Principles of Electrical Troubleshooting

To properly troubleshoot an electric circuit, you must know how each electrical component in the unit should function and be able to evaluate the performance of each component. Electrical records, prints, schematics, and manufacturers’ literature—combined with your knowledge and experience—will help you determine how each component is expected to operate. After determining the expected operating characteristics, use electric meters to obtain the current operating characteristics of the circuit.

Some situations also require testing for power, power factor, frequency, phase rotation, inductance, capacitance, and impedance. Before beginning any test, answer the following five questions:

  • Is the circuit on or off?
  • What is the condition of the fuses or breakers?
  • What are the results of a visual inspection?
  • Are there bad terminations?
  • Is the meter working?

Meters and test equipment, as well as print tools, such as operating logs and schematics, will all help you diagnose and solve electrical problems. The fundamental diagnostic tools and test equipment are the voltmeter, ammeter, and ohmmeter. The basic functions of these meters are combined in a multimeter.


Use a voltmeter to test the voltage potential at the motor. With the generator running, the switch closed, and the voltmeter probes attached to the current conductor and neutral conductor connections of the motor, the voltmeter will indicate the voltage potential at the motor. The voltmeter test shows only the presence of voltage. It will not indicate that the motor is turning or that current is flowing.


A clamp-on ammeter is used to test the amperage in a motor circuit. With the generator running, the switch closed, and the ammeter jaws clamped around either lead, the ammeter will indicate the amperage draw, or current, being used by the circuit. To obtain an accurate reading when using a clamp-on ammeter, clamp the meter jaws around just one wire, or lead, at a time, and make sure the jaws are completely closed.


An ohmmeter tests the resistance of a motor. Before beginning an ohmmeter test, open the switch that controls the motor, attach the appropriate lockout/tagout device, and isolate the motor from the circuit. An ohmmeter test can identify a short or an open circuit.

Quick-Test Instruments

Several specialized, practical, and inexpensive electrical tools are available for use in troubleshooting electric circuits. Before using any electrical test tools, be certain that they comply with current OSHA regulations.

Voltage indicators are pen-like pocket tools used to check for the presence of AC voltage over 50 volts. Voltage indicators are useful when checking for breaks in AC wiring. When the plastic tip of the indicator is applied to any connection point or next to a wire with AC voltage, the tip will glow or the tool will emit a chirping sound. Voltage indicators do not measure the AC voltage directly; they indicate a voltage potential.

Circuit analyzers plug into standard receptacles and can function as a basic voltage tester, indicating the available voltage. These plug-in devices are commonly used to test for a lack of ground, a reversed polarity or neutral, and a voltage drop. They are also used to check the GFCI. Sophisticated versions of this device can also check for voltage surges, false grounds, current capacity, impedance, and safety hazards.

Infrared scanners are used regularly to check for potential electrical problems. As amperage passes through an electrical device, heat is generated in proportion to the resistance created. An infrared scanner highlights temperature differences between elements and can be programmed to show the actual temperatures. If any circuit or element is hotter than the components immediately surrounding it, that device or connection will appear as a hot spot on the scanner. Any hot spots are candidates for additional analysis or troubleshooting. Hot-spot problems can usually be solved by adjusting the torque on the suspect electrical connections to the proper level or by cleaning and tightening all connectors. These procedures may also correct phase imbalances.

Circuit Tracers

A circuit tracer is a device that, when attached to any accessible point in the circuit, can trace circuit wiring through the building—all the way to the service entrance, if necessary. Circuit tracers have two parts:

  • Signal generator: Attaches to the circuit wiring and creates a radio-wave-type signal throughout the circuit.
  • Signal receiver: Locates the circuit wiring by receiving the radio signal through the wiring.

Electrical Records, Prints, Schematics, and Manufacturers’ Literature

As useful as some of these tools are, documentation is often equally or more important. Inspection records and operating logs include information such as amperage draws and operating temperatures and pressures of components. A change in any of these parameters could indicate voltage potential problems. When there is an obvious problem, inspection records and operating logs can help you compare the present operation of the equipment to normal operating conditions. This comparison can further help you pinpoint specific problem areas.

For example, an increase in the operating amperage draw of a motor driving a pump indicates a potential problem. Noting a change from the normal amperage draw, you can conduct additional tests, such as checking the operating temperature of the bearings. Furthermore, if the temperature of the bearings is above the operating temperature, some type of repair could soon become necessary and should be planned for. Without referring to the operating logs, you may not notice such issues. This type of oversight could result in an equipment breakdown.

Prints, drawings, and schematics are useful in determining the location of equipment, identifying its components, and specifying the proper sequence of operation. You will use three basic types of prints and drawings in electrical troubleshooting and repair.

  • “As-built” blueprints and electrical drawings indicate the location and size of power supply control devices, such as switches and circuit breakers, and the location of wiring and cables. Most items are represented by standard symbols. Nonstandard or unusual components are generally identified on the drawing or in a separate electrical drawing key.
  • Installation drawings are pictorial representations of electrical devices useful for locating connection points, wiring, and specific components. Standard electrical symbols are not required, but some may be used for convenience.
  • Schematics, or ladder diagrams, are detailed drawings that show how a device functions electrically. These rely heavily on standard symbols and have little written explanation.

Manufacturers’ literature may include installation and schematic drawings, as well as instructions and tables describing specific performance or operating parameters. All of this information should be readily available to you.

This article is excerpted from the BOMI International course Electrical Systems and Illumination, part of the SMT® and SMA® designation programs. More information regarding this course and BOMI International’s education programs is available by calling 1-800-235-2664. Visit BOMI International’s website,