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Home / Technical Articles / What’s That Hissing, Cracking, Buzzing Noise?

Corona or Partial Discharge with Buzzing Noise

Corona can be visible in the form of light, typically a purple glow, as corona generally consists of micro arcs.

What's that hissing, cracking, buzzing noise?
What’s that hissing, cracking, buzzing noise? (on photo: Corona discharge on insulator string of a 500 kV transmission line)

Darkening the environment can help to visualize the corona. We once attached a camera (set to a long exposure time) to a viewing window in a vacuum chamber to confirm that corona was indeed occurring, and thereby confirming our suspicions.

You can often hear corona hissing or cracking. Thus, stethoscopes or ultrasonic detectors (assuming you can place them in a safe location) can be used to find corona.

In addition, you can sometimes smell the presence of ozone that was produced by the corona. (Who said you don’t use all your senses when troubleshooting?)

The corona discharges in insulation systems result in voltage transients. These pulses are superimposed on the applied voltage and may be detected, which is precisely what corona detection equipment looks for. In its most basic form, the following diagram is a corona (or partial discharge) measuring system.

Partial discharge activity on sharp metal edge
Partial discharge activity on sharp metal edge (photo by

When corona occurs it creates ozone (detrimental to the human lungs, eyes, etc.), ultraviolet light, nitric acid, electromagnetic emissions and sound.

Ozone is a strong odorous gas that deteriorates rubber-based insulation.

If moisture or high humidity conditions exist nitric acids can also be formed that attacks copper and other metals. The electromagnetic emission can be heard as interference on AM radios and the corona sound can be heard by the human ear and be ultrasonic scanning devices.

Damaged electrical cable due to the partial discharge
Damaged electrical cable due to the partial discharge (photo by JIM CAHILL at

One important point to consider is that unlike infrared that detects heating due to current flow, corona indicates voltage problems and can be present without current flow. High potential in the electrical field is the major dictating factor for its presence.

Corona activity is at its strongest on the positive (+) and negative (-) peaks of the 60Hz cycle.

The effects of corona are cumulative and permanent, and failure can occur without warning.

Corona causes:

  • Light
  • Ultraviolet radiation
  • Sound (hissing, or cracking as caused by explosive gas expansions)
  • Ozone
  • Nitric and various other acids
  • Salts, sometimes seen as white powder deposits
  • Other chemicals, depending on the insulator material
  • Mechanical erosion of surfaces by ion bombardment
  • Heat (although generally very little, and primarily in the insulator)
  • Carbon deposits, thereby creating a path for severe arcing

How Corona works?

Focussing on corona discharge and surface discharge, i.e. electrical gas discharges occurring in ambient atmosphere, ionisation phenomena are initiated in the high electrical field region respectively resulting from the conductor geometry (wire, point, sharp edge) or from a triple point (metallic conductor / insulation material /gas).

According to the conductor geometry, applied voltage amplitude and polarity, ionisation phenomena will be confined in the vicinity of the high field region or will propagate in gas from this region as transient successive ionisation waves (streamer regime with current pulses associated with the development of filamentary discharges).

In any case, electron in elastic collisions will also lead to gas molecules dissociation and excitation resulting in chemical active species formation and light emission; elastic collision between ions and neutral molecules will result in local gas heating.

Surface partial discharge effect
Surface partial discharge effect (photo by

Light, gaseous chemical reactants and heating are consequently evidence of gas discharges.


  1. Corona and Tracking Conditions in Metal-clad Switchgear Case Studies By James Brady, Level-III Certified Thermographer

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Allan Dave Sargento
    Jun 27, 2022

    What should be the solution to kind of problem?

    I was already encounter this kind of hissing sound with odorous gas that deteriorates rubber based insulation in MV switchgear.

    And once it occurs, it could not be remove unless replacement will be done.

    Can you recommend that controlling humidity inside the switchgear is possible solution. or replacement should be done.

  2. Saifullah
    Nov 15, 2018

    I love this Engineering Portal.. Please share study material for Over head Transmission lines upto 220KV.

  3. Golden
    Mar 26, 2017

    Quite interesting and great information.

  4. vince
    Feb 21, 2017

    Excellent article, I normally explain to my students how low level ozone can be produced with high voltage power lines and the amount of humidity. Your are a very informative engineer who explained clearly how this is achieved. Many thanks. Vince

  5. sunit18236
    Jul 27, 2014

    nice portal.. this gave me great info… even it improves my iq in this field

  6. laith
    Jul 11, 2014

    Hi guys,
    Is there any technique for reducing corona discharge broadband crackling noise?

  7. michael mazuba
    Jun 17, 2014

    i think that this portal is really a good idea

  8. [email protected]
    May 31, 2014

    “Buzzing” 11kV cable terminations on a high voltage cable network are investigated in this blog post by Thorne & Derrick

  9. JIG
    Jan 06, 2014

    Please dont copy material available on Net. It sounds not good to other who did hard work for their paper..
    I like this site but article should be of your own study, material and knowledge..
    Good luck guys

    • Edvard
      Jan 06, 2014

      JIG, did you notice the listed references used for this article?

  10. hassan.shahbeigi
    Nov 07, 2013

    Thanks, that’s useful and helpful for electric al engineer

  11. rohk
    Oct 31, 2013

    Great infor

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