Breaker Tripping With A Hot Smell: Damaged Isolator, Not Compressor

• •Concern: Risk appeared to be an internal compressor short requiring a full and expensive replacement.
• •Key check: Checked isolator and terminal heat marks before condemning the compressor

Breaker trips with a localised smell need the power path checked before the compressor is opened. We started at the isolator and worked inward along the cable run. The key observation was the trip pattern: intermittent under load, not instant on every start attempt. A true compressor short circuit trips the breaker the moment power is applied, every single time, with no variation. An intermittent trip under load points to a resistive fault that builds heat gradually until the breaker threshold is exceeded. We followed the smell to the isolator box and opened it for inspection.

• •Trips were intermittent under load, not instant on every start attempt.
• •Isolator terminal showed visible heat discolouration and pitting on the contact surface.
• •Hot smell concentrated at the isolator box, not at the compressor housing.

One isolator terminal had loosened over twelve years of thermal cycling, the repeated expansion and contraction of the metal as the connection heats under load and cools when the unit stops. Each cycle loosened the terminal fractionally until the contact pressure dropped below the threshold needed for clean current flow. The loose contact created high resistance at that point, and under compressor load current, that resistance converted electrical energy into heat. The hotter the connection got, the more the metal expanded and the worse the contact became, creating a runaway feedback loop. This is why the trips grew more frequent over the week: each episode left the terminal slightly worse than before. The hot electrical smell was localised at the isolator box because that was the only point generating excess heat. The compressor, wiring, and breaker were all functioning correctly.

We explained the findings and recommended replacing the heat-damaged isolator terminal and cleaning all contact surfaces in the switch assembly. The existing terminal was too pitted and deformed to re-torque reliably. Tightening a damaged contact only buys a few months before it loosens again under thermal cycling. After fitting a new terminal and torquing every connection in the isolator box to the manufacturer's specification, we ran the unit under full compressor load for thirty minutes continuously. We monitored the isolator for any heat buildup using a thermal reading at the contact point and confirmed the breaker held steady with no trips or temperature rise throughout the test.

Trips stopped, the hot smell did not return, and full cooling resumed with the original compressor still in service.

Intermittent trips with a hot smell usually live in the power path.

• •A true compressor short circuit trips the breaker instantly on every start attempt, with no variation. Intermittent trips that come and go under load point to a resistive fault building heat somewhere in the power path, usually a loose connection that worsens as it heats up.
• •Follow the smell. Electrical odour localised near the isolator or cable entry narrows the search to the power path before any compressor testing is needed. Ask your technician where the smell is strongest, that location is almost always where the fault sits.
• •Isolator terminals and cable lugs are service items that wear over time. Thermal cycling (the repeated heating under load and cooling when off) gradually loosens connections. A torque check during routine servicing catches this early, before it becomes a safety concern.
• •If a contractor diagnoses compressor failure based on breaker trips alone, ask whether the trip pattern was instant or intermittent. Instant means the compressor may indeed be shorted. Intermittent under load points to the wiring path, and the compressor should not be condemned without a winding test.