Selection and Operation of Megohmmeters
If a multimeter is used to measure the insulation resistance of equipment, only the insulation‑resistance value under low‑voltage conditions can be obtained, which cannot truly reflect the insulation performance under high‑voltage operating conditions. Different from a multimeter, a digital megohmmeter is equipped with a built‑in high‑voltage power supply whose output voltage ranges from 500 V to 5000 V. Therefore, measuring insulation resistance with a digital megohmmeter can yield insulation‑resistance values consistent with actual working conditions.
Operation and Maintenance of Digital Megohmmeters
Before measurement, cut off the power supply of the equipment under test, and connect its conductive parts to earth for full discharge to ensure safety. Electrical equipment that has been tested by a digital megohmmeter shall also be grounded and discharged promptly before re‑measurement.
Check whether the digital megohmmeter is in good condition prior to measurement. With no‑load connection to the object under test, turn on the power switch and check the battery status. Replace the battery in time if the battery voltage is low; otherwise, the measured data will be invalid.
Insert the test leads into the Line (L) and Earth (E) terminals, select the test voltage, disconnect the test leads, and press the test button to verify whether the display shows infinity "∞". Short‑circuit the Line (L) and Earth (E) terminals, then press the test button and check whether the reading shows "0". If "0" is not displayed on the LCD screen, the digital megohmmeter is faulty and shall be repaired before use.
Correct wiring must be performed. A digital megohmmeter is generally fitted with three terminals marked L (Line), E (Earth) and G (Guard). Terminal L connects to the conductive part of the test object insulated from earth; terminal E connects to the housing of the test object or ground; terminal G connects to the shield or non‑measured portion of the test object.
Terminal G serves to shunt surface leakage current. When measuring the insulation resistance of cables, leakage current exists on the surface of insulating materials and may cause inaccurate results. Measurement errors will become considerable especially under high‑humidity conditions or when cable‑insulation surfaces are contaminated.
To eliminate the influence of surface leakage current, fit a metal shielding ring on the surface of the test object and connect it to the Guard (G) terminal of the digital megohmmeter. In this way, surface leakage current \(I_B\) flows from the positive terminal of the built‑in generator, returns to the negative terminal via terminal G and forms an independent loop. Since \(I_B\) bypasses the measuring mechanism of the megohmmeter, the interference from surface leakage current can be fundamentally eliminated.
Do not use twin‑core or twisted insulated wires for connections between terminals and the equipment under test. Separate single‑core wires shall be adopted instead to avoid errors caused by defective insulation of twisted cables. For reliable measurement results, wipe the surface of the test object clean with a dry cloth or cotton yarn.
When measuring insulation resistance of equipment with large capacitance, do not disconnect the megohmmeter immediately after taking readings. Otherwise, the charged capacitor will discharge towards the megohmmeter and may burn out the instrument. Disconnect the test leads first after reading, then terminate the test. Do not touch the conductive parts of the equipment under test until both the megohmmeter and the test object have been fully discharged.
While measuring insulation resistance, record ambient temperature, humidity and relevant conditions of the test object for subsequent result analysis.
