By now you know heat follows resistance, and that where the heat sits tells you what kind of problem it is. This lesson turns that into a short list of patterns you will actually meet.
The pattern: a bright, tight hot spot right at the termination. The conductor behind it is close to normal. Heat fades within a few inches.
Why: extra resistance at one point. Same current, higher R at that spot, so watts are dumped there and nowhere else.
Most common causes: never torqued to spec, torqued and then relaxed, corrosion, dissimilar metals, a strand count that was wrong at install.
This is the bread and butter of electrical thermography. It is also the most satisfying, because the repair is usually cheap and the consequence of ignoring it is usually expensive.
The pattern: the whole conductor runs warm, evenly, end to end. Often all three phases together. No single bright point.
Why: resistance is spread along the conductor, so the heating is too.
What it is not: a connection problem. If you call an overload a loose lug, someone retorques a perfectly good connection, the heat comes back, and the report loses credibility.
Check the actual load current against the conductor rating before you write this one up. The camera suggests overload; a clamp meter confirms it.
The pattern: one phase consistently warmer than the other two along its whole length — not at one joint, along the run.
Why: that phase is carrying more current than the others. Single-phase loads distributed unevenly across a three-phase panel is the usual cause.
This is a design or loading issue, not a hardware fault, and the fix is usually rebalancing circuits rather than replacing anything.
The pattern: one phase noticeably *cooler* than the others.
Why: it is not carrying current. An open fuse, a failed contact, a dropped phase.
New thermographers miss these constantly because they are scanning for bright spots. Train yourself to notice a phase that is too quiet. On a three-phase motor circuit, a cold phase is a serious finding — single-phasing will cook a motor.
Fuses: a fuse running hot at its clips is usually a connection problem at the clip, not the fuse element. A fuse hot along its body may be undersized or near end of life. A cold fuse in a bank of warm ones is open.
Breakers: heat at the lug is a connection issue. Heat in the case body is more concerning — it suggests something internal, and internal breaker problems do not get better.
A 600 amp three-phase service to a plastics plant. Annual route scan, covers on, through IR windows, at roughly 70 percent of rated load.
Readings at the main breaker line side: A 44 °C, B 47 °C, C 45 °C. Normal.
Readings at the load side going into the distribution bus: A 44 °C, B 46 °C, C 78 °C.
The heat appears at the C phase load-side termination, nowhere else, and the bus beyond it comes back down. Line side is normal, so the problem is not upstream. The bus is normal, so it is not a downstream load.
That isolates it to the joint. Delta T against the other two phases: about 33 °C. The plant's own severity criteria put that in their highest urgency band, so the finding was written up for repair at the next available outage rather than at the next annual.
On teardown: the load-side lug had been reused during a breaker swap two years earlier and the hardware was the wrong length, so the clamping force was coming from two threads.
You can see these patterns. You can describe them accurately. You can measure and record them.
What you cannot yet do is make the severity call. "How hot is too hot" depends on the employer's written criteria, on load at the time of measurement, on ambient, on emissivity, and on the uncertainty in the measurement itself. That judgment belongs to a qualified thermographer working to a written practice — Level II work, and the quantitative skills behind it are the backbone of VET-2.
Report what you measured and what you saw. Do not guess at urgency. A careful observation handed to a qualified person is worth far more than a confident guess.
Take the free 15-question electrician readiness checkVoltmark Thermography Academy is Voltmark's own training program. It is not ASNT certification and it is not affiliated with, endorsed by, or sponsored by ASNT, NFPA, ICC, or any certifying body or licensing authority. Certificates issued by this program attest to completing Voltmark training and passing Voltmark examinations. Employer qualification under NFPA 70E and any written practice remains the employer's responsibility. Always follow your employer's safety program and your supervising electrician.
Electrician licensing exam prep: practice questions, timed exam simulations, and step-by-step help finding every answer in the NEC.