Electrical discharge is more common than we’d like to believe; even in new installations. Detecting these defects poses a serious challenge, particularly at an early stage when there is still time to initiate corrective actions. Vigilant technologies help reduce the risk of arc flash explosion while enhancing the overall reliability of electrical assets. 

While there is no one perfect inspection technique or technology which can, on its own, detect and localize every defect, ultrasound performs best. Especially when the goal is to find the defect safely and early. Ultrasound’s suitability is characterized by its versatility, ease of use, practicality, detection accuracy, measurement repeatability, and diagnostic ability. 


Any discussion about risk must include health and safety. But it should not ignore asset reliability. One certainty is that safety and reliability risks are both linked to unscheduled downtime, costly legal exercises, and ultimately, lost profit. A winning solution identifies safety and reliability as one and the same and marginalizes their impact to the other three. 

Ultrasound presents a win/win solution. It reduces the risk of arc flash exposure while ensuring equipment reliability. It does this by detecting defects which, when left to deteriorate, could lead to an arc flash event. This doesn’t suggest that ultrasound removes the need for personal protective equipment and other safety measures. But I’m yet to meet an arc flash suit which can detect an arc flash at its inception. If an arc flash suit is the last line of defense, then ultrasound is the first. 

Origins of Electrical Defects

Figures produces by the IEEE point to insulation failure in as much as 90% of switchgear defects. Age has very little influence over reliability. In fact, 80% of failures are associated with poor workmanship or quality issues. IEEE concluded that the most likely cause of electrical asset failures include water ingress, cracked insulators, and incompetent workmanship during installation or maintenance. 

Types of Partial Discharge

To understand why ultrasound is so effective it’s important to look more closely at electrical discharges. What are they, what causes them, and how they behave? The terms “partial discharge” and “arcing” are commonly used to describe electrical defects, but they can be broken down into further subsets. 

Partial Discharge (PD) is a localized electrical discharge in an insulation system that does not completely bridge the electrodes. PD is an atomic reaction which ionizes the air molecules at locations of high stress. Ionization splits the oxygen molecule to form ozone and nitrous oxide which, in their normal states, are generally harmless. But when mixed with water vapor in the air, they become corrosive. The smell of ozone in a substation is usually the first indicator of the presence of PD. 

Corona is a type of PD where the discharge is directly airborne with no insulation between the conductors. Corona PD produces distinct ultrasound signals that are strongest at their source. Corona levels are influenced by humidity levels. Moisture in the air captures some of the free electrons which would otherwise initiate the discharge. The directional nature of ultrasound and the ability to super focus sensors makes these defects detectable from long distances, with pinpoint accuracy. 

Tracking is another form of PD. It is characterized by the flow of electricity over an interface of different insulation materials. Most commonly it involves solid type insulation and is often sourced on the surface of an insulator contaminated with hydroscopic dust. An increase in moisture – or even just humidity – reduces the insulation resistance on the surface and enhances the tracking type discharge. Humidity levels can serve as a way to differentiate between Corona and Tracking activity. 

Arcing, and in some instances Partial Arcing, is the flow of electricity through the air from one conductor to another object which is capable of conducting electricity. Arcing is characterized by a rapid expansion of gas that can cause fires and explosions. It can be extremely violent, generate extreme temperatures, and consequently the rapid destruction of all equipment connected. Obviously, it is a huge risk to personnel safety. 

Why Ultrasound is Effective

Ultrasound refers to any sound signal above the range of human hearing, but technically, that value is defined as 20kHz. Sound and ultrasound need a medium to move from source to detection point. That medium might be a solid, liquid, or a gas so that applies to cable termination boxes, oil-filled transformers, or any system with suitable air gaps. 

To better understand where ultrasound can help, we resort to some simple word play. The capabilities of ultrasound can be abbreviated with the word FIT. That is to say that ultrasound works extremely well finding any defect that produces Friction, Impacting, and Turbulence. 

Partial discharge generates ozone which produces an unmistakable smell. While our nose may tell us PD is present, the human olfactory sense is not reliable, repeatable, or directional. We need a detection technique that detects, identifies, and pinpoints. PD also emits a steady frequency produced by the turbulent flow of molecules at the source of ionization. These frequencies have peaks at the operating range of SDT ultrasound detectors (38-40kHz). 

40kHz sound pressure waves are low energy signals that do not spread widely through their medium and remain closely isolated at their source. Signals from PD defects are quickly and safely detected and pinpointed with ultrasound sensors. Near distance signals are revealed with close-up focused sensors – ideal around switchgear closets — while longer distances associated with transmission and distribution lines are amplified with parabolic sensors. 

With the right ultrasound detector, inspectors are able to very quickly locate a defect in a high noise environment. They can hear the difference between corona discharge, tracking and arcing, which means knowing exactly which defect type he is dealing with and how to plan a corrective maintenance strategy. 

Advanced ultrasound detectors allow inspectors to record the sound, measure the amplitude, analyse the signal for diagnosis, and compare the multiple defects’ amplitudes using scalable data. Condition indicators are a recent innovation offered exclusively from SDT. These useful tools provide advances insight about system condition by providing certain flexibility to inspectors. Prior to collecting a data set, inspectors decide just how much data they want to sample. They can choose between 1 and 80 seconds. Ultrasound data is then sampled at a frequency of 8,000 measurements per second for the duration of this acquisition time. 

Final Thoughts

Partial Discharge presents risks to both safety of personnel and reliability of electrical assets. Ultrasound provides an easy to use handheld technology to detect and isolate sources of partial discharge. Through operator training and a relatively short competency curve, ultrasound technicians can soon use analysis skills and experience to differentiate between different PD sources. 

Differentiating between corona, arcing, and tracking helps to not only eliminate electrical defects, but also identify potential root causes. This promotes our philosophy of not only finding, but also removing defects from our electrical asset systems. 

There is no perfect single technology when it comes to finding electrical system defects. Ultrasound may find defects earliest in the failure curve, but it is limited to the ability of the sensor to access the source of the defect. In some installations, no matter which technique or technology you use, you may still come up empty-handed. That’s why it is important to have access to more than just one. Ultrasound, however, is clearly the best, most versatile, and most cost-effective place to start.

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