NFPA 70B Guidelines: Partial Discharge Testing Can Include the Use of Acoustic Imaging

In our previous post, we covered the importance of software to fulfill the documentation requirements of NFPA 70B.  In this post, we consider the requirement for PD testing in NFPA 70B and how ultrasonic imaging can be used to address this demand in an easy way.

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Partial Discharge, or PD, is electrical discharge often caused by a localized reduction in the effectiveness of an electrical insulator. There are three modes of partial discharge:

  • Corona (conductor to air)
  • Tracking (surface or internal discharge through or across a physical insulator)
  • Arcing (phase to phase or phase to ground)

PD can create several hazards to plant safety and equipment reliability, the most impactful being arc flash, but also evolution of electro-magnetic waves, nitrous oxides, and volatile compounds from the degeneration of insulating materials. PD faults can also affect power quality.


FLIR Si2-PD and Si2 Pro acoustic cameras can pick up the audible and ultrasonic noise created by partial discharge faults, even at very long distances. They use artificial intelligence to classify the type of PD and assess its severity so you can know exactly what to fix and how to prioritize the repair.

The most common way to detect PD is through the use of ultrasonic acoustic tools. The use of ultrasound wavelengths helps to eliminate a lot of the background noise that exists in most plants and partial discharge faults typically radiate significant amounts of ultrasound.  PD in components can eventually result in a local temperature rise that can be detected with a thermal camera, however this does not always occur prior to functional failure.

FLIR-Si2-PD Prodcut Image.png

It is important to be aware that thermally visible faults, often caused by electrical power transforming to heat due to electrical resistance, and partial discharge faults, are two different modes of failure, with different modes of detection and quantification. A tool made to detect heat cannot be relied on to detect PD, and vice versa.

transformer-thermal copy.jpgtransformer-acoustic copy.jpg
Acoustic Issue not visible in a thermal image vs. Thermal Issue not visible in an acoustic image.

PD faults are most common with higher voltage equipment, which is why NFPA 70B only requires PD inspections on equipment over 1000 Volts, with the exceptions of busways and medium voltage (MV) power and distribution transformer windings, which must be inspected if >600 V.

Use of airborne ultrasound technologies, including the FLIR Si124 and the new Si2, may be appropriate for use cases classified in NFPA 70A as Category 1A (Online Enhanced Test), for these applications, and for some uses cases is the required inspection technology. Like thermography, PD testing and surveys must be conducted at least yearly for most applicable electrical assets and is detailed in Chapter 9 of NFPA 70B.

Assets where airborne ultrasound technologies could be used as per NFPA 70B



Inspection Interval

NFPA 70B Ref. Table.


>600 V

60 months – Cond. 1.

36 months – Cond. 2

12 months – Cond. 3


Power & Distribution Transformer Windings (Dry type, air cooled)

>600 V

60 months – Cond. 1.

36 months – Cond. 2

12 months – Cond. 3


Power cables & conductors (airborne ultrasound mandatory inspection technique)

>1000 V

60 months – Cond. 1.

36 months – Cond. 2

12 months – Cond. 3


Substations & Switchgear

>1000 V

60 months – Cond. 1.

36 months – Cond. 2

12 months – Cond. 3


HV Substation Insulator Corona Testing & Surveys

>1000 V

12 months – Cond.1

6 months – Cond. 2

4 months – Cond. 3


*All electrical equipment requires thermography to be performed at least every 12 months.


Historically, airborne ultrasound (UT) technology has been limited to single-transducer detectors, sometimes with parabolic cones to help scan UT sources at long distances. While effective, the technology required significant time for training and experience. 

In recent years, the use of ultrasonic acoustic imaging has grown rapidly, leveraging the inherent safety benefits of airborne ultrasound, but with scanning speed and ease of use that are unheard of in single-transducer systems. This rapid evolution has culminated in the FLIR Si2, our new flagship ultrasonic acoustic imager, which launched on March 5, 2024.

The Si2-PD and Si2-Pro ultrasonic acoustic imaging systems for PD inspections and surveys provides many unique benefits:

Fast scanning with few misses. Wide-area scanning, combined for very high sensitivity means scans of large areas, such as substations, can be conducted very quickly with low risk of missed faults;

Fault severity assessment and recommended actions. Faults are not only detected, they are also measured and evaluated for severity, resulting in minimal training and experience requirements for effective use;

Strong Return on Investment (ROI). In addition to getting time back and catching more faults, the Si2 Pro provides improved ROI by finding and measuring mechanical bearing faults, compressed air, and compressed gas leaks, each of which has their own ROI for early fault detection.

Made for scaling. The Si2 is unique capable of supporting scaled benefits to organizations through the ability to manage tool fleets across organizations. Through tool health metrics and organization data and image sharing, inspection managers can ensure their equipment fleet is maintained and data is being effectively gathered and shared. 

FLIR Si2-Series acoustic cameras provide measurement and severity assessment tools you can review on-camera while still in the field.

Si2-screen1.jpg Si2-screen2.jpg Si2-screen3.jpg

PD severity assessment for FLIR Si2-PD, Si2-Pro

Compressed gas quantification for FLIR Si2-LD, Si2-Pro

mechanical fault measurement for FLIR Si2-LD, Si2-Pro

In summary, partial discharge testing and surveys are mandated as part of NFPA 70B, part of the National Electrical Code of the USA. The FLIR Si2 is an easy-to-use tool to perform some of these inspections, with great return on investment and organization scalability.

For more details regarding NFPA 70B 2023, read our article on the new guidelines.
Review the specific thermography regulations within NFPA 70B
Learn how to document thermal inspections findings.
And for a more thorough discussion including a Q&A session, watch an Infrared Training Center webinar on NFPA 70B recommendations 

Contact your local FLIR sales manager

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