A megohmmeter, also called a megger or insulation tester, measures electrical insulation resistance in megohms (millions of ohms) to detect moisture and degradation in motor windings and cables. The device applies a high DC voltage (typically 500-5,000 volts) and measures the resistance of insulation layers. This test prevents electrical failures and ensures technician safety.
Technical Details
Megohmmeters typically apply test voltages of 500V for low-voltage circuits, 1,000V for motors, and 2,500-5,000V for high-voltage equipment. They measure resistance from 0 to 20 gigohms, with resolution to 0.1 megohms. Test duration is typically 60 seconds, though some standards require readings at 15-second intervals to detect absorption trends. Standards specify minimum insulation values: 5 megohms for motors at 1,000V and 100 megohms for cables.
Applications in HVAC
Megohmmeters test motor windings before and after repairs, verify cable integrity in outdoor units, and detect moisture in compressor chambers.
Practical Significance
Regular megohmmeter testing prevents dangerous electrical shock and equipment damage. Poor insulation readings indicate the need for motor replacement or cable remediation before system failure occurs.