Electrical Systems and Controls Study Guide
This domain, Electrical Systems and Controls, tests safe electrical measurement and the control circuits that sequence HVACR equipment. Expect Ohm’s law, series and parallel loads, transformers, contactors, relays, thermostats, pressure and temperature switches, defrost and safety controls, motor windings, capacitors, and ladder-diagram logic.
Core rules and concepts
- Ohm’s law: E = I × R; power: P = E × I in watts or VA. In series, current is common and voltages add; in parallel, voltage is common and currents add.
- Measure voltage across a load in parallel; measure current by placing the meter in series or by using a clamp.
- A 24-V control transformer has a VA rating; total connected VA must not exceed it, leaving margin for pickup and inrush.
- Ladder diagrams show controls in their unpowered state. Trace from the transformer through safeties, thermostat, and load back to common.
- Lock out and tag out; discharge capacitors; verify absence of voltage with a meter rated for the installation category.
Worked example
A 40-VA, 24-V transformer supplies a contactor coil rated 8 VA, a control board at 6 VA, and a thermostat at 2 VA. Connected load = 16 VA. Current ≈ 16/24 = 0.67 A, and the transformer has 24 VA reserve. Adding a 30-VA accessory would overload it. The reasoning matters: VA adds in parallel, and the transformer must cover steady-state load plus pickup or inrush.
Traps
Assuming a wire is dead because the thermostat is off; measuring current in parallel; using a low-category meter on line voltage; ignoring transformer VA; misreading normally open and normally closed symbols; replacing a capacitor without discharging it.