Which of the following precautions should be taken when measuring high voltages with a voltmeter?

When measuring high voltages with a voltmeter, it is crucial to ensure that the voltmeter and its leads are rated for use at the voltages to be measured. This precaution is important because using equipment that is not rated for high voltages can result in equipment damage, inaccurate readings, or even pose a safety risk, including the possibility of electric shock. Voltmeters and leads typically have specific voltage ratings that indicate the maximum voltages they can safely handle. By using a voltmeter that is properly rated, you help to ensure accurate measurements and maintain safety standards during electrical work.

The other options do not provide the necessary safety or measurement assurance when dealing with high voltages. Using a voltmeter with low impedance may lead to incorrect readings, particularly in high-voltage circuits where impedance can influence the voltage being measured. Grounding the circuit through the voltmeter is generally not a standard practice and could compromise safety. Finally, while setting the voltmeter to the correct frequency is important in some applications (especially when dealing with AC signals), it is not a primary concern when simply measuring high DC or AC voltages. Hence, ensuring that the voltmeter is properly rated remains the top priority for effective and safe measurements.