Bringing anomalies to light with IR inspection

Figures 7 and 8 are examples of deteriorated connections on an enclosed overhead bus duct. If an overload or load imbalance condition existed here, one would expect to see a warm thermal anomaly along the entire length of the bus. However, the warm thermal pattern is restricted to a small section of bus.

Figures 7 and 8. These show deteriorated connections on an enclosed overhead bus duct. If an overload or load imbalance condition existed here, one would expect to see a warm thermal anomaly along the entire length of the bus.

Figures 9 and 10 are examples of loose connections obstructed by the components inside the electrical enclosure. Figure 9 is a wire harness on a three-pole circuit breaker inside a motor control bucket. The actual loose connection is a spring tension clip that attaches to the bus behind the bucket enclosure. Figure 10 is suspected to be a loose bolted bus connection that is obstructed from view by a piece of insulation board.

Figures 9 and 10. The left image shows a loose bus connection on a wire harness feeding a circuit breaker inside a motor control bucket. The right image shows a suspected bus/fuse holder connection problem hidden behind the insulation board inside a switchgear cabinet.

Typical blocked line of sight connection problems include:

• internal contacts on breakers, contactors, and fuse blocks
• bus to bus bolted connections on enclosed overhead bus work
• pressure/blade contacts on transfer switches and other disconnects
• various connections hidden from view inside switchgear enclosures
• any other type of connection that is obstructed from view.

Open circuits

Conductors displaying a colder-than-normal pattern under normal operating conditions, when compared to adjacent conductors under similar loads, usually signify an open circuit somewhere along the line. The reason a cold thermal pattern exists is the lack of current flow. This can happen for various reasons, but the most common situations arise from broken conductors and single phased motors.

During a recent inspection at an industrial site, a 25 °F degree temperature rise was observed on one of two 400 A rated parallel feed conductors that linked an 800 A three-phase breaker to a motor control center. When performing infrared inspections of parallel feed conductors, it is important to understand that paired conductors are sharing load and therefore should have identical thermal patterns. An Ampere reading showed that the warm conductor was carrying 450 A, while the paired conductor, which showed a much cooler temperature, had less than 1 A (Figure 11a). An infrared inspection at the main lug compartment of the motor control center showed the same thermal relationship as observed at the main breaker but also showed a deteriorated connection that no longer was capable of carrying load (Figure 11b).

Figure 11. An infrared inspection at the main lug compartment of the MCC showed the same thermal relationship as observed at the main breaker but also showed a deteriorated connection that no longer was capable of carrying load.

A similar situation was encountered during an inspection of a motor control center bucket. It was noticed that the C-phase thermal overload unit appeared appreciably cooler than the A- and B-phase thermal overload units (Figure 12a). An amperage check showed no current flow on the C-phase circuit, running under a single phase condition. The motor that was being fed from this bucket was inspected and found to have an operating temperature of 248 °F, as compared to a tandem motor operating at 156 °F (Figure 12b). The motor was taken out of service and replaced.

Figure 12. The left image shows a C-phase thermal overload unit that is cold and carries no load. The right image shows the single-phased motor with internal short.