangerous high-energy transient voltage, such as a lightning strike, is attenuated or dampened as it travels through the impedance (AC resistance) of the system and the system grounds.
  • A CAT m-1000 V (steady-state) DMM provides better protection than a CAT nI-600 V (steady-state) DMM.
  • Between categories, a DMM with a higher voltage rating might not have higher transient voltage protection. A CAT III-600 V DMM has better transient protection compared to a CAT II-1000 V DMM.
  1. Explain how to set a multimeter to test diodes.
  • The polarity of a diode can be determined with a DMM, and the diode can also be checked for opens and shorts.
  • The forward and reverse bias of an unknown diode can be determined if the diode is placed between known polarities one way, and then placed in the opposite direction. The diode indicates a low resistance in forward bias and a high resistance in reverse bias.
  • Since the polarity of the source is known, the end connected to the negative lead during forward bias must be the cathode and the end connected to the positive lead must be the anode.

Specialty Test Instruments

  1. Explain how an oscilloscope is used.
  • An oscilloscope is a test instrument that provides a visual display of voltages. An oscilloscope displays the waveform for the voltage in a circuit and allows the voltage level, frequency, and phase to be measured.
  • The two types of oscilloscopes are bench and handheld. Both types of oscilloscopes include the same basic features.
  • Oscilloscopes are used to troubleshoot digital circuits, communication circuits, factory process instrumentation, machine control circuitry, and computers. Besides showing a voltage waveform in a circuit, oscilloscopes also allow the voltage level, frequency, and phase to be measured.
  1. Explain how a digital logic probe is used.
  • A digital logic probe is a special DC voltmeter that detects the presence or absence of a signal.
  • Displays on a digital logic probe include logic high, logic low, pulse light, memory, and TTL/CMOS.
  • The high LED lights when the logic probe detects a high logic level (1). The low LED lights when the logic probe detects a low logic level (0). The pulse LED flashes relatively slowly when the probe detects logic activity present in a circuit. Logic activity indicates that the circuit is changing between logic levels.
  1. Explain how a megohmmeter is used.
  • A megohmmeter is a device that detects insulation deterioration by measuring high resistance values under high test voltage conditions. Megohmmeter test voltages range from 50 V to 5000 V.
  • A megohmmeter detects insulation failure or potential failure of insulation caused by excessive moisture, dirt, heat, cold, corrosive vapors or solids, vibration, and aging.
  1. Define ground resistance tester.
  • A ground resistance tester is a device used to measure ground connection resistance of electrical installations such as power plants, industrial plants, high-tension towers, and lightning arrestors.
  1. Define infrared temperature meter and noncontact temperature probe.
  • An infrared temperature meter is a non-contact temperature probe that senses the infrared energy emitted by a material.
  • A noncontact temperature probe is a device used for taking temperature measurements on energized circuits or on moving parts.
  1. Explain how a phase sequence indicator is used.
  • A phase sequence indicator is a device used to determine phase sequence and open phases. Phase sequence indicators help protect motors, generators, and other equipment from damage due to an incorrect motor rotation. They also ensure that the three-phase power lines are present and in the correct order.
  1. Explain how an optical time domain reflectometer is used.
  • An optical time domain reflectometer (OTDR) is a test instrument that is used to measure fiber optic cable attenuation.
  • An OTDR uses a laser light source that sends out short pulses into a fiber and analyzes the light scattered back. The light source decays with fiber attenuation. Attenuation is produced by reflections from splices, connectors, and any areas in the cable that cause problems.
  • Based on the amount of signal reflected back, the type and location of a fault are displayed.
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