Electrical Systems and Controls
Ohm's law and meters
Every electrical question reduces to E = I × R, P = E × I, and P = I² × R for single phase; three-phase power is 1.732 × E × I × power factor. Voltage is measured in parallel with the load; current is measured in series by clamping one conductor only; resistance and continuity are measured with the circuit de-energized. Clamping a cable containing both hot and neutral gives a false zero.
Components you must recognize
A residential air handler typically runs on 240 V with a 24 V control circuit from a step-down transformer. Know the thermostat color code: R (24 V hot), C (common), W (heat), Y (cool), G (fan), O/B (reversing valve). Safety devices - high-pressure switch, low-pressure switch, float switch, limit, and freeze stat - are wired in series in the 24 V circuit so any one opens and stops the call.
Motors: PSC motors use a run capacitor plus a start assist; ECM/constant-torque motors are electronically commutated and programmed, so they are not tested like PSC motors. Compressor terminals are C, R, S; the start winding reads highest resistance, and C-to-S = C-to-R + R-to-S. Capacitors are rated in microfarads and must be discharged before testing; a bulged top or a reading more than about 10% off rating means replacement.
Traps
Confusing a blown low-voltage fuse (no 24 V anywhere) with an open safety (24 V present at the transformer but not at the contactor coil). Assuming a contactor is bad when the coil simply is not receiving 24 V. Testing a capacitor while it is still charged.
Worked example: a compressor drawing 28 A on a 240 V single-phase circuit consumes 240 × 28 = 6,720 VA. If nameplate RLA is 22 A, that reading signals a problem worth investigating before it trips the overload.