A connection that is failing electrically is failing thermally first. Increased resistance produces heat that is invisible to the eye and imperceptible until the failure is already advanced. An infrared camera shows it directly. A substation that gets scanned annually under load gives the maintenance team a complete picture of what is heating abnormally, ranked by severity, without taking anything out of service.
Loose or corroded connections are the most common finding. A bolted connection that has worked loose, corroded at the contact interface, or was never properly torqued at installation will show elevated temperature compared to the identical adjacent connection carrying the same current. The temperature rise is proportional to I²R — so a high-resistance connection on a high-load circuit heats dramatically, while the same connection on a lightly loaded circuit may show only a modest elevation that worsens significantly when load increases.
GOAB and disconnect switch contacts that have lost contact pressure or developed oxidation show hot jaws relative to the blade and the connecting conductor. A jaw that is 15°C above the conductor temperature under normal load is already a problem. One that is 40°C above is approaching a maintenance emergency.
Transformer bushings with internal degradation sometimes show thermal anomalies detectable by IR before the power factor test result climbs to an alert level. A bushing that is hotter than its adjacent phases at the same load, or one that is hotter at the flange than at the upper end, can indicate an internal problem worth investigating with a tap test.
Overloaded conductors and cables show up as uniformly elevated temperature over their length. This is a different pattern from a connection problem, which produces a localized hot spot. A cable that is 20°C above ambient along its entire run is operating above its rating. This is a capacity planning finding rather than a maintenance finding, but it is important to capture.
Failing insulators in a string can show abnormal temperature distribution. A single bad insulator in a multi-disk string takes a disproportionate share of the voltage and dissipates more power than the adjacent units. The thermal signature of a failing disk is subtle compared to a hot connection, but it is detectable with a quality camera at appropriate distance.
LTC compartments — particularly the drive mechanism and contact area — sometimes show externally detectable thermal patterns when internal problems are developing. A transformer where the LTC compartment surface is significantly hotter than the main tank in a region that does not correlate with normal heat rejection patterns warrants a closer look.
An IR scan performed at low load produces results that cannot be compared to one performed at peak load. Temperature rise at a resistive fault is proportional to I² — doubling the current quadruples the heating. A connection that shows a 5°C rise at 30% of rated load may show a 45°C rise at full load. For this reason, NETA and NFPA 70B recommend performing IR scans when the circuit is loaded to at least 40% of rated capacity, with higher load producing more definitive results.
Scanning at night or on overcast days eliminates solar loading, which can heat conductive surfaces and mask or amplify the thermal signature of a fault depending on orientation. A bus connection on the south face of a structure will appear hotter in afternoon sun than an identical connection on the north face regardless of their electrical condition. Night scanning removes this variable entirely.
Wind is a complicating factor — convective cooling from wind reduces the measured temperature of a hot connection, potentially masking a problem. In locations where afternoon thermal winds are common, morning scans in calm conditions give more reliable results. Document ambient temperature, wind speed, and load at the time of the scan for every report so that future scans can be compared under equivalent conditions.
The most widely used severity classification for electrical IR findings compares the temperature of the anomalous component to a reference temperature — either an identical component on an adjacent phase or a similar connection not showing the anomaly. NETA MTS and NFPA 70B both provide severity frameworks:
A ΔT of 1–3°C above a similar component warrants monitoring — rescan at the next scheduled interval and track the trend. A ΔT of 4–15°C indicates a possible deficiency and should be investigated and scheduled for repair at the next planned outage. A ΔT greater than 15°C indicates a serious problem that requires prompt repair, with the repair priority increasing as the temperature rise increases. A component showing a ΔT above 40°C relative to its reference should be considered for immediate repair or load reduction while a repair is scheduled.
These thresholds are guidelines, not hard rules. A ΔT of 12°C on a lightly loaded circuit that will see significantly higher load seasonally may need faster attention than the scheduling bracket suggests, because the actual temperature rise at peak load will be much higher. Conversely, a ΔT of 20°C on a component that is already near its thermal rating at current load may be more urgent than one at the same delta on a lightly loaded circuit. Use the classification as a starting point and apply engineering judgment about where the equipment is in its thermal margin.
An IR report with findings ranked by severity gives the maintenance planner a prioritized work list. The highest-priority items drive outage planning — which equipment needs to come out of service first, and which work can be batched with other planned maintenance to minimize outage count.
After any repair is made — a connection retorqued, contacts cleaned and lubricated, a component replaced — the repair should be verified with a follow-up IR scan before the circuit is returned to service and then again at the next scheduled load condition. A connection that still shows elevated temperature after repair was either not fully repaired or has a more fundamental problem than a loose bolt.
Trend the results across multiple years. A substation with consistent annual IR scan data builds a baseline that shows which equipment is stable, which is slowly degrading, and which has suddenly changed. A connection that was clean for three scans and is now showing a ΔT of 8°C is more urgent than one that has been at 8°C for three years — the new onset suggests an active change in condition rather than a long-standing but stable deficiency.
We perform infrared thermography scans on substations across Florida and the Southeast as part of our field maintenance and diagnostic services. Send us your site details and we’ll respond within one business day.