Which device is essential for measuring insulation resistance in electrical systems?

The megohmmeter is essential for measuring insulation resistance in electrical systems because it applies a high voltage to the insulation material and measures the resistance to current flow. This type of measurement is crucial for assessing the integrity and performance of insulation, especially in cables and electrical equipment, where insulation failure can lead to short circuits and equipment damage.

In contrast, a multimeter typically tests lower resistance values, suitable for circuit measurements but not designed for high-resistance insulation testing. An ohmmeter, while it is a device specifically for measuring resistance, is also limited to lower voltage measurements and cannot adequately evaluate insulation against high voltage stress. An ammeter is used to measure current flow in circuits but does not provide any information regarding insulation resistance. Thus, the megohmmeter is the most appropriate instrument for this purpose.