Bringing anomalies to light with IR inspection

Conductors

Conductors serve as pathways for current flow and consist of either copper, aluminum, or some hybrid of these two elements combined. Common examples of conductors are wire (stranded, solid), bus bars (usually flat, rectangular shaped, although outdoor substations commonly have tubular bus systems), and miscellaneous components that are present in breakers, disconnects, contactors, thermal overloads, motors, transformers, and other devices where electricity flows.

Compromised connections

All electrical systems have various mechanical connections that join conductors together so that current can flow uninterrupted. Examples of connection types include screw-in or bolted receivers, clips, blades, crimps, sleeves, pressure contacts, and others. Any or all of these types of connections can be encountered during a typical infrared inspection. Some of these connections are visible with direct line of sight, such as a load side wire connection on a single pole breaker. Others may not be visible, such as internal contacts on a three-pole circuit breaker or contacts on a motor contactor. Regardless of their accessibility to sight, the one common function that connections have is to provide a seamless path of electrical continuity from one conductor to the next.

Connections can be compromised either through looseness, breakage, corrosion, or any other means whereby current flow is retarding either partially or completely. Heat is generated at these sites through the physical reduction of conductive surface area created by the compromised connection. Figure-2 shows a schematic cross section of three different wires with increasing degrees of resistance as a result of compromised connections (light blue areas). Assuming that load conditions remain constant for each wire below, the figure shows that current flow becomes more concentrated in conductive areas as areas of resistance increase, resulting in localized hot spots. In effect, a hot spot is a localized overload condition in a conductor’s pathway.

Figure 2. The cross-section view of a conductor under normal load conditions increases in degrees of resistant. As resistant area increases, conductive pathway area decreases and the amount of heat will rise.

Compromised connections are one of the most common problems encountered by thermographers. They are recognized by having a localized heat pattern at the highest point of resistance that dissipates away from the loose connection. When a direct line of sight to the component is available it’s easily recognized (figures 3 and 4).

Figures 3 and 4. When a direct line of sight to the component is available it’s easily recognized. Show typical heating patterns associated with a loose line side conductor connection on a three-pole circuit breaker and a loose bus connection on a two-pole circuit breaker, both showing a direct line of sight to the problem.

Typical line-of-sight connection problems include:

• line and load side conductor connections to breakers, contactors, thermal overload devices, bus bars, and other electrical components
• bus to bus bolted connections
• breaker to bus connections
• fuse clip connections
• knife blade and pivots on disconnect switches
• wire connections at terminal blocks 
• wire nuts and other splices.

When connections are not readily visible from a direct line of sight, determining thermal problems can be challenging. The heating pattern on these types of problems can vary, depending upon the component and the barriers that prevent a direct line of sight. Two examples of internal contact problems are shown in figures 5 and 6. Figure 5 shows centralized heat in the middle of a breaker where the internal connection is located. Figure 6 shows an enclosed fuse block with the middle phase showing a heat pattern that propagates through the fuse block and along the line and load side conductors. When evaluating a thermal pattern such as this, it is important to check amperage to make sure that the thermal pattern you are seeing is not load-generated heat.

Figures 5 and 6. The left image shows a centralized heating pattern on a three-phase circuit breaker with an internal contact problem. The right image shows an internal contact problem on the A-phase with a thermal pattern that propagates through the entire breaker.