Megger. ohmmeter

Megger

The Megger is a power tool used for checking the insulation of various materials and equipment or facilities.In fact, measures the resistance in mega ohm (million ohms).It has an internal voltage generator. 
The one in photo it has 500 volts DC generator but with very little power (that's is harmless), which is achieved by turning with constant speed  the crank.
There are also with more than 500 volts but 500 volts is quite acceptable for most applications in domestic electrical installations.

The instrument has two dots at 1 and 100 mega ohm.
Left from the first dot means poor insulation
Measurement over the second dot is good insulation.
Mid term means that there is doubt about the quality of insulation.The main use of the instrument is to check the insulation in the motor.Putting the black connector on earth (earth terminal on the motor) and red alternately in the three windings an three-phase motor, we see the quality of insulation.Multimeter have also ohmmeters with voltage source for measuring ohms, usually up to 9 volts, which is why they can not show whether an insulation volts in normal operation is acceptable.
Ideal for insulation consider the infinite resistance.