Why there are two different numbers
Every finding is measured two ways, and the worse of the two sets the rating.
The temperature difference — phase B against phase A on the same board, under the same load — is the more sensitive measure, because both components should be doing identical work. If one is hot and its twin is not, the cause is the component rather than the duty.
The absolute fault temperature is the backstop. A connection at 110 °C is dangerous whether or not its neighbours are hot too, and it is the only measure available when there is no equivalent component to reference.
This is why a report that quotes a single temperature is not a finding. 91 °C on a lug means nothing until you know what the other two phases were doing.
The priority ratings in full
| Priority | ΔT vs reference | Fault temperature | Recommended action |
|---|---|---|---|
| 1 — Critical | > 30 °C | > 100 °C | Serious abnormality. Immediate repair recommended — failure may be imminent. |
| 2 — Significant | 20 – 30 °C | 75 – 100 °C | Should be corrected — could fail suddenly. Repair within one week. |
| 3 — Caution | 10 – 20 °C | 50 – 75 °C | Minor problem. Rectify when convenient, then re-inspect for the trend. |
| 4 — Checked | < 10 °C | < 50 °C | No abnormality at inspection. Re-inspect at regular intervals. |
What can make the number wrong
- Load below 40%. A fault under light load may not generate enough heat to register. This is the most common reason a real defect is missed.
- Emissivity error. Shiny copper and painted steel emit very differently; an uncorrected reading on bare copper can be out by tens of degrees.
- Reflection. Polished surfaces mirror the thermographer, the lighting and the camera, producing hot spots that are not there.
- Spatial resolution. If the target is smaller than the camera's measurement spot, the reading averages it with everything around it — always low.
Every one of those errors runs in the same direction: the component reads cooler than it is.
Frequently asked
Is 60 °C on a switchboard connection dangerous?
It depends on what sits beside it. If the other phases are at 55 °C that is normal load heating, and the fault temperature alone puts it in Priority 3. If they are at 25 °C, the 35 °C difference makes it Priority 1 — immediate repair.
Who is qualified to classify a finding?
A Category 2 thermographer certified to ISO 18436-7 or equivalent. A Category 1 holder can capture images; setting and applying criteria is a Category 2 function.