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Getting (dis)Charged Up

Picture a thief on a hot, windless summer night, a night that is so hot you might think that you can hear the sound of eggs frying atop a transformer.  So hot, that your electrical equipment has maxed out its cooling capacity.  It is so hot this thief is robbing you of precious energy, tearing at the very heart of your electrical system. This thief is waiting for that perfect moment to flash over and shut you down, rendering an entire plant or community without power.

This thief is better known to you and me as Corona.  How familiar are you with this phenomenon known as corona?  Can you recognize corona if you hear it?  Can you visually detect the presence of corona?  If not, by all means, read on.

Until relatively recent times, corona hadn’t been talked about, hadn’t been routinely identified, nor readily understood.  Here are some of the characteristics of Corona;

•  Corona is more prevalent at night.
•  Corona is more prevalent during high ambient temperatures.
•  Corona is most intense as it moves towards tracking in the
    300-400nm range.
•  As corona exceeds 400nm, and there is no interference
    from background light, it may become visible to the
    human eye.
•  Corona is typically responsible for the occurrence of
    flash over.
•  Corona can be heard with the ultrasonic receiver at 1kV
    or higher.
•  Corona is not visible to the infrared spectrum.

Ultraviolet light is a by-product of corona and/or corona discharge.   Today corona can be seen in its natural state with the use of corona cameras, which are equipped with special lenses that detect ultraviolet light.  Couple that with a video camera and you have a two channel video of the ultraviolet light. 

Corona is visible in the ultraviolet spectrum, but can only be found in the infrared spectrum if arcing and tracking have started to occur.  So, corona (without arcing and tracking) cannot be seen by the infrared technicians and electrical workers when working under 240kV.  However, corona can be heard and detected with an ultrasonic receiver at 1kV or higher.  I suggest you build yourself an audio library of electrical wave files to familiarize yourself with nuisance corona, destructive corona, arcing and tracking.  Nuisance corona in an outdoor substation, for example, can only be identified with UV camera.  In metal clad switchgear, there will be no nuisance corona.


Corona is nothing new.  It was actually identified (along with nearly every other natural phenomenon) by Pliny the Elder in the 1st century A.D.  It’s often seen atop ship’s masts by sailors, including the crews of Magellan and Columbus, and is referred to as “Saint Elmo’s Fire”.  If you happen to drive down a dark highway, especially during a thunderstorm, and observe a blue faint glow atop an electrical line, it will most likely be corona.  If you have your AM radio on while driving those same roads, you may hear an egg-frying sound of nuisance corona, usually because of the dirt and dust upon insulators of the power line.

Corona is a continuous decay, and its by-products are nitric acid (or nitric oxide), ozone, carbon and ultraviolet light.  Left unchecked, corona will continue to deteriorate insulation values and decay metals until grounding or short circuiting occurs.  I have actually seen, due to the effects of corona, the solid core rod of a row of insulators that snapped in two, causing a “line drop.”

While researching the detection of corona using airborne ultrasound, I remember seeing the lab technicians apply voltage at 980 to 999 volts, with no detectable ultrasound signature.  But, as soon as 1000 volts (1kV) was reached, the frying egg sound of corona could be heard immediately.

I recently sat down with Chuck Humphrey’s, President of HighVec, Inc. of Timmins, Ontario, Canada.  Chuck has been an avid user of airborne ultrasound for corona inspection for many years.  I asked him to share some of his experiences with detecting corona and corona discharge.  I told him that for several years now I have been promoting the idea of scanning the switchgear door seams prior to opening, based on the premise that technicians should be able to identify the sounds of corona as well as locate the source.

Chuck told me that he and his crew use ultrasound first to detect the presence of corona.  Ultrasound is particularly useful with metal clad switchgear, because with the many cables inside, you may not have line of sight for corona cameras to view corona.  Ultrasound receivers can hear the high frequency short wave, which enables the user to locate the corona.
Talking with Chuck about the actual inspection of metal clad equipment, I was most interested in how they approach the gear. He said that his crew “does not” scan the panels with ultrasound before they open the cabinet, that it is a waste of their time.  Now this goes against my beliefs, but he went on to explain that his crew is already prepared for whatever is going to happen.  They are outfitted in a  Category 4, 95 Cal Personal Protective Equipment (PPE) suit (full moon suit).


Chuck also explained how inspecting metal clad gear is like a cat and mouse game.  You have to be able to hear it, then locate it, identify the root cause and offer a remedy.  Airborne ultrasound detection gives them the ability to find the corona discharge both in the early stages, before any serious damage is done, and in the more advanced stages, but before tracking.  The service they provide is all about safety to the worker and eliminating downtime.

