Checking single or three-phase systems
Megger testing cables and auxiliary devices of a single-phase system
Figure 1Disconnect P and N from the supply side, as well as from the other end.
Now we have isolated our test circuit, making it dead. Short P and N with a temporary
short link. Close switch and protection devices.
As shown in Figure 1, open motor terminals, so that the motor remains isolated from
the test circuit. Check resistance with the insulation tester between the neutral link and
earth. If the value shown in the meter is less than 1 M?, then there is a fault with either
the cable insulation or device terminals.
Megger testing cables and auxiliaries of a three-phase system
Figure 2Disconnect L1, L2, and L3 from the supply side, as well as from the other end. This makes it a dead circuit.
Short L1, L2, and L3 terminals with a temporary link. Close the breaker device and
protection devices. As shown in figure 2, open motor terminals T1, T2, and T3, so
that the motor remains isolated from the test circuit.
Check resistance with insulation tester between each conductor and earth.
If the meter shows a low value less than 1 M?, there is a fault in either cable insulation
or device terminals.
Megger testing of motor
A pre-condition for megger testing of a motor is to isolate the motor from the supply totally. Take the megger value of a motor between each conductor and earth, as shown in figure, to check the earthing of the stator winding. 
This will help us to conclude on earthing status of the stator winding. Similarly, check for shorting between two windings by checking the megger value between two stator-winding terminals, as shown in figure below.
Thus, a low reading can identify insulation failure of any winding inside the motor.