Bringing anomalies to light with IR inspection

The magnitude and intensity of inductive heating should not be underestimated when performing infrared inspections of electrical switchgear. Inductive heating is derived from the proximal interaction of non-carrying electric devices with the magnetic field around conductors.

Inductive heating affects ferrous metals causing inexplicable heating on non-current-carrying components (figures 23, 24, and 25). The intensity of heating is a function of the amount of current passing through the conductor and not so much the voltage class. The affected components can reach temperatures in excess of several hundred degrees.

Figures 23, 24, and 25. The left image shows inductive heating on a corner of a switchgear cabinet. The center image is an example of inductive heating on ferrous metal bolts on main bus supports. The right image is inductive heating resulting from tie-wrapped cables in a 480 V main switchgear cabinet.

Common components that can be affected by inductive heating include:

• parts of switchgear cabinets
• bolts and other hardware used to support insulation between conductors
• conductors of one phase that are bound together
• bus-work that travels though walls and other tight areas.

During a recent inspection at a power generation plant, two examples of inductive heating were observed near the plant’s step-up transformers. Images captured show intense heating on a non-current-carrying support pole and bus transition box, both close to iso-phase bus entering a13 kV to 230 kV step-up transformer (figures 26 and 27). Temperatures documented on these devices were in excess of 400 °F. Being the starting point of transmission service, a heavy current load would be expected on energized equipment.

Figures 26 and 27. The left image shows inductive heating on a metal pole located between iso-bus. The right image is an example of inductive heating on transition box.

Often, engineering designs on switchgear enclosures and other electrical equipment do not take into consideration the interaction of non-current-carrying ferrous devices within electromagnetic fields. In some cases, these situations can pose safety hazards when the affected component is in contact with combustible materials or heat structures that are accessible to human contact. When faced with perplexing heat patterns on components that shouldn’t be hot, inductive heating may be to blame.

Defective equipment

It’s quite common that defective electrical components display abnormal heating patterns. Examples of these somponents include:

• UPS batteries
• ribbon bus circuit boards
• current transformers (CTs)
• lightning arresters 
• variable frequency drive (VFD) fans
• metering devices/meter cans 
• stand-off bushing inside main switchgear cabinets.

Examples of the above mentioned defects are shown online in figures 28-34.

	Figure 28. Elevated heating pattern on a UPS battery cell, most likely caused by an internal short causing high resistance.
	Figure 29. Elevated heating pattern on a current transformer in a 480 V main switchgear cabinet, most likely caused by an internal short causing high resistance.
	Figure 30. Elevated heating pattern on an impregnated ribbon circuit board in a 13 kV main switchgear relay cabinet — a weak solder connection.
	Figure 31. Elevated heating pattern on lightning arrester in a 13 kV main switchgear relay cabinet.
	Figure 32. A hot cooling fan motor inside a VFD cabinet. Visual inspection showed the fan blades stationary but the motor still operating.
	Figure 33. A hot spot on a metering can. Mostly there is an electrical short causing high contact resistance or a weak connection.
	Figure 34. This image shows the mounting bracket for a stand-off bushing inside a 13 kV load interrupter switch cabinet. This insulator is allowing current to leak to ground.

Conclusion

Infrared technology is a powerful tool that can be used to detect thermal anomalies in electrical systems. Understanding the relationship between current flow through conductors and the heat pattern associated with current flow is critical for properly diagnosing electrical problems. Furthermore, an understanding of the operating nature of electrical components and the construction details of electrical systems is essential. These skills are gained over time by working closely with qualified electricians who are wiling to share their knowledge and through mentorship programs with experienced thermographers.

While loose connections are the most commonly detected thermal exceptions, overloaded circuits, open circuits, and other electrical defects can pose a great threat to the safety and reliability of electrical systems. Learning to recognize the significance of abnormal heating and the ramifications it has on the operation of an electrical system legitimizes the use of infrared technology and solidifies the role of an infrared thermographer as a key player in the electrical maintenance industry.