When measuring resistance of a component connected in a circuit, which statement is incorrect?

• A. Connect the meter leads across the component
• B. De-energize the circuit before measuring
• C. The component should be isolated from live components
• D. The power supply should be on

Answer: (D)

Measuring resistance relies on injecting a small current and reading the resulting voltage drop across the component. If the circuit is powered, that injected current from the meter combines with live voltages and other paths in the circuit, giving an inaccurate reading and potentially damaging the meter or components. Therefore, the statement that the power supply should be on is incorrect. The proper approach is to de-energize the circuit, isolate the component from any live parts, and place the meter leads across the component so the meter can measure its resistance directly. In-circuit measurements can still be attempted, but they require removing one end of the component or ensuring no parallel paths interfere, to get a true reading.