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Ultrasonic Electrical Inspection: Corona, Are You Listening, or Pretending That It’s Not There

I feel far too many electrical inspectors are ignoring the 800-pound gorilla in the room named Corona. They are also ignoring the use of ultrasound for electrical inspections. Yes, it’s a hard stance. But I cannot tell you how many technicians I have interviewed at trade shows, conferences, or in my classrooms that admit they aren’t interested in using ultrasound. Or, not interested in changing their current inspection practice. Still others admitted they just aren’t familiar with ultrasound for electrical inspections to detect arcing, tracking, or corona discharge.

So, let’s start with simple theory. Imagine a picture illustrating sound waves that are moving through the air, vibrating over great distances. These waves are known as sonic waves. The number of sonic waves determines the frequency. Frequencies are measured in hertz (Hz); one cycle is 1 Hz. Sound perceived by the ear is aural or sonic and ultrasonic is sound above the human hearing range. In general, most humans hear in the range from 20 Hz to as high as 20 kilohertz (kHz). The high-end, of course, can be limited by interference or competing sounds. Well, the same applies when using an ultrasonic receiver/instrument. Ultrasound is sound above 20,000 Hz. When performing an ultrasonic electrical inspection, the ultrasound technician may select 40 kHz to detect an electrical anomaly (sound waves moving through the air). However, if background noise or other competing sounds are present, raising or lowering the frequency 1 kHz to 3 kHz may limit the interference, allowing the user to detect and define the electrical anomaly.


Scanning substation for electrical anomalies using an ultrasound instrument with a long-range horn (Photo courtesy of Ultra-Sound Technologies, Woodstock, Ga.)


Student using an ultrasound instrument with a long-range horn, left, and another using a parabolic dish and ultrasound instrument, right, to detect corona discharge (Photo courtesy of Ultra-Sound Technologies, Woodstock, Ga.

The comments from above tell me how much the electrical technicians use infrared (IR) cameras -- a lot. That’s not a bad thing, but, implementing ultrasound into your infrared program will complement your IR program. Ultrasound, unlike IR, does not require line of sight. Ultrasound also can be used to scan door seams prior to opening a cabinet (depending upon the voltage) or may be used to scan a substation prior to entering. Experts tell me that all electrical cabinets 480v and above should be scanned with ultrasound prior to opening.

Say you work in a steel mill. After getting properly attired in your personal protective equipment (IAW NFPA-70E), you walk onto the production floor and aim your IR camera at the bushings atop a high-voltage, step-down transformer and detect no heat. No anomaly. Yet, you aim your ultrasound instrument with a long-range adapter or parabolic reflector at that same bushing atop the transformer and detect the presence of corona. That’s because IR does not detect corona under 240 kilovolts (kV); however, ultrasound detects corona at 1 kV or higher!

I visited a General Electric laboratory in Hartford, Conn., 20 years ago. I was there to provide an ultrasound instrument to the lab for them to test the theory of when ultrasound detects the presence of corona. The test took place in a soundproof lab behind protective glass panels. Inside the lab, electricity was applied to a strand of one-inch braided electrical cable, not once, but several times. At 950 volts, 975 volts and at 1000 volts (1 kV), a sound resembling bacon frying could be heard, yet corona was detected every single time, even at 1000 volts (1 kv).

Familiar with Saint Elmo’s fire? I learned when the electrical field potential strength reaches 1000 volts (1 kV) per centimeter (.39 inches), point discharge or corona discharge occurs on objects, particularly on pointed objects, such as a lightning rod, or even a burr (pointed object) on a bolt of an electrical cable connector. That's a good reason to perform those periodic maintenance tasks.

The by-product of corona is ozone, carbon, nitric oxide1 and ultraviolet light. Corona is a continuous decay, therefore, corona should be removed or remedied when detected.

Did you know that in most cases, electrical anomalies can be verified through ultrasound?2 Ultrasonic Down Conversion™ (UDC™) is about taking the high frequency signal that has been converted to a low frequency signal and diagnosing a condition through signal analysis. UDC may be used to monitor a condition of a bearing, such as the inner or outer race or ball defects. Recorded sound of an electrical discharge can be used to identify corona, arcing, or tracking. For instance, when using an ultrasound instrument, sound is taken from the headphone jack or output jack of most ultrasound instruments and this low frequency signal (20 kHz and below) through signal analysis (FFT or time waveform) can be analyzed much like a vibration analyzer.

In my ultrasound courses, I go to great lengths to identify corona, arcing and tracking through in-class demonstrations. Using a neon sign transformer behind a protective shield, attendees can hear corona, tracking and arcing. Video is shown from a corona camera that identifies corona as it is seen through the ultraviolet lens. More importantly, attendees see a number of white dots in the video that represent the ionization. I like to describe this as a “corona spray” or “corona shower.” The techs get to “see” what they are hearing. Although a corona camera provides a phenomenal view of the corona, it, as with IR, requires line of sight.

The corona video inherently warns technicians not to get too close, not to go beyond the front of the panel or too close to an anomaly. Yet, many are not aware of the ionization field that may lurk several inches away from the anomaly. I estimated in some cases when viewing corona video examples that the field, depending upon the voltage and severity, is sometimes 12 to 14 inches away from the anomaly.

Corona not only occurs at switchgear or substations in above ground utilities, but below ground, as well. We see it where the aging infrastructure is rapidly deteriorating. Take for example a spring 2013 underground fire in downtown Atlanta, Ga. It caused a powerful explosion that blew a half dozen manhole covers several feet into the air. The utilities spokesperson said crews were working underground making repairs when cables caught fire and the buildup of pressure from the fire caused several manhole covers to blow. Fortunately, there were no injuries.

It’s actually happening more than you think or read about. Perform a web search using keywords “manhole fires.” Like me, you will find events happening more frequently than you think. But, that’s a story for another day.

Stop ignoring the 800-pound gorilla -- start an ultrasound electrical inspection today.


Ultrasonic electrical scanning requires slow and deliberate movement, whether scanning switchgear or substations. Ultrasound-detected corona is shown in metering cabinet prior to opening. (Video credit: Jim Hall and Roger Beaty, Ultra-Sound Technologies (substation), and Holland Board of Public Works, Holland, Mich. (corona video).)


Ultrasonic Down Conversion™ used to show corona through signal analysis. Notice the high peaks at 120 Hz and the half peaks at 60 Hz. (Photo courtesy of Ultra-Sound Technologies, Woodstock, Ga.)

Note: I’d like to congratulate the Arizona Public Service Company (APS) for the Uptime Award for the Best Electrical Maintenance Reliability Program that included an unbelievable acoustic program. There’s no 800-pound gorilla at APS!

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