Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

Home / Technical Articles / Why is continuous on-line monitoring of partial discharge in the switchgear necessary?

What’s the condition of your switchgear?

Not sure? You know that periodical maintenance test like partial discharge test can still leave switchgear in virtually unknown condition. Insulation defects and deterioration may very well develop in service within maintenance cycle.

Why is Continuous On-line Monitoring Partial Discharge in the Switchgear Necessary?
Why is Continuous On-line Monitoring Partial Discharge in the Switchgear Necessary? (on photo: 11kV voltage transformer spout failure in progress – Located by Partial Discharge survey; by

These defects are often not detectable with traditional off-line tests and yet, traditionally, on-line or off-line partial discharge tests have been performed on a periodic basis commonly twice a year. Think this is often enough?

Advantages over periodic partial discharge (PD) testing

Continuous PD monitoring has the following advantages over periodic PD testing:

1. Periodic on-line PD test could miss significant PD activities since PD activities vary by time. On-line continuous monitoring eliminates the inherent flaw of interval-based testing.

2. Trending of PD activity is one of the most important parameters for predictive diagnostics. Periodic tests will not be able to provide sufficient information for diagnostics based on trending.

3. On-line monitoring provides more accurate information than off-line testing since off-line testing conditions can differ greatly from real operating conditions.

4. Continuous on-line monitoring effectively reduces labor costs. In addition, the PD data saved in the instrument can be accessed anytime, anywhere with modern communication means.

Partial discharge test performed on site
Partial discharge test performed on site (photo credit:

Degradation of Insulation in Switchgear

Electrical insulation is subjected to electrical and mechanical stress, elevated temperature and temperature variations, and environmental conditions especially for outdoor applications.

In addition to normal operating conditions, there are a host of other factors that may trigger accelerated aging or deterioration of insulation.

Switching and lightning surges can start ionization in an already stressed area. Mechanical strikes during breaker operation can cause micro cracks and voids. Excessive moisture or chemical contamination of the surface can cause tracking.

PD Between Bus and Cubicle Wall
PD Between Bus and Cubicle Wall

Any defects in design and manufacturing are also worth mentioning. Both normal and accelerated aging of insulation produce the same phenomenon in common – Partial Discharge (PD).

Partial discharge (PD) is a localized electrical discharge that does not completely bridge the electrodes. PD is a leading indicator of an insulation problem. Quickly accelerating PD activity can result in a complete insulation failure.

Partial discharge mechanism

PD mechanism can be different depending on how and where the sparking occurs:

  1. Voids and cavities are filled with air in poorly cast current transformers, voltage transformers and epoxy spacers. Since air has lower permittivity than insulation material, an enhanced electric field forces the voids to flashover, causing PD. Energy dissipated during repetitive PD will carbonize and weaken the insulation.
  2. Contaminants or moisture on the insulation induce the electrical tracking or surface PD. Continuous tracking will grow into a complete surface flashover.
  3. Corona discharge from sharp edge of a HV conductor is another type of PD. It produces ozone that aggressively attacks insulation and also facilitates flashover during periods of overvoltage.

Reference //  Predictive Diagnostics for Switchgear – EATON

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
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. Yvon
    May 28, 2015

    So if some of these switchgear discharge tests are not always accurate, do you have to actually have someone go out and look at the thing? With so much electricity going through, you need to be sure that the insulation is intact and in good repair. Being able to keep track of the maintenance and the wiring conditions online would help a lot to lower labor costs.

  2. Franc
    Mar 05, 2015

    Thanks for this article, colleague.

  3. Chris Dodds
    Jan 20, 2015

    The following Video Tutorial is used by the UK utilities to provide insight and training into Partial Discharge :

    Video Tutorial Masterclass – PD Partial Discharge & Electrical Cable Breakdown – See more at:

  4. rodney p mero
    Dec 14, 2014

    Iam not yet in technology but I try to folow you in order to know many things

  5. PK Sahoo
    Oct 26, 2014

    Nice article with some practical snaps. We faced similar instances several times. We find one thing that Cast Resin/epoxy body High Voltage equipment are more prone to tracking phenomenon. Then how to maintain those. Would you suggest some maintenance procedure to improve the condition? Thank you a lot.

  6. Mark Bollard
    Jun 24, 2014

    Extremely interesting article, well written and informative! Thank you for this hard to find electrical industry articles that don’t put you to sleep…. Keep up the good work!!

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

  +  thirty  =  thirty five

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge