Wiring and usage of megger and precautions

1. Selection of shake meter

When selecting a megohmmeter, its rated voltage must be compatible with the operating voltage of the electrical equipment or circuit being measured, and the measurement range should also be consistent with the range of the insulation resistance being measured. The following table lists some requirements for selecting a megohmmeter in different situations.

Selection of megohmmeters with different rated voltages Measurement object Rated voltage of insulation to be measured (V) Rated voltage of selected megohmmeter (V) Coil insulation resistance Below 500 500 Above 500 1000 Motor and power transformer coil insulation resistance Above 500 1000~2500 Generator coil insulation resistance Below 380 1000 Electrical equipment insulation Below 500 500~1000 Above 500 2500 Insulators—2500~5000

2. Wiring and usage of megger

The megohmmeter has three terminals, which are marked with line (L), ground (E) and shield or guard ring (G). The connection method for measuring insulation resistance with a megohmmeter is shown in the figure

1. Measurement of insulation resistance of lighting and power lines to ground.

Connect the megohmmeter terminal E to the ground reliably, and the terminal L to the circuit under test. Shake the generator handle of the megohmmeter clockwise from slow to fast for about 1 minute, and read the value after the megohmmeter pointer stabilizes. The value indicated by the megohmmeter is the insulation resistance value of the circuit under test to the ground, in megohms.

2. Measurement of motor insulation resistance

Disassemble the Y or Δ-shaped connection of the motor winding. Use the two terminals E and L of the megohmmeter to connect the two-phase winding of the motor respectively, as shown in the figure. Shake the generator handle of the megohmmeter to read the reading. This connection method measures the phase-to-phase insulation resistance of the motor winding. The measurement wiring of the motor winding to ground insulation resistance is shown in the figure. Terminal E is connected to the motor housing (the paint or rust on the contact point of the housing should be cleared) Terminal L is connected to the motor winding, shake the handle of the megohmmeter to read the reading, and measure the motor to ground insulation resistance.

3. Measurement of cable insulation resistance

The wiring method for measurement is shown in the figure. Connect the terminal E of the megger to the cable shell, the terminal G to the insulation layer between the cable core and the shell, and the terminal L to the cable core. Shake the generator handle of the megger to read the reading. The measurement result is the insulation resistance value between the cable core and the shell.

3. Precautions for use

1. When measuring the insulation resistance of equipment, the power supply of the equipment must be cut off first. Equipment containing inductance and capacitance (such as capacitors, transformers, motors and cable lines) must be discharged first. Non-duty personnel must apply for the corresponding work ticket before working.

2. The megohmmeter should be placed horizontally. Before connecting the wires, you should first shake the megohmmeter to see if the pointer is at "∞". Then short-circuit the two terminals L and E, and slowly shake the megohmmeter. The pointer should be at zero. After the open and short circuit tests, the megohmmeter can be measured only after it is confirmed to be intact.

3. The leads of the megohmmeter should be made of multi-strand soft wires, and the two leads should not be twisted together to avoid inaccurate measurement data.

4. After the megohmmeter measurement is completed, the object to be measured should be discharged immediately. Before the megohmmeter stops rotating and the object to be measured is discharged, do not touch the measuring part of the object to be measured or remove the wires to prevent electric shock.

5. The surface of the object to be measured should be wiped clean and free of dirt (such as paint, etc.) to avoid inaccurate measurement data.