What type of power supply is typically used to operate a megohmmeter?

A megohmmeter is specifically designed to measure high resistance values, typically in the megohms range. For this purpose, it requires a reliable and consistent power source to generate high voltage needed for effective testing. The internal power supply is the correct answer because it is built into the megohmmeter itself, allowing it to produce the necessary testing voltage without relying on an external circuit or any other device.

This internal power supply ensures that the measurements are accurate and consistent, as it can provide the required voltage levels to overcome the insulation resistance of the circuit under test. In contrast, other options like a current transformer or potential transformer are not appropriate because they serve different functions in electrical systems and cannot provide the high voltage necessary for megohm measurements. Similarly, using the circuit being tested as a power source would be ineffective, as it could lead to inaccurate readings or even damage to the measuring device. Thus, the internal power supply is essential for the operation and functionality of a megohmmeter in insulation resistance testing.