Partial Discharge Detection

Detect and localize electrical Breakdowns. Early.

Partial discharge (PD) testing is a failure mode detection method that trained technicians can perform on energized equipment. It provides an extremely early indication that a critical electrical asset may fail. PD inspections can help confirm liquid testing results or serve as a standalone detection technique for failure modes such as:

  • Insulation deterioration
  • Arcing/electrical discharges
  • Breakdown in transformer liquid

What is partial discharge?

In plain terms a partial discharge is the result of an electrical breakdown that—over time—degrades an asset’s insulation or conductive components, leading to failure. Partial discharge can affect transformers, bushings, arrestors, switchgear, cables, and many other substation components over 1000V.

How does PD testing find failure modes so early?

Partial discharge inspections can pinpoint electrical issues in your network early before the condition leads to catastrophic failure. Partial discharges emit energy in several ways: electrical pulse currents, dielectric losses, light, sound, increased gas pressure, and chemical reactions. Based on the energy emitted, there are multiple detection methods, such as Ultrasound, Transient Earth Voltage (TEV), High Frequency Current Transformer (HFCT) and Ultra-high Frequency (UHF).

Ultrasound, the first line of defense, is based on pattern changes in the ultrasonic waves, which can be detected long before heat or other indicators monitored in other preventive maintenance tests.  

Types of partial discharge


A surface discharge that builds a conducive path across the surface allowing the condition to grow and and lead to an eventual failure.


The air around a connection becomes ionized and starts to break down the Oxygen into Nitrogen Oxide, leaving a residue on the connections and insulators that can change to Nitric Acid with exposure to humidity and water.


The continuous impact of partial discharges in solid dielectrics from the breakdown area forms discharge channels which grow in a tree-like formation.


Appears at the boundary of different insulation materials.


Occurs in voids or cavities within solid or liquid dielectrics.

Utilizing IR Windows for Ultrasound

IR windows allow for safe IR thermography on energized equipment and should be installed on switchgear, breaker panels, and cabinet transformer doors to avoid the risk of arc flash inherent in opening energized equipment. IR windows can include an optional ultrasound port and allow for the extra benefit of collecting ultrasonic data when performing regular IR scans. SDMyers can help install IR windows on your cabinet transformer doors, as well as your metal-enclosed and metal-clad switchgear.


Partial discharge is an important puzzle piece.

Reliability-centered substation maintenance is centered on gathering and analyzing as much condition data as possible before assets fail. Broadly, there are five core planned maintenance activities that can be deployed in a transformer program: visual inspections, oil testing, infrared scanning, electrical testing, and partial discharge testing. Partial discharge offers especially powerful insight into your transformer health when combined with other maintenance activities, such as oil testing.

Contact an SDMyers expert to add partial discharge testing to your routine testing plan.


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