Which instrument is used to conduct an insulation-resistance test on 480-volt feeder conductors?

A megohmmeter is the appropriate instrument for conducting an insulation-resistance test on 480-volt feeder conductors because it is specifically designed to measure the resistance of insulation over a wide range of values. When performing insulation tests, high voltages are applied to assess the integrity of the insulation material without causing damage, and megohmmeters typically operate at test voltages of 250V, 500V, or even up to 1000V, which is ideal for verifying the insulation quality against electrical breakdown.

Using a megohmmeter allows electricians to determine if the insulation of the conductors is intact and free from wear, moisture, or other conditions that could lead to potential faults or failures. This test is a crucial preventive measure in ensuring overall electrical safety and system reliability.

In contrast, other instruments such as voltmeters, clamp meters, and ohmmeters are not suited for insulation testing at high voltages. Voltmeters are designed to measure voltage rather than insulation integrity. Clamp meters measure current and are not intended for insulation testing. Ohmmeters measure resistance but typically do not apply high enough voltage to effectively test insulation resistance in the way a megohmmeter does.