Teaching an old dog new (electrical) tricks

There’s a better way to approach electrical problems than using a digital multimeter.

05/15/2008


There’s a better way to approach electrical problems than using a digital multimeter. Use a non-contact tool, first. It’s safer and, in the end, faster. A non-contact tool can be a voltage detector, an infrared thermometer or a thermal imager.

 

If you’re working on a “dead” circuit, it’s always wise to quickly test it first with a non-contact voltage tester and verify that power is really disconnected. Sounds obvious, but you don’t have to look far to uncover too many accidents that could have been avoided.

 

If you’re troubleshooting a live electrical problem, chances are high that the cause of your problem will have a tell-tale heat signature associated with it. Take a minute to scan the electrical equipment with an infrared thermometer or a thermal imager. Even if you see nothing abnormal, you’ve still learned something: what’s not wrong. If you do see an abnormally hot or cool spot, you’ve determined where to start your investigation. And in the process of thermal inspection, you’ll often notice other developing issues with the panel/motor/control that you can address preemptively.

 

In any live electrical measurement situation, the technician must still wear appropriate PPE. The benefit is that since no direct contact is necessary with infrared and thermal measurement, the risk of accidental arc flash is lower.

 

When choosing between an infrared thermometer and a thermal imager, the choice is situational. Most technicians now carry a small infrared thermometer in their pocket for on-the-go checks. However, the infrared thermometer gives you only a “spot” temperature reading. If you know what you’re looking for, infrared may be enough. But if you’re hunting an elusive problem, a full thermal map of the entire unit is more effective.

 

More about thermal imaging

Thermal imagers detect and measure surface temperature differences and assign colors based on temperature. Rather than seeing light, thermal imagers create pictures of heat. They measure infrared energy and convert the data to images corresponding to the temperature.

 

The biggest trick with using a thermal imager effectively is understanding emissivity. Emissivity describes how well an object’s surface emits IR energy, or heat. This affects how well a thermal imager can accurately measure the object’s surface temperature.

 

Most painted objects, ceramic, rubber and most electrical tape and conductor insulation, have high emissivities. This makes them ideal for thermal imaging. Aluminum bus, however, is very reflective, and so are copper and some kinds of stainless steel.

 

The good news is that most thermal imaging performed for electrical inspection purposes is a comparative, or qualitative, process. You don’t usually need a specific temperature measurement. Instead, look for a spot that is hotter than similar equipment under the same load conditions, spots that you do not expect.

 

Commonly inspected components with thermal imagers include:

 

  • Three-phase power distribution and switchgear
  • Fuse boxes
  • Cables and connections
  • Relays and switches
  • Insulators
  • Capacitors
  • Circuit breakers
  • Electric motors
  • Motor controllers
  • Transformers
  • Battery banks
  • Substations.

Typical reasons for temperature hotspots or deviations include:

 

  • Unbalanced loads
  • Harmonics (third harmonic current in neutral)
  • Overloaded systems/excessive current
  • Loose or corroded connections* increasing resistance in the circuit (typically one side of components heats up)
  • Insulation failure
  • Component failure
  • Wiring mistakes
  • Underspecified components (like fuses, would heat up on both sides of the fuse).

* Note: Not all electrical problems discovered by thermal imaging are high-resistance connections. Follow and analyze the thermal pattern of the conductors and other components to assess other possibilities.

 

Scope of possibilities

 

The possible applications for thermal imaging are broad but not endless. Constraints and cautions include:

 

  • Equipment must be operating (ideally at the highest possible load, minimum 40%)
  • Direct view: Open enclosure doors, when possible. You cannot see through enclosure doors
  • Understand present and future loading conditions
  • Only qualified persons should access live electrical equipment
  • Avoid working in direct sunshine, as the sun’s heat can mask some problems
  • Be aware of air convection and cooling, and account for windy conditions when outside
  • You can detect only surface temperatures
  • Be aware of low emissivity materials
  • Look for both hot and cold abnormalities, since sometimes “cold” also indicates a problem
  • Check all connections and points of electrical contact
  • Compare all phases to each other, and compare items under similar loads. They should have similar thermal signatures when operating properly.

What happens next?

 

Thermal imagers work best in the hands of skilled technicians who understand the equipment in question. A hot phase could indicate an unbalance, overload, bad connections and so on. If the problem isn’t immediately identifiable as a bad connection, use a properly-rated digital multimeter or power quality analyzer to trace the problem to the root.

 

As with any measurement technology, there is no substitute for proper training and experience. Although thermal imaging is a powerful tool, and it has become even easier-to-use and more affordable in recent years, it does require some basic knowledge. Invest in proper thermal training to build your inspection skills, increase electrical measurement safety and improve problem-solving efficiency well into the future.

 

Emissivity

 

Different materials emit infrared energy in different ways. Every object and material has a specific emissivity rated on a scale of 0 to 1.0. A thermal imager can accurately calculate the surface temperature of an object only if the emissivity of the material is relatively high (close to 1.0), and/or the emissivity level on the imager is set close to the emissivity of the object. 

 

 

 

Author Information

Michael Stuart manages thermal imagers for Fluke Corp. He has trained with the Snell Group and has spent extensive field time documenting electrical and industrial thermal applications. Previously, Stuart mastered the somewhat arcane knowledge field of insulation resistance test and managed electrical testers of all types for Fluke Corp. He can be reached at michael.stuart@fluke.com.

 



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