While using Megger equipment to test insulation, DC voltage is applied to the winding or device that is to be tested. The main reason for such a testing is to evaluate whether the insulation is damaged or not. This testing is important because damaged insulation can lead to a short circuit or damge to electrical equipment during normal power applications.
Generally, Megger instruments vary in output ranges, out of which 1000V, 750V, and 300V are the most common. Using the most appropriate voltage for the device during testing is very essential, otherwise the device might get damaged. This is why you must understand how to operate electrical test equipment before using it.
Usually such devices come with user manuals, but even then for a first timer, support of an expert electrician or tester is important so that testing is done safely. If you are an amateur tester, make sure you test electrical devices in the guidance of an expert.