Here are some telltale signs of corona:

•  The smell of ozone when opening the switchgear (Ozone is
    a by-product of corona).
•  Residue from the nitrogen oxide (white powdery residue).
•  Air patterns forming in the contamination.
•  Discoloring of areas in the switchgear near or on
    conductors or buss work.
•  Oxidation or corrosion.

Even though Chuck and his crew do not scan the doors before they open them, this should still be  standard practice for all technicians and inspectors prior to opening any electrical cabinet.  Please remember that Highvec is a company specializing in cleaning and inspecting “hot” substations and metal clad switchgear.  You will most likely not be in a Cat 4, 95 Cal suit.  So, if you want to live a long and happy life, it would be wise to scan before ever opening any cabinet or door (480v or higher).


I mistakenly thought that once corona is present, it’s there until removed. But Chuck explained that corona may be there today, but gone tomorrow. Moisture, air movement, air quality and heat all have a role in the accumulation of dust and dirt particles that form where corona is present.  Opening a cabinet door too quickly can cause enough air movement  to change the position or the characteristics of the corona (or eliminate it altogether). Ionization can also create air movement.  Air quality can be a problem , not just the ambient air particles but how much air can or cannot pass around and between cables of the cabinet structure or hardware (electrical fittings, etc.).  The lower the air quality, the higher the chance of corona occurring.


Chuck also cautions anyone that “only” listens to switchgear panels with an ultrasound receiver needs to confirm corona - particularly those technicians that do not find corona while scanning the door seals or openings without opening the doors. Yes, there are technicians out there that will simply stop the inspection if they don’t hear corona using the ultrasound receiver.  That’s simply wrong.  You should not rely simply on sound to hear corona.  You should always open panels and inspect visually as well, using all the tools available to you today, infrared, corona cameras, etc.  Always remember that corona can be present at low amplitudes, yet left unchecked, can and will become a problem later.

Some electricians and inspectors who are not familiar with the proper use of airborne ultrasound have misinterpreted the its application when inspecting for corona.  Some believe that if they did not hear or detect corona inside a cabinet while scanning the door seam, then there must not be any corona in the cabinet. Wrong. The practice is to scan the door using your ultrasound receiver placed perpendicular to the door or cabinet door seam or seals.  This will help eliminate hearing competing ultrasound or reflected ultrasound from another source or door.  If corona is heard, you should make note of the area, and once you are wearing the Personal Protective Equipment (PPE) to be in compliance with the NFPA-70E, you should open the cabinet door/doors “slowly”.  Once the doors are open you should scan with the ultrasound receiver keeping the instrument from extending into or inside the cabinet door boundaries.  Since, ultrasound is very directional, you should be able to locate the source of the corona.  Sometimes an attachment such as a non-conductive cone can be used to aid you in pinpointing the corona. But you should never stop an inspection if you do not hear the corona!  A visual inspection may still reveal corona activity.


Chuck likes to use a custom-made, long straw-like cone attached to his ultrasonic instrument that allows him to reach into tight spaces while thoroughly examining a cabinet while the cabinet is energized.   CAUTION - Do not mistake the “long straw-like cone” with the regular rubber cone that comes standard in your ultrasound kit.  The straw like cone is a special cone built for this end user only.

Since ultrasound does not require line of sight, you can follow the sound to locate, and to visually identify, the corona or signs of corona discharge.  Signs include carbon tracks, a white powdery residue and/or dust and dirt accumulated at the point of the ionization.


Chuck told me that on one occasion they had to enter a cabinet that had smoke coming from the vents.  They were able to open the cabinet and subdue the flash over potential using their cleaning equipment without an incident.

Many of you have probably seen some remarkable video clips on, where someone has filmed an arc flash or flash over.  Now, that’s pretty scary stuff.  I cannot imagine being inside a cabinet when a flash over occurs, let alone standing there and watching the fireworks as it happens — even if I was wearing a Cat 4, 95 Cal suit!

A digital corona camera may be used in the daytime or at night to detect corona in switchgear and or in a substation.  The low light analog type camera is used at night, and will give a view of the corona discharge as it actually fires.  Ultraviolet cameras can detect the corona wherever it may exist, which is normally 1,000 volts (1kV). 

Chuck performs all substation scans at night and only during the summer months, because the conditions (moisture in the form of snow & ice and low ambient temperatures) in winter months, are less favorable for corona detection.  However, he does inspect metal clad switchgear year round.

