Corona or Infrared Technology FAQ

Corona or Infrared Technology FAQ

The differences between corona and infrared can be explained within the following frequently asked questions.

Q: We already do infrared scanning, so we don't need to do corona scans do we?

A: Infrared locates heat sources, heat is produced during current flow (arcing is current flow, arcing usually occurs at failure), problems detectable with infrared are when the spot temperatures exceed the overall normal surface temperature. The infrared heating may be present during the last 10% (or less) of component life. Infrared cameras measure energy which is radiated in the 1 to 13 micron wavelengths (1000 nm to 13000 nm).

Corona is different, corona is the indication of a voltage problem, corona indications are present from day one until failure. When the circuit is energized voltage is present and a corona inspection can be done before applying load, at maximum overvoltage condition or after load is applied. Corona inspections will show missing hardware, damage, poor installation practices, defective insulating components, "bird-caged" wires (typically vertical conductors where the aluminum spreads away from the steel center conductor), and many more things.

An infrared camera with the right spot (spatial) resolution can see an arc at a short distance, a corona camera with a solar-blind filter can see an arc at over a mile away line-of-sight.

An infrared camera can not see electrical corona, which is the ionization of the air around a high voltage device although due to the chemical reaction occurring heating must occur of the air molecules.

A solar-blind (sensitive only to 240 to 280 nm, 0.24 to 0.28 microns) corona camera senses the ultraviolet light and thus can see the corona during bright sunlight and at night. During dusk or cloud covered conditions, indoors or after dark when ultraviolet, night vision cameras and sometimes the human eye can see corona. Solar-blind can be achieved by using a special filter only passing the wavelengths 240 to 280 nm, the ozone layer surrounding the earth completely absorbs the suns energy in the UVc wavelengths of 200 to 300 nm. The UVc wavelengths would be pitch-black -on earth- except for nearly entirely man-made events.

When corona occurs it creates ozone (detrimental to the human lungs, eyes, etc.), ultraviolet light, nitric acid, electromagnetic emissions and sound. The nitric acid over time removes the plating and causes corrosion of the steel parts of the conductors and insulators. The electromagnetic emissions can be heard on an inexpensive AM radio and the corona sound can be heard by the human ear or a focused ultrasonic device even though the sound is directional, may be muffled, reflected or cancelled and provides lilttle objectivity or discrimmination to which component is the cause of the sound. The electromagnetic emissions are in all directions but directional sensitivity to potentially random sources is time consuming and may be inaccurate.

Now lets talk real world where the objects with corona emitting are of many shapes, the corona may only be on one side or down in a "bell", common on insulators, the light emission and sound emission are strongest in a line-of-sight direction and may not be present in other or all directions.

The damage sequence is corona, flash-over and sustained arcing. The Corona Technology Course will address the science and explanations over a 14 hour period followed by 1 hour of hands-on corona detection testing with the latest test equipment for corona, flash-over, arcing detection.

Q: Why is corona present sometimes and not others?

A: Purchase the Corona Technology Course on DVD or attend the Corona Technology Course to learn the science.

What can corona tell us about substation switches?

A: Which type?

1. manually operated with a history of falling apart, multiple hot-sticks to operate

2. 345 kV with remote operator, sharp points on castings, loose to insulator, corrosion (conductive) staining insulator?

3. why don't birds and most nest materials always have corona?

What can corona tell us about the many transmission and distribution lines (many over roadways) with radio noise?

1. broken strands

2. contamination by road salt

3. other contamination

4. damaged insulators

5. structural damage

What can corona tell us to improve construction practices to reduce radio noise?

1. stub ends at cable connectors

2. stub ends at cable splices

3. forgetting to install/remove hardware during new construction

4. freeze and thaw damage

5. clamp damage on conductors during splice installation or temporary connections

What design practices cause corona?

1. bus stubs to future? Inspect new construction at start-up.

2. conductors thru insulator clevis without a shoe on cable

3. no corona ring on some HV insulators

4. building downwind of roadways

What interference signals (continuous/random) are present that interfere with:

Ultraviolet detection

1. Welding, direct and airborne water droplet reflected

2. Propane flames, forklift exhaust, direct and airborne water droplet reflected

3. High pressure steam, direct and airborne water droplet reflected


1. Industrial noises, machinery, belts, bearings

2. Compressed air leaks

3. Vehicle noises, random from passing vehicles

Electromagnetic emission

1. Radio stations

2. Some diesel trucks and autos, alternators or power converters

3. Burst antennas, communications, cell phones, etc.


1. Lack of spatial resolution at distance with common lenses.

2. Emitted corona signal is too weak or not present in infrared wavelengths.

3. Lack of thermal sensitivity in long wave infrared cameras used outdoors.

Q: What voltage does corona occur at?

