A technician opens the electrical panel of a rooftop unit after pulling and locking out the disconnect at the curb. What inside that panel still has to be treated as live?
Correct Answer
C) The run and start capacitors, which can hold a dangerous charge after power is removed
Locking out the disconnect removes the incoming supply but does not empty the capacitors. A run or start capacitor can hold enough stored energy to injure or kill after the power is gone, and start capacitors on larger equipment are particularly dangerous. Capacitors must be discharged through an appropriate resistor before terminals are touched, and the absence of voltage should be verified with a meter that has been proven on a known live source. There is also often a second power source feeding controls from a separate circuit.
Why This Is the Correct Answer
Capacitors store energy by design and hold it after the supply is removed. That stored charge is available at the terminals whether or not the disconnect is locked out, so it has to be deliberately discharged and then verified as absent before any work begins.
Why the Other Options Are Wrong
Option A: The low-voltage thermostat wiring, which carries line voltage whenever the disconnect is open
Thermostat wiring runs at low voltage and it does not become energized because the disconnect was opened. That description reverses how the circuit works.
Option B: The compressor terminals, which stay energized by the crankcase heater circuit
A crankcase heater is fed from the same supply that the disconnect removes, and it does not energize compressor terminals for service purposes.
Option D: The contactor coil, which remains energized until the unit finishes cooling down
A contactor coil is de-energized when its supply is removed. It has no stored energy and does not stay hot while equipment cools.
Memory Technique
The lock stops the power coming in. It does nothing about the power already parked in the capacitor.
Reference Hint
Locking out the disconnect removes supply voltage only; capacitors retain stored charge and must be discharged and verified before contact.
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