What Is The Theory Of Ultrasound?

Your company has just decided to replace the calendar based, open panel electrical inspections with safer, closed panel Condition Based Maintenance programs. Historically, your maintenance team has only relied on infrared thermography to inspect electrical components and equipment. As a result of moving to a Condition Based Maintenance program, the decision was made to add ultrasound inspection capabilities to the program. As the director of the maintenance team, you are researching the topic of ultrasound to understand how this technology can benefit the maintenance inspection program. The first step is to understand the theory behind ultrasound. Let’s see what information you have found?

Sound is a mechanical wave that travels in a straight line.  A mechanical wave is a local oscillation in a medium and it transfer its energy through that medium. These waves can move over long distances and the energy propagates in the same direction as the wave travels through the medium.  Common examples of mechanical waves are seismic waves, sound waves and water waves.

Sound is a vibration that travels through the air or another medium and can be heard when that vibration reaches the human or animal’s ear.  Sound is a type of energy made by vibrations.  When an object vibrates, it causes movement in the medium particles.  These particles start to bump into particles that are close, and this continuous bumping makes them vibrate into more medium particles – a sound wave is born.

                                                                             The Critical Angle of Ultrasonic Electrical Inspection of Transmission Lines

There are several components of a sound wave:

  • Amplitude:   the height of a wave’s crest which determines its loudness.
  • Compression: a region in a sound wave where particles have been pushed together making the densest part of the wave.
  • Frequency:  the number of waves produced in a given period of time and usually measured in Hertz (Hz) or cycles per second named after the German physicist who produced the first artificial electromagnetic waves.
  • Loudness: the intensity of the pressure wave resulting in the levels intensity perceived by someone and measured in Decibels (dB).
  • Pitch: describes the perceived highness or lowness of a sound wave and the difference between them Higher frequency sounds have a higher perceived pitch.
  • Rarefaction: the region in the sound wave where the particles have been spread out and are the least dense part of the wave.
  • Wavelength: the measurement of a sound wave from compression to compression or from rarefaction to rarefaction.

An ultrasound wave is a wave with a frequency exceeding the upper limit of human hearing greater than 20 kHz (20,000 Hertz).  Ultrasound waves can be produced by friction inside a motor, gear box, bearing housings, pistons, hydraulic rams and fan housings.  Exterior friction can create ultrasound waves on conveyor rolls, belts, belt splices, finger guards, chains and sprockets.  Internal turbulence occurrences within steam traps, vacuum leaks, valve leaks, pump cavitation and back pressure issues can create ultrasound waves.  On electrical equipment, ultrasound is generated due to Partial Discharge phenomenon including arcing, tracking and corona.  So, on electrical assets, Infrared inspections lets you see heat from loose or overloaded connections that you could not see with the naked eye while ultrasound allows you to hear signs of defects that would be impossible to hear otherwise but that may not generate any appreciable heat.  The two inspection technologies work extremely well together to ensure that the most common problems with electrical equipment can be detected early.

Detecting ultrasound waves using handheld ultrasound testing devices play a critical role when used in a Condition Based Maintenance Inspection program.  Anomalies in equipment or components of equipment may create ultrasound waves, when detected, could alert the maintenance team that a component or machine needs to be further assessed for deterioration.  The goal is to find the anomaly and determine the health of the equipment and fix it if needed before that equipment fails.  Condition Based Maintenance programs are growing in popularity as companies look for ways to proactively detect and fix equipment before costly repairs or full replacements are needed.  


Detecting ultrasound waves is gaining popularity as a tool within Condition Based Maintenance programs.  Companies recognize that preventing equipment failures by performing proactive maintenance inspections saves them time and money in the long run.


How to Create A Safe and Effective Electrical Maintenance Program

To comply with NFPA 70E 2018 Edition regulations and to reduce overall operating costs, your company has just decided to replace the calendar based, open panel electrical inspections with safer, condition based maintenance programs. As the director of the maintenance team, you are researching the topic to help achieve this company goal. You have been exposed to condition based maintenance and NFPA 70E discussions at past educational seminars and you know there is a library of information on the subject. What information have you found?
An electrical condition based maintenance program is a schedule of regularly scheduled maintenance testing events and data collection on the critical assets to evaluate the health of those assets. Performing frequent inspections and trending the data over time may help pinpoint an asset that is beginning to show signs of deterioration. Once an asset has been identified as a potential failure risk, a preventive maintenance event can be scheduled and performed before a failure occurs.

                                                                                  How to Create A Safe and Effective Electrical Maintenance Program

Based on your research, you have outlined an action plan identifying the steps required to achieve this goal:

  1. Conduct a thorough analysis to identify the critical electrical assets where a failure would be catastrophic to the company’s operation
  2. Analyze what technologies are commercially available to inspect and collect surveillance data specific for determining the health of your assets. Common technologies available are infrared thermography, airborne ultrasound, structure borne ultrasound, motor current analysis, partial discharge testing, visual inspections and online temperature monitoring.
  3. Consult with a vendor that specializes in electrical maintenance safety devices (EMSDs) such as IRISS. Vendors are willing to assist you with a walk around of your facility and make recommendations on products and services needed to achieve the goal.
  4. Perform a ROI comparison between the costs of adopting technologies to support a condition based program vs. the cost of planned inspection downtime, unplanned downtime, repair or replacement of failed assets and impact to the company’s end users
  5. Schedule product installations and training for your team

Your research also confirmed that adoption of a condition based maintenance program utilizing EMSDs falls under the Second Control of the NFPA 70E Hierarchy of Risk Control Methods called SUBSTITUTION. Most electrical assets must be inspected under full load.  EMSDs are new “Safety by Design” products that enable safe electrical inspections to be performed under normal load without opening the equipment doors or covers that would expose workers to risk.  Such products are redefining how inspections of electrical assets can be performed. With the NFPA 70E 2018 regulations in place, EMSDs are the logical choice for moving towards a condition based maintenance program. 


