What ΔT is considered a critical electrical fault?
Justin Dravsnik · CAT 2 Thermographer (ISO 18436-7) · Updated 14 August 2026 · 6 min read
A temperature difference greater than 15 K between a component and a similar component under similar load, or greater than 40 K above ambient, is classified as a major discrepancy requiring immediate repair. Below that, 4–15 K indicates a probable deficiency to be repaired as scheduling permits, and 1–3 K warrants investigation at the next outage.
Why there are two different numbers
Every thermographic finding can be measured two ways, and they do not agree. Comparing against a similar component — phase B against phase A on the same board, under the same load — is the more sensitive measure, because both should be doing identical work. Comparing against ambient is cruder but is the only option 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 severity bands in full
| ΔT similar component | ΔT ambient | Classification | Recommended action |
|---|---|---|---|
| 1–3 K | 1–10 K | Possible deficiency | Investigate at next scheduled outage |
| 4–15 K | 11–20 K | Probable deficiency | Repair as scheduling permits |
| — | 21–40 K | Deficiency | Monitor until corrective measures taken |
| >15 K | >40 K | Major discrepancy | Repair immediately |
What can make the number wrong
- Load below 40%. A fault under light load may not generate enough heat to register.
- 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.
- Spatial resolution. If the target is smaller than the camera's measurement spot, the reading averages it with everything around it — always low.
Frequently asked
Is 60 °C on a switchboard connection dangerous?
On its own, unknowable. If the other phases are at 55 °C it is normal load heating. If they are at 25 °C it is a major discrepancy needing immediate attention.
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.