Why airborne ultrasound?  Corona is ionization. The ionized air particles create friction. A disruption of the molecules in the air creates the friction, which, in turn, creates a high frequency sound.  Ultrasound is defined as sound above 20 kHz or 20,000 Hz, and most ultrasound receivers are centered at 40 kHz; others may have tunable frequency selectors enabling them to select between 20-200 kHz.

The typical airborne ultrasound detector you use for air leaks, bearing analysis and steam traps diagnostics is the same instrument that can be used to scan your switchgear cabinet for arcing, tracking and corona activity.  However, the trick is identifying the sound of nuisance corona, destructive corona, tracking and arcing.

Chuck has many years of experience using airborne ultrasound as part of his cleaning and inspecting of high voltage equipment and substations. He is one of only a few leading authorities that inspects high voltage equipment while HOT.

You may have seen pictures of men in “moon suits” (Category 4 PPE), inspecting high voltage cabinets.  But seldom do you see pictures of men like the crews of Highvec inspecting high voltage cabinets with the doors open.  This is very dangerous work and only a few will actually perform this work while the equipment is energized.

These men will see more in one inspection and cleaning of high voltage switchgear or apparatus than most will see in a lifetime working at their plant.  Highvec’s crew are more than just glorified dust and dirt cleaners...they are highly skilled technicians.

My first encounter with this group was at a trade show many years ago while selling airborne ultrasound equipment.  This company, and particularly their knowledge regarding locating and understanding the phenomenon known as corona, had greatly impressed me. At that time, I had not come in contact with anyone else who understood what they were seeing, or even what to actually look for, much less how to remedy corona.  Even way back then, Highvec was already inspecting substations at night using analog corona cameras, which were just coming into the marketplace.

Chuck was kind enough to allow me to post a 60-second mpeg video of corona as seen through the ultraviolet camera on my website.  Go to to view this truly amazing clip.

Just remember those Fourth of July celebrations where you might have held a sparkler in your hand that was supported by a thin piece of wire.  That is what the corona spray reminds me of.  Sometimes those little sparks would end up down the collar of my shirt or, fall on my bare feet (southern boy) and burn like heck.  Ouch.

Well, just imagine how much trouble you would be in if you were holding, oh, say 100,000 sparklers.  That will give you a rough idea of what would happen if you open an electrical cabinet and flash over or an arc flash occurs?  One gentleman in one of my Ultrasound Level I classes had, in fact, experienced an arc flash occurrence. Luckily, he happened to be exhaling at that moment, and not inhaling.

I have a personal mission to educate as many technicians as I can about just what is in the switchgear and metal clad cabinets that they cannot see with their own eyes.  Today’s technicians are learning about infrared and how to use the infrared systems, but few actually have seen the photos, the videos, or have actually viewed this phenomenon with a corona camera.

When I think back a few years, I am amazed at how irresponsibly (although at the time, we didn’t know better) we inspected switchgear using airborne ultrasound.  I can thank my lucky stars I am still here among the living.  I can remember years ago seeing training video clips of instructors reaching through and inside the switchgear cabinet door with an ultrasound instrument.  Granted, this was before NFPA-70E, but even then, that should not have been a recommended practice.  You should never, at any time, reach inside an electrical cabinet with a metal instrument like an ultrasonic receiver to inspect a connection for arcing, tracking or corona.  Always stay outside the perimeter of the door.

I want to thank Chuck Humphrey and the men of Highvec for taking the time to talk to  me and provide these photos and information, so that others can benefit from their experience in the detection of corona.  As Chuck mentioned earlier, “It’s all about safety to the worker and eliminating downtime.”

Jim Hall is the president of Ultra-Sound Technologies, a vendor-neutral company providing on-site predictive maintenance consultation and training.  UST provides an Associate Level, Level I & II Airborne Ultrasound Certification. Jim is also a regular provider of on-line presentations at and is a contributing editor for Uptime Magazine (ultrasound segment).  Jim has been in the airborne ultrasound industry for 20 years. Jim has provided airborne ultrasound training for several Fortune 500 Companies in electrical generation, pulp & paper, petro-chemical and transportation (marine, automotive, aerospace).  A 17 year civil service veteran, Jim served as an aerospace engineering technician for Naval Aviation Engineering Service Unit (NAESU) and with the Naval Aviation Depot Jacksonville Florida (NADEP).  Jim can be reached at
or (770) 517-8747


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