Once your company fully adopts the condition based maintenance program, the recognized benefits will be reliable company operations, the highest level of safety in the workplace and controlling operating costs by eliminating the risk of an unplanned power failure.  Compliance with NFPA 70E 2018 regulations is confirmed by using EMSDS to inspect energized electrical assets while eliminating the risk of injury in the workplace.

Using the Sonus PD for Partial Discharge Detection

Partial Discharge (PD) can occur in medium and high voltage switchgear and represents a breakdown between two conducting electrodes. If PD is not detected, the damage to electrical equipment can be catastrophic and cause serious safety events in the workplace. PD can easily be detected with ultrasound and ultrasound inspections should be included in a routine predictive maintenance strategy. This video illustrates how PD can be detected with the use of the Sonus PD handheld ultrasound testing device.


What Specifications Must Be Considered When Choosing Maintenance Inspection Windows?

Your company is evaluating purchasing and installing maintenance inspection windows to comply with NFPA 70E 2018 Edition. As the manager of the small maintenance team, you are working with a consultant to write a specification document for incorporating these windows into both your existing equipment and new equipment specifications as part of your condition based maintenance protocols. Your company realizes the benefits of assessing the health of its critical electrical assets. The consultant provides insight into all of the criteria that must be considered when specifying maintenance inspection windows. Let’s summarize these criteria that need to be considered.

Maintenance Inspection Windows come with different applications. Let’s summarize:

  • Infrared (IR) Windows: IR windows allow infrared cameras to see and measure temperature of objects and connections inside energized electrical equipment with the covers and doors in place (maintaining a safe and guarded condition). IR Windows allow visual, infrared and UV inspections. These windows are increasing in popularity as companies comply with the NFPA 70E 2018 regulations because they provide a safe and cost-effective way to conduct inspections on energized electrical equipment.
  • Multi-technology Maintenance Inspection Windows: These windows go beyond just infrared inspection by adding ultrasound capability. In addition to allowing for thermal imaging, visual and UV inspections, ultrasound and partial discharge inspections can be performed – all from a single window. This approach is cost-effective for the company overall as one type of window can accommodate multiple inspection modalities.

       How to Specify Maintenance Inspection Windows

One vendor, IRISS, can provide custom maintenance inspection windows in any size, shape and even color. All IRISS’s windows also are either UL Recognized or UL Listed. Custom windows and replacement panels enable previously “un-inspectable” assets to be “inspectable”.

Other Criteria for Specifying Windows:

  • Field of View (FOV)- Field of View takes into consideration the viewing angles and distance from the object of the camera and window working simultaneously. Are all the internal objects visible to the camera or is something blocking the camera’s view? The FOV calculation determines the size of inspection window needed for each piece of equipment. When specifying windows on new equipment, make sure to specify the targets you want to be able to see and put the responsibility for selecting appropriately sized windows onto the OEM’s design team.
  • Window Design – compliance with flammability, impact and load requirements and OEM specifications for materials and color. The Window cover protects the window itself and the maintenance team.
  • Viewing Lens Materials – the lens material must withstand mechanical impact and load requirements with the cover open. The lens must also have a fixed and stable transmission rate for the life of the unit and not change over time. Likewise, the lens material should not change with exposure to humidity or other environmental factors such as vibration. Outdoor lens material should withstand exposure to the sun and not change when exposed to water.
  • Certifications Required – Windows, depending on application, may need to comply with multiple certifications such as UL, IP Ratings, CSA for Canada, IEEE Standards for specific equipment types and specialized certification for marine applications such as Lloyd’s of London Approval, ABS or DNV.
  • Performance specifications – specify the environment temperature where the Window will reside. Arc Containment testing may be required depending on the application of the Window or switchgear that contains the Window – on new equipment this becomes the responsibility of the OEM as part of their “type testing” protocols. Specify the lens material transmission rate be stable over the window’s operating life. Insure that the Windows are compatible with all makes and models of IR and ultraviolet cameras. If you choose a Window with IR, Ultrasound and Partial Discharge applications, the Window must be IP2X compliant.
  • The benefits of using Maintenance Inspection Windows outweigh the detailed planning process thereby creating a safe environment for inspecting electrical assets under full load conditions. With Windows, a single inspector can easily and safely perform a route and quickly gather data and perform a trending analysis to determine each asset’s health. The Windows also provide access to inaccessible targets that could never be inspected before which provides inclusive data on more of your critical assets that require inspection. The goal of condition-based maintenance is to trend the health of the assets over time and determine when an asset is showing signs of deterioration, so it can be easily fixed before the asset fails.



Planning for and determining Maintenance Inspection Window’s specifications may seem cumbersome; however, once the specification is determined and the Windows are installed, the maintenance team will be able to perform routes faster and more frequently and most important, safer. This ultimately limits unscheduled downtime for the company and reduces unforeseen operating costs. Maintenance inspection windows will ultimately reduce the Total Cost of Ownership (TCO) of your electrical infrastructure assets.