Infrared Thermography Severity and Emissivity Reference
Infrared inspections only help if you can rank what you find and read temperatures correctly. This reference gives typical repair priorities by temperature rise and typical emissivity values by material, in one searchable table.
| Group | Item or basis | Value | Guidance |
|---|---|---|---|
Temp rise vs similar component | Priority 4 | 1 to 3 deg C | Possible deficiency, warrants investigation at the next outage |
Temp rise vs similar component | Priority 3 | 4 to 15 deg C | Probable deficiency, repair as time permits |
Temp rise vs similar component | Priority 2 | 16 deg C and above, or lower with evidence of damage | Major discrepancy, repair as soon as possible |
Temp rise vs similar component | Priority 1 | Severe and rising quickly | Repair immediately, consider de-energizing if safe to do so |
Temp rise vs ambient air | Possible deficiency | 1 to 10 deg C | Investigate at next scheduled outage |
Temp rise vs ambient air | Probable deficiency | 11 to 20 deg C | Repair as time permits |
Temp rise vs ambient air | Deficiency | 21 to 40 deg C | Repair at the next opportunity |
Temp rise vs ambient air | Major discrepancy | Above 40 deg C | Repair immediately |
Inspection conditions | Load | At least 40 percent of rated load preferred | Low load hides faults, record the actual load at the time of the scan |
Inspection conditions | Wind and sun | Avoid outdoor scans in strong wind or direct sun | Wind cools the surface and sun adds solar heating, note both on the report |
Inspection conditions | Covers and doors | Open panels using the site arc flash procedure | Cameras do not see through steel covers or glass |
Emissivity (typical) | Matte black paint | 0.94 to 0.97 | Good target for thermal imaging, treat as about 0.95 |
Emissivity (typical) | Black electrical tape | About 0.95 | Put a small piece on bare metal to get a stable reading |
Emissivity (typical) | Painted metal surface | 0.90 to 0.95 | Reliable for enclosures and motor frames |
Emissivity (typical) | Porcelain insulator | 0.92 to 0.94 | Reads well, watch reflections from nearby sources |
Emissivity (typical) | Concrete | 0.92 to 0.95 | Useful for floor and structure scans |
Emissivity (typical) | Rubber | 0.90 to 0.95 | Applies to cable jackets and belts |
Emissivity (typical) | Water | About 0.96 | High emissivity, but only the surface layer is seen |
Emissivity (typical) | Oxidized steel | 0.70 to 0.90 | Varies with rust level, verify with tape |
Emissivity (typical) | Oxidized copper | 0.60 to 0.80 | Bus bars and lugs vary a lot, use tape to confirm |
Emissivity (typical) | Bare copper, clean | 0.02 to 0.10 | Very low and reflective, readings are unreliable without tape |
Emissivity (typical) | Polished aluminum | 0.04 to 0.10 | Very low, a bare shiny surface looks colder than it is |
Emissivity (typical) | Glass | 0.90 to 0.95 at the surface | Opaque to long wave infrared, you cannot see through it |
Temperature rise tables follow the widely used NETA style of priorities. Many sites adopt their own limits, so confirm against your site standard and manufacturer data. Emissivity figures are typical ranges, not calibrated values.
Turn inspection findings into scheduled work
Optional: we can review how thermography and other findings become work orders in your maintenance system.
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How to use it
Scan under load, then measure the temperature of the suspect connection and of a similar component carrying the same load, for example the other two phases. The difference is the temperature rise against a similar component. If no comparison exists, use the rise over ambient. Look up the rise in the matching group to get a priority and the typical repair timing. Record the load, emissivity setting, reflected temperature and distance in the report so the finding can be repeated.
- Compare phase to phase before comparing to ambient
- Set emissivity before reading temperatures
- Record load current at the time of scan
- Re-scan after the repair to confirm the fix
Why emissivity matters
A thermal camera measures radiated energy and converts it to temperature using an emissivity setting. Shiny metals have low emissivity and reflect their surroundings, so a hot bare copper lug can look cooler than a painted enclosure beside it. Using black tape or a painted spot with known emissivity solves most problems. Never trust absolute temperatures from bare polished metal.
Worked example
A motor control centre shows 78 deg C on the B phase lug and 52 deg C on the A and C lugs under 60 percent load. The rise against similar components is 26 deg C, which sits in the highest group of the table above. That is a repair soon item, so raise a work request, tighten or replace the lug under a proper isolation, and re-scan. Note that current rises with the square of load, so the temperature rise at full load will be larger still.
Common mistakes
Scanning at low load gives a false pass. Ignoring reflections from nearby hot objects leads to false positives. Using a single fixed emissivity for every surface gives wrong absolute values. Finally, thermography finds only what is visible from outside, so closed enclosures need an approved procedure or an infrared window.
Frequently Asked Questions
What temperature rise is considered serious?
Under common electrical guidelines, a rise of about 16 deg C or more over a similar component under load is a major discrepancy that needs prompt repair. Your site standard or insurer may set tighter limits, so check those first.
What emissivity should I use for a bus bar?
It depends on the surface. Painted or taped bars read near 0.95. Bare oxidized copper may be 0.6 to 0.8 and clean copper far lower. Put a small piece of electrical tape on the target and set emissivity to 0.95 for a reliable reading.
How often should electrical thermography be done?
Annual is a common baseline for critical electrical equipment, with higher frequency for heavily loaded or unstable circuits. Many insurers also expect an annual scan. Always scan after major repairs or load changes.
Can I use this table for mechanical equipment?
The priority rows are intended for electrical connections. For bearings and motors, trend the temperature against the machine baseline and against similar machines, and combine the result with vibration and lubrication data.
We can help connect inspection findings to work orders in your ERP or CMMS so each priority becomes a scheduled job. Ask for a short expert review of your inspection workflow.
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