Overview
A megohmmeter is a common electrical test instrument used to measure the insulation resistance of electrical equipment. Correct selection, proper use, and periodic maintenance are essential to ensure safe operation of electrical systems. The following sections summarize key points for selecting, using, and maintaining a megohmmeter.
Selection
When selecting a megohmmeter, consider two main factors:
- Rated test voltage: The megohmmeter's rated test voltage must match the operating voltage of the equipment or circuit under test. A rated voltage that is too high or too low can produce inaccurate readings or may damage the instrument.
- Measurement range: The meter's measurement range should cover the expected insulation resistance values of the equipment under test to avoid large reading errors.
Using a Megohmmeter
When using a megohmmeter, observe the following points:
- Placement: Place the megohmmeter on a stable, firm surface, away from high-current conductors and strong external magnetic fields.
- Inspection and preparation: Before use, perform open-circuit and short-circuit tests to verify the instrument is working properly. Clean the measurement points on the equipment and the meter probes to minimize contact resistance and ensure accurate readings.
- Connections: The megohmmeter typically has three terminals labeled L (line), E (earth), and G (guard). Use the appropriate terminals for the measurement. When measuring insulation resistance to earth, connect L to the equipment under test and connect E to a reliable earth.
- Taking readings: Stabilize the megohmmeter with one hand and crank the handle with the other, increasing speed gradually to the rated speed of 120 r/min. Insulation resistance varies with test duration; the reading after 1 min is commonly used as the standard value.
Maintenance
Periodic maintenance extends the service life of the megohmmeter and helps maintain accuracy. Common maintenance measures include:
- Cleaning: Regularly clean the instrument's exterior and internal components to prevent dust and dirt from affecting performance.
- Storage: When not in use, store the megohmmeter in a dry, cool location, avoiding direct sunlight and high temperatures.
- Shock protection: Avoid subjecting the instrument to vibration or impact during use and storage to prevent damage.
Summary
Proper selection, correct operation, and regular maintenance of a megohmmeter help ensure the safety of electrical equipment and extend the meter's service life.
