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Ultrasound Inspection vs. Infrared (Thermographic) Scanning of Electrical Equipment

Can ultrasound inspection be used in lieu of infrared scanning to detect faults in electrical equipment? The simple answer is no: An ultrasound inspection is not a substitute for infrared scanning. Rather, it is a complementary technology.


At voltages of 1000 or greater, an ultrasound inspection may be used to detect corona, partial discharge, tracking, arcing, and mechanical vibrations on:

  • Switchgears
  • Load interrupter switches 
  • Breakers 
  • Transformers 
  • Motor control centers 
  • Terminal transition cabinets
  • Overhead transmission lines
  • Switchyards


There are many benefits to utilizing ultrasound technology: 

  1. Ultrasound instruments are versatile and easy to use. The equipment is relatively inexpensive and plant personnel could perform the inspection in between the interval of infrared scans to see if any emissions are heard.  
  2. The ultrasound inspections can be performed without opening the cabinets or enclosures.  
  3. They are highly effective in detecting certain failures like corona, tracking, and arcing.
  4. It is an excellent method to detect faults within an enclosure prior to conducting infrared scanning to confirm the severity of the defect detected by the sound intensity.


There are two disadvantages to utilizing ultrasound inspection: 

  1. Ultrasound inspection can only be used at 1000 volts and greater; air becomes a conductor only at higher voltage ranges, where the air surrounding a connection becomes ionized. 
    • Ultrasound scanning is not suitable for low voltages. For corona and partial discharge to occur, the voltages have to be greater than 1000V. The greater the voltage, the better the results for presence of corona and partial discharge.
  2. Although ultrasound inspection is quite effective in detecting certain failures like corona, tracking, and arcing, it is unable to detect the condition of a conductor – which infrared scanning can do. 



Infrared scanning detects heat that is generated by current flow or amperage-related problems that are often caused by loose connections. So, infrared detects the condition of the conductor.


To perform a proper infrared (thermographic) scan, the electrical equipment should be energized and (preferably) at full operating load. Due to high voltage levels, there are significant safety concerns in opening the locked doors of the switchgear/enclosures to perform the scanning unless the switchgear has infrared windows installed on the covers/doors. 


To minimize the safety risk, an ultrasound inspection may be performed on both enclosed and open access electrical equipment. The technology utilizes sound waves that would be blocked by a solid surface. With an ultrasound inspection, the instrument is simply pointed at the equipment to be inspected. 


To utilize ultrasound technology during an inspection, take the following steps: 

  1. Position the ultrasound instrument at the access, through a capped hole or the removal of a few bolts, to achieve the best results. 
  2. Then, listen. If tracking or corona is present, one will hear a buzzing noise similar to static on a radio. 
    1. Ionization disturbs the air molecules at high frequency, thus producing the buzzing sound. The ultrasound instrument detects the high frequency noise produced by this effect and translates it into the audible ranges; the intensity is analyzed by utilizing headphones as well as observing the strength of the signal on a meter.  
    2. Technicians performing the ultrasound inspection are trained to differentiate the noise from the normal 60-cycle hum or other constant mechanical noises. 



Infrared scanning has been used for years to detect defects in electrical equipment that, if not detected quickly, could result in catastrophic damage. The infrared method may be utilized in all applications and can provide detail to the severity of the problem. 


In recent years, ultrasound instruments have been added to inspections. The ultrasound method is advantageous from a safety perspective where the voltages are high – 1000 volts or greater. 


Ultrasound inspection should not be considered an alternative to infrared scanning; instead, using both technologies in conjunction generates better results and better predicts fault within the equipment.

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