A: I have been told corona can occur at 2.5 VAC, I have seen it consistently at 3 to 5 kV in ambient air at sea level. I have seen corona, in the manufacturers test laboratory, on the insulated surface of a new uninstalled electric motor winding at 4 kV, normal operating voltage of 13 kV and test voltage of 35 kV+.

Q: What is the best time to look for corona?

A: Outdoors, the day after a rain storm corona will occur until the energized surfaces dry. After drying corona is less likely and indicates the real problem areas. Wind can reduce corona activity. Cloudy days can have more reflections from interference sources producing "snow" on the ultraviolet cameras, then you change your viewing angle to stop the UV reflections or use integration of the ultraviolet image.

Corona can occur from icicles. I have seen icicles on insulators in a 115 kV substation that had recently had several flash-overs resulting in breaker operations and line outages.

Indoors electric motors, generators and switchgear can also have corona more severe then outdoors. The build-up of ionized air in an enclosed space or space with no air movement accelerates corona and flash-over and reduces the voltage at which corona can occur. Motors and generators can have multiple air pressures (air insulating value) present due to rotating fans moving air within an enclosure or in the winding slots.

What corona is damaging?

A: The insulator "doctor" is Prof/ Dr. Ravi S. Gorur at Arizona State University ( who will be presents an Insulator Course and writes a monthly article in "Insulator News and Marketing Report" ( describing insulators, testing, test equipment, testing practices and standards.

A2: The The OK You Found Corona CD® includes many corona camera and visible camera images showing the many places where corona is damaging.

Typically corona on hardware is ignored, except for it being a radio interference source; corona on insulators is an indication of ionization voltage at that point due to contamination or damage either of which can lead to "flash-over" and outage.

Q: What can I see with a Zoom visible inspection?

A: Some ceramic insulators have "cement growth" which causes the cement holding the insulator together to grow and fracture allowing the insulator to fail. The condition of NCI insulators. Heat stressed, discolored areas on ceramic insulators.

Q: Where does corona occur in a bare-hands working environment?

A: Get the Understanding Corona Series on CD® to see where corona and arcing occur when a bucket truck , helicopter, and service men intentionally make contact with an energized 735 kVAC line to do service work.

Look at the Corona Technology Course Home Page

What materials are available to learn more about corona, testing, improving reliability?

Q: When is the next Corona Technology Conference?

A: Today look at the presentation about Corona, Flash-Over and Arcing on the webpage and sign up for the Corona Technology Conference taught by the corona experts.

You may also be interested in getting the "Corona Technology Course on DVD®" to get instruction "right now."

Why should I order the "Corona Technology Course on DVD®"; at 1-800-531-6232 or by Email at

Daylight corona detection provides safety for the testing personnel whether on the ground, in a vehicle, in a helicopter or a fixed-wing aircraft. The UV corona signal is very weak and decreases rapidly with distance, thus the available power (voltage, more importantly amperage) determines the distance at which corona (voltage) and arcing (amperage) can be seen. Distribution voltage corona has been seen at 100+ feet, 240 kVAC at several hundred yards, 735 kVAC corona at over 1/4 mile. Sustained arcing UV can be seen easily at over a mile.

Have you ever considered - When looking for the source of AM radio or TV interference (seldom is there FM interference); a radio or TV is tuned to a specific frequency with an antenna which has a specific direction of sensitivity and effective reception range determined by the source frequency: ultrasound is similar in that the better units are "tuneable" and can be made very directional with a parabolic dish, focusing at a maximum sensing distance of 50 (without dish) to 70 feet (with dish)*; compared to a camera which usually looks at a specific range of wavelengths (ultraviolet, visible, nightvision, infrared) and is also directional/line-of-sight with the observed area increasing with distance, the resolveable determined by the lens, pixels in the array or detector type, signal delta from surroundings and variable by technology with infrared effectively limited to about 100 yards**, ultraviolet low-light corona cameras to about 100 yards, and daylight(all-light, bispectral) corona cameras limited by signal strength (decreases with distance) but with a far more sensitive detector able to image arcing at over one mile and corona at distances of five hundred yards with probable detection of corona at distances of 5,000 feet. *with a minimum corona signal; **due to the size of conductors, pixels per unit diameter

©2017 Ox Creek Energy Assoc Inc

Last update: September 3, 2017