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

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

• A. Yes, there should be power on the circuit
• B. No, there should be no power on the circuit
• C. Only during measurements
• D. It doesn't matter

Answer: (B)

Testing resistance with an ohmmeter relies on the meter’s own small test current flowing through the component. If the circuit is energized, voltage sources and active currents create other paths and states, so the reading won’t reflect the component’s true resistance. The measurement becomes distorted and the meter could be damaged by live power or by interacting sources. To get a meaningful reading, power must be removed and the component isolated from the rest of the circuit, with capacitors discharged if present. In short, de-energize the circuit before measuring resistance to obtain an accurate value.

Testing resistance with an ohmmeter relies on the meter’s own small test current flowing through the component. If the circuit is energized, voltage sources and active currents create other paths and states, so the reading won’t reflect the component’s true resistance. The measurement becomes distorted and the meter could be damaged by live power or by interacting sources. To get a meaningful reading, power must be removed and the component isolated from the rest of the circuit, with capacitors discharged if present. In short, de-energize the circuit before measuring resistance to obtain an accurate value.