When measuring resistance of a component with an ohmmeter, should the circuit be energized?

• A. Yes, there should be power in the circuit.
• B. It doesn't matter.
• C. Only if testing for continuity.
• D. No, there should be no power in the circuit.

Answer: (D)

Measuring resistance with an ohmmeter relies on the meter sending a small test current through the component and reading the resulting voltage drop. If the circuit is energized, external power drives current and voltage through the circuit, which interferes with the meter’s measurement, can give an inaccurate reading, and may damage the meter or components. To get a valid resistance reading, you must have no power in the circuit (and, if needed, isolate the component and discharge any capacitors). This is why the correct approach is that there should be no power in the circuit.