Electrical Power and Control Components
This domain tests the devices that switch, protect, and regulate appliance loads. Expect questions on contactors, relays, thermostats, limits, switches, ignition components, cords, and bonding.
Switching and protection
- Contactors and relays: coil voltage, normally open/closed contacts, pull-in and drop-out. Pitted or welded contacts can read continuity but fail under load; check voltage across contacts while energized.
- Overloads: thermal and bimetal types, manual or automatic reset. Never bypass a safety.
- Thermostats: mechanical and electronic; cycling differential, anticipation, and calibration. A thermostat that is out of calibration may still "click."
- Limits and fuses: thermal fuses, high-limit thermostats, and fusible links protect against overheating. Replace only after finding the cause.
- Pressure, float, sail, and door switches: verify the switch changes state, not just that it has continuity.
- Timers and defrost boards: check inputs and outputs; a board can have power and still not switch a load.
Ignition and valves
- Spark modules, hot-surface igniters (often 40–100 Ω), and flame sensors (microamp signal). A dirty flame sensor causes short cycles.
- Gas valves and solenoids: coil resistance, voltage at the operator, and correct manifold pressure.
- Water valves: inlet screens, coil resistance, and flow.
Cords, receptacles, and bonding
- A 240 V dryer or range uses a 3-wire or 4-wire cord matching the receptacle. On a 4-wire installation, the neutral is separate and the bonding strap is removed; on a 3-wire, the frame is bonded to neutral as permitted by the installation instructions and code.
- Confirm polarity and ground at the receptacle; a reversed neutral/ground is a shock hazard.
- GFCI protection is required in specified locations; do not nuisance-trip troubleshoot by removing it.
Worked example
An electric dryer runs but does not heat. You measure 240 V between L1 and L2 at the terminal block, 120 V from each leg to neutral, and 0 V across the element when the control calls for heat. The element is open. But you also check the thermal fuse and high-limit, and the blower wheel and vent, because an open element may be the result of restricted airflow. You replace the element, clean the vent, and verify current draw against nameplate.
Traps
- Replacing a blown thermal fuse without correcting airflow.
- Fitting a 3-wire cord to a 4-wire receptacle.
- Assuming a clicking thermostat is good.
- Measuring contact continuity only, not voltage drop under load.