Infrared Thermography in Electrical Maintenance

Editor’s note: With the fourth edition of ASNT’s Nondestructive Testing Handbook, Vol. 4: Thermal & Infrared Testing now in print—the first full update to the volume in more than 20 years—and updates to NFPA 70B making infrared inspection a mandatory rather than recommended practice, electrical maintenance is where much of that renewed attention is landing. The following excerpt, from Sonny James’s chapter on infrared thermography in electrical maintenance, walks through what a thermographer is actually looking at in the field: how reflections and shiny surfaces distort a reading, why load conditions and ambient temperature change what a scan can and can’t catch, and what separates a technically correct measurement from a useful one. Readers will notice a reference to testing equipment at “40% of design load” per NFPA 70B—that guidance predates the code’s 2023 shift to mandatory inspection and reflects the load-testing practice in place at the time this chapter was written; it’s included here as part of the original text, not as a citation of current code language. It’s reprinted here as a working reference for readers who want the fundamentals in one place.

Thermography is one of the most powerful tools available for electrical maintenance. With professional training and experience, a thermographer can quickly locate high-resistance connections, load imbalances, and overloads while the system is in operation. This can all be accomplished without direct contact to the energized system (Snell 1996).

Substations can be easily and quickly inspected for discontinuities that have the potential to cause equipment failure and process disruptions. Figure 1 shows a connection problem of a bus isolator, while Figure 2 shows a defective lightning arrestor that requires replacement, and Figure 3 shows overheating bushing connections coming from the generator supply to the main step-down transformer (Figure 3b) (James 2009).