Question T7D11
From subelement T7 - T7D
Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?
Why is this correct?
The correct answer is B because ohmmeters supply their own small test current to measure resistance using Ohm's Law (R = V/I). If the circuit is powered, external voltages will interfere with this measurement, giving false readings and potentially damaging the meter. Power must be removed before resistance measurements. Options A, C, and D are irrelevant to resistance measurement fundamentals.
Memory tip
Remember the measurement rule pattern: voltage measurements need parallel connection to 'see' potential difference, current measurements need series connection to 'feel' the flow, and resistance measurements need unpowered circuits to avoid interference. Each measurement type has its own connection method and safety requirement.
Learn more
In practical troubleshooting, this safety rule protects both your equipment and measurement accuracy. Powered circuits can damage sensitive ohmmeter circuits and mask component failures. Professional technicians always verify power-off conditions before resistance testing. This principle extends to all low-power test equipment - spectrum analyzers, antenna analyzers, and SWR meters can all be damaged by unexpected RF or DC voltages during measurements.
Think about it
Why do you think an ohmmeter's internal test voltage would give meaningless readings in a powered circuit, and what could happen to the meter's sensitive input circuits?