What happens
An internal pole or terminal fails to deliver reliable power to the branch.
Typical symptoms
Equipment is dead or has a missing leg despite a handle that looks on.
Checks that distinguish this cause
Qualified electrical personnel compare the supply and branch voltage under the specified conditions and inspect isolated terminations. Panel work requires the appropriate electrical work procedure.
What can look similar
A blown line fuse, utility problem or open downstream wire can also remove power.
Repair direction
Replace the confirmed defective approved device or repair the connection; identify fault damage before re-energizing.
Verify the repair
Confirm the complete equipment supply and operation under load.
Equipment-specific limits
Use the nameplate maximum overcurrent protection, minimum circuit ampacity and installation instructions together. Breaker size is not selected from a single running-current reading.
Practice this diagnosis
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
- Carrier: HVAC installation and commissioning overviewSizing, airflow, charge and A2L installation considerations; obtain the exact unit manual.
Editorial review: 2026-09-29. Verify the current manual for the installed model.