Electrical panel thermography: what the camera sees before the failure
A loose connection, an overloaded phase and a breaker with worn contacts show up as heat weeks before they become downtime. NETA temperature-difference criteria, the minimum load for an inspection, and what safety requires of whoever opens the panel.
Every electrical problem turns into heat before it turns into a failure. A loose connection has higher resistance, and resistance under current heats up. A breaker with worn contacts heats up. A phase carrying more than the others heats up. A thermal camera sees all of this weeks before anyone smells something burning, which is why thermographic inspection of panels is one of the highest-return routines in electrical maintenance.
This article explains what thermography detects in a panel, which temperature criterion to use when deciding what to do, and what safety requires of the people doing the inspection. It is written for the maintenance manager and for the electrician who will open the panel door.
What the camera sees
A thermal image shows the temperature distribution across a surface. In a panel, the typical patterns are:
- Loose or oxidized connection: a hot spot concentrated at the terminal, with the temperature dropping along the conductor. It is the most common defect and the most dangerous, because contact resistance grows over time and the heating feeds on itself. Retightening does not always fix it: arcing inside the thread leaves the wire loose even with the screw tight.
- Phase unbalance: one of the three phases hotter than the other two under the same load. The cause may be uneven load, a bad connection on one phase or a problem in the downstream equipment. Heating grows with the square of the current, so even a modest current difference shows up in the image.
- Breakers and disconnect switches: heat at the contacts or terminals, from contact wear, insufficient contact pressure or a stuck mechanism. A contact that welds will not open on a short circuit.
- Undersized or overloaded conductor: the whole cable is hot, evenly, not just the terminal.
- Transformers and drives: asymmetric heat patterns between phases or windings.
What the camera cannot see: the inside of a closed component, or anything behind a steel door. Thermography is a surface technique.
The criterion: temperature difference, not absolute temperature
A terminal at 60 °C can be normal in a panel at full load and alarming in a panel at 30% load. That is why the criterion used in practice is delta T, the temperature difference between the suspect spot and a reference.
The most widely used reference is the suggested-actions table in ANSI/NETA MTS (Maintenance Testing Specifications), which compares in two ways: between similar components under similar load (phase A against phase B, for example) and between the component and ambient air.
- 1 to 3 °C between similar components, or 1 to 10 °C above ambient: possible deficiency, investigate.
- 4 to 15 °C between similar components, or 11 to 20 °C above ambient: probable deficiency, repair as time permits.
- 21 to 40 °C above ambient: monitor until corrective measures can be taken.
- Above 15 °C between similar components, or above 40 °C over ambient: major discrepancy, repair immediately.
NETA itself points out that these values are guidance in the absence of equipment-specific criteria: cables, busbars and breakers have different temperature ratings, and when the manufacturer specifies something, the manufacturer rules.
Load changes everything
Because heat grows with the square of the current, a panel inspected at low load hides defects. The practice recommended by NFPA 70B is to inspect with the equipment at at least 40% of rated current, preferably above 80%, after half an hour or more in that condition. Inspecting a pump panel on a day the pump is off is worthless. Record the load along with the image: without it, the comparison with the next inspection does not add up.
Emissivity and reflections
Polished copper and bare aluminum have low emissivity: they reflect their surroundings and look cooler than they are. The rule of thumb is to compare identical components, made of the same material, in the same image, and to be suspicious of a shiny busbar that looks cold next to an insulated terminal.
Safety: the open panel is the hazard
Thermography is done with the panel energized and under load. That means opening the door with live busbars. NFPA 70E treats removing covers and opening doors as exposure to energized parts: it requires an arc flash risk assessment, defined approach boundaries and PPE rated for the panel's incident energy. The thermographic inspection itself, performed outside the restricted approach boundary, carries a low likelihood of an arc flash; the act of opening the door does not. Two people, arc-rated PPE, and the person who opens the panel is a qualified electrician under your local electrical-safety rules, not the thermographer.
The alternative that removes much of the risk is infrared inspection windows installed in the panel door: the camera sees through them without anyone opening anything. For critical panels that will be inspected frequently, that is the way to go.
From periodic inspection to continuous monitoring
An annual or semiannual inspection finds whatever is hot that day. What it misses is the evolution: a terminal that was 2 °C above its neighbors in March and 8 °C in June is telling a story, and that story is what lets you schedule the repair. Fixed thermal cameras aimed at the panel or at the bearing record the pattern at regular intervals, and the platform compares each image with the history of the same spot. A new hot spot becomes an alert by email and WhatsApp; a stable spot is simply logged. Camera position and type are defined per asset, in the proposal, because a closed panel, an open busbar and a mill bearing call for different solutions.
What to do now
- Rank your panels by criticality: MCCs for lines that cannot stop, the main power entrance, pump panels.
- Do the first inspection with the load above 40% and record the current of each phase along with the images.
- Classify each finding using the NETA table and schedule the repair according to its class.
- On panels that will be inspected regularly, install an infrared window or a fixed camera.
- Repeat, compare, and treat a growing delta as a priority, even if it is still in the "investigate" range.
If you want to know what your plant needs, talk to one of our engineers.
Sources: ANSI/NETA MTS, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems, Table 100.18 (suggested actions for thermographic surveys); NFPA 70B, Standard for Electrical Equipment Maintenance; NFPA 70E, Standard for Electrical Safety in the Workplace; Fluke, application note Electrical inspections using thermal imaging (Jim White, Shermco Industries).