What must be done to ensure accurate measurements with an ohmmeter?

• A. Leave the circuit powered during measurement
• B. Electrical power must be disconnected in the circuit
• C. Increase the circuit voltage
• D. Short the circuit

Answer: (B)

When you use an ohmmeter, it sends a small test current through the path you’re measuring and uses the resulting voltage to determine resistance. If the circuit is powered, that external voltage and current will distort the meter’s reading, making the resistance measurement inaccurate. There’s also a safety and equipment risk: applying power while measuring can damage the meter or other components and create shocks or arcs. So the circuit must be de-energized and isolated from any power source before taking an ohmmeter reading, and any stored energy (like capacitors) should be discharged. Increasing the circuit voltage or shorting the circuit would further distort results or cause danger, and leaving the power on would defeat the purpose of the measurement.