Thermography Fundamentals
All objects emit infrared radiation proportional to their temperature (Stefan-Boltzmann law). An IR camera detects this radiation and displays it as a false-colour temperature map (thermogram). Abnormal heat generation — from electrical resistance, friction, chemical reaction, or loss of insulation — appears as a hot spot relative to adjacent components.
Key technical parameter: emissivity. Emissivity is the fraction of radiation that an object emits relative to a perfect black body (1.0). Shiny metallic surfaces have low emissivity (0.05–0.3) and require correction in the camera settings — failure to set emissivity correctly causes large temperature measurement errors. Painted or oxidised surfaces typically have emissivity 0.85–0.95 and cause fewer measurement errors.
Electrical Applications
Electrical thermography surveys are conducted with panels under load (typically >40% of rated current for meaningful results). Open panel doors safely per electrical safety procedures. Common findings:
- Loose connections: Increased resistance at bolted, crimped, or clamped connections generates heat proportional to current squared. Progressive loosening leads to arcing, carbon tracking, and fire.
- Overloaded conductors: Cable carrying more than rated current shows uniform heating over its length.
- Failing switchgear contacts: Elevated temperature at contactor or breaker contact block.
- Harmonic heating: Neutral conductors carrying high harmonic current overheat even when phases appear normal.
- Unbalanced loads: One phase significantly hotter than others — check load balance.
Severity Classification
| Temperature rise (ΔT above reference) | Severity | Recommended action |
|---|---|---|
| <5°C | Negligible | Monitor at next scheduled survey |
| 5–10°C | Low | Record and monitor; schedule repair within 3 months |
| 10–25°C | Medium | Schedule repair within 1 month; increase survey frequency |
| 25–50°C | High | Repair within 1 week; consider load reduction in interim |
| >50°C | Critical | Immediate action — isolate or reduce load now |
ΔT is measured as the hottest point in the anomaly minus the temperature of a similar component in normal operation (reference temperature), both under the same load conditions.
Mechanical Applications
- Bearing overheating: Trend bearing housing temperature as a simple screening tool — rise of >40°C above ambient indicates lubrication problem or bearing damage
- Blocked heat exchangers / cooling fins: Uniform hot or cold zones indicate fouling
- Refractory and furnace wall inspection: Hot spots indicate refractory damage or insulation loss
- Steam trap condition: A passing (failed-open) steam trap shows elevated temperature on the downstream pipe compared to a functional trap (which stays cooler on the downstream side between trap actuations)
- Pipe insulation quality: Cold bridges and missing insulation sections visible as hot patches in winter conditions
Survey Reporting
A professional thermography report for each anomaly should include:
- Thermogram (IR image) and visible-light photograph side-by-side
- Maximum temperature at hot spot; reference temperature at similar component
- ΔT and severity classification
- Location: panel/circuit reference; equipment ID
- Probable cause
- Recommended corrective action and priority
- Load at time of survey (% of rated)
- Ambient temperature; humidity; camera settings (emissivity, distance)
This documentation is required for insurance purposes and to demonstrate due diligence. Many electrical insurance policies now require annual thermographic surveys on HV/LV switchgear.