Which of the following is essential when using an AC voltmeter in a live circuit?

The correct choice highlights the importance of the correct placement of test probes when using an AC voltmeter in a live circuit. This aspect is crucial because proper placement ensures that the voltmeter accurately measures the voltage across the points of interest in the circuit. Incorrect placement can not only lead to inaccurate readings but also pose safety risks to the user and equipment.

When using any measuring instrument in a live circuit, especially an AC voltmeter, it is essential to connect the probes to the right points: the positive (or red) probe should be at the higher potential point, and the negative (or black) probe should be at the lower potential or ground. This ensures that the voltmeter effectively completes the circuit for measurement, providing reliable and meaningful voltage readings.

The other choices involve common misconceptions or practices that could compromise safety or measurement accuracy. For example, limiting the voltmeter to low voltage, waiting for the circuit to turn off, or having sensitivity to high voltage do not adequately address the operational needs of measuring voltage safely and accurately while the circuit is live. Correctly positioning the probes is the critical factor in obtaining valid voltage readings from the AC voltmeter in an active circuit.