What tool should be used to test a switchboard for grounds?

The megohmmeter is the correct tool to test a switchboard for grounds because it applies a high voltage, typically between 500 to 1000 volts, to the circuit being tested. This high voltage helps to identify low resistance paths to ground, which could indicate insulation failures or unwanted grounding. The megohmmeter measures resistance in megohms, which is crucial when assessing the integrity of electrical insulation in various components of a switchboard. This testing ensures safety and functionality within electrical systems.

In contrast, while an ohmmeter and multimeter measure resistance, they do not provide the high voltage needed for insulation testing, which is essential in determining whether grounding conditions are safe. A voltmeter is primarily used to measure voltage rather than resistance and cannot effectively assess grounding issues. Therefore, the megohmmeter stands out as the most appropriate tool for this specific purpose.