Introduction:- Megger is used for measuring insulation resistance (IR) value of motors, cables, and panel mounted components and bus bar etc. Megger are available for different voltage range like 50 volt, 500 volt, 1000 volt, 2500 volt(2.5kv), 5 kv . IR value measurement is very important in electrical work to ensure safety of working personnel's and equipment.
Construction and working:- it consist of a permanent magnet cross coil dynamo meter type indicating instrument and a hand operated DC generator. There is permanent magnet as shown in fig. Two moving coil are placed within the magnetic field of magnet supported by pivots. The two coil are placed at 90 degree to each other. First coil is known as pressure coil connected in parallel to output voltage of generator. it provide torque to spindle in anti clock wise direction hence moves the pointer towards infinity. The second coil is connected in series with testing resistance, it produce torque in opposite direction which moves pointer towards 0 ohms. So when Megger is used for measuring resistance , Megger pointer indicates the ratio of V/I that is R of the circuit.
Hand operated generator:- it is very small generator with permanent magnet field. armature is rotated with the the help of handle and gear system. A clutch is provided so that handle start slipping after the 120 rpm. So when handle is rotated with full speed the armature speed is limited to 120 rpm and the output voltage remains constant.
Uses:- LT motor are Meggered with 1000 volt Megger. HT motors are meggered with 2.5 kV Megger. Minimum IR value should not be less than voltage rating of machine into 1000 ohms that means 1000 ohms per volt of machine rating. In HT motors and HT cables R15/R60 ratio is also considered. R15 and R60 is meggering values at 15 seconds and 60 seconds respectively. R60 value must be 1.3 to 1.5 times higher than R15 value.
Construction and working:- it consist of a permanent magnet cross coil dynamo meter type indicating instrument and a hand operated DC generator. There is permanent magnet as shown in fig. Two moving coil are placed within the magnetic field of magnet supported by pivots. The two coil are placed at 90 degree to each other. First coil is known as pressure coil connected in parallel to output voltage of generator. it provide torque to spindle in anti clock wise direction hence moves the pointer towards infinity. The second coil is connected in series with testing resistance, it produce torque in opposite direction which moves pointer towards 0 ohms. So when Megger is used for measuring resistance , Megger pointer indicates the ratio of V/I that is R of the circuit.
Hand operated generator:- it is very small generator with permanent magnet field. armature is rotated with the the help of handle and gear system. A clutch is provided so that handle start slipping after the 120 rpm. So when handle is rotated with full speed the armature speed is limited to 120 rpm and the output voltage remains constant.
Uses:- LT motor are Meggered with 1000 volt Megger. HT motors are meggered with 2.5 kV Megger. Minimum IR value should not be less than voltage rating of machine into 1000 ohms that means 1000 ohms per volt of machine rating. In HT motors and HT cables R15/R60 ratio is also considered. R15 and R60 is meggering values at 15 seconds and 60 seconds respectively. R60 value must be 1.3 to 1.5 times higher than R15 value.
