Learn the practical step-by-step guide to test insulation resistance for distribution transformer, dry type transformer and power transformer. Follow correct megohmmeter setup, wiring and safety procedures for reliable readings.
When performing routine maintenance for a power transformer, measuring insulation resistance is one of the most straightforward yet critical tests. It helps technicians identify hidden insulation degradation, moisture contamination and potential failure risks before unexpected outages occur. Whether you are working on an oil immersed distribution transformer or a dry type transformer, following standardized testing steps directly affects personal safety and the authenticity of test data.
First, select the proper megohmmeter according to the equipment voltage class. For transformers rated from 10 kV to 35 kV, a 2500 V megohmmeter is required. Before any measurement, completely cut off power supply to the unit. Carry out full short-circuit discharge to eliminate residual voltage; this step protects operators and avoids distorted measurement results. Use two separate, well-insulated single wires for megohmmeter connections. Never twist the test leads together or let them touch the ground, as poor lead insulation will introduce unnecessary errors.
Complete open-circuit and short-circuit checks to verify megohmmeter functionality prior to testing. With leads open, crank the handle and confirm the pointer points toward infinity. Briefly short the two leads; the pointer should move to zero. If both responses check out, the instrument is ready for field use.
To test primary-winding insulation against secondary winding and transformer enclosure: short all three-phase primary terminals A-B-C and connect them to the megohmmeter L terminal. Short secondary terminals together along with the transformer tank and connect to the E terminal. To minimise surface leakage current that skews results, wrap bare copper wire around primary-side bushing skirts and link this wire to the megohmmeter G guard terminal. For secondary-winding insulation tests against primary winding and enclosure, swap the setup: short secondary terminals to L terminal, short primary phases plus tank to E terminal, and fit the guard wire on secondary bushings when needed.
Place the megohmmeter on a flat horizontal surface. Maintain a steady cranking speed of roughly 120 revolutions per minute. Record readings at 15-second and 60-second timestamps. Keep one hand steady on the megohmmeter housing to prevent shaking. If the pointer falls to zero, stop cranking immediately to avoid internal instrument damage. Always disconnect the L-side test lead before stopping the crank. After finishing every insulation test, use a discharge rod to fully discharge transformer windings to ground.
Correct insulation resistