Bringing anomalies to light with IR inspection

Overload conditions

Electrical systems are designed and sized to meet anticipated load demands based on a facility’s operations. Load demand is calculated based on the number of motors, lights, receptacles, machinery and other load devices present in a facility. Ideally, electrical systems are engineered around a facility’s specific needs and include additional capacity to accommodate future expansion. Over the course of 10 years of infrared inspections for any given facility, load capacities can range from 25% up to 70%, with an ideal system operating at just less than 50% capacity.

National Electric Code 220-10(b) states that circuit load should not exceed 80% of conductor ampacity or 80% of the over-current device rating. This includes main branch circuits as well as individual single-pole circuits. If this criterion is exceeded, an overload condition develops.

Overload conditions arise in electrical systems for various reasons. Common reasons include:

• the expansion of a facility’s electrical needs outgrows the original design capacity
• conductors and other components are not sized properly in the original design to meet load demands
• permanent and/or temporary load devices are added to a circuit after the original design that causes loads to exceed the capacity of the circuit.

Infrared cameras are capable of detecting load-generated heat in electrical systems. Load-generated heat develops through the interaction of electrical particles as they move through a conductor. A typical load heat pattern is uniform in appearance and propagates through an entire circuit. The amount of heat generated depends upon the amount of load and the ambient operating temperature of the equipment. Figure 13 shows a circuit breaker panel under normal operational load. Notice the uniform heat pattern on each breaker and the slight thermal differences between breakers of varying loads. Figure 14 shows a three-phase transformer bank and service feed to a building that shows load-related heat responding to the building’s operations. Under normal conditions, load-generated heat should show small to moderate temperature rises above no-load circuits and not pose an unsafe condition to the electrical system.

Figures 13 and 14. The left image shows a typical thermal pattern of load-generated heat in a circuit breaker panel. The right image shows a load-related heat on a three-phase transformer bank feeding a building.

Circuits with elevated heat patterns as shown in figures 15 and 16 are examples of over-load conditions. To determine if a circuit is operating within its rated ampacity, the amount of load on a circuit must be measured using an ammeter. Simple calculations can be run to determine if a circuit is operating below an 80% load capacity. Document a circuit as a thermal exception when its load is at or near 80% load capacity. As a precautionary measure, service and/or replace components that have been under high load conditions once the abnormal load condition is rectified, as extended periods of elevated load and subsequent heat can weaken connections.

Figures 15 and 16. The left image shows a three-phase circuit breaker with an overload condition on the top phase. The right image shows are two contactors and thermal overloads under operating conditions with the right component appearing warmer than the left. A load reading on the right component showed it operating above 80% load capacity.

Determining the difference between thermal patterns caused by load and those created as a result of a compromised connection is established by comparing load readings. Load-related thermal exceptions are only present under abnormal high circuit loads. Connection problems will show abnormal heat patterns under normal load conditions, as well as elevated load conditions. If a component displays an abnormal heating pattern under normal load conditions, a compromised connection, either internally or externally, is most likely the cause.

Figure 17. Elevated heating on the A-phase fuse was caused by an undersized fuse for the given load.

Improperly sized electrical components can also create overload conditions. This is common in fused components where a fuse is inadvertently replaced by a smaller size fuse. Figure 17 shows a three-phase disconnect operating at 70 A of current per phase. The A-phase fuse has a rating of 75 A, and the B- and C-phase fuses are rated at 100 A and 125 A, respectively. The thermal pattern for each fuse reflects the load capacity for each circuit: A-phase operating at 93% load capacity, B-phase at 70%, and C-phase at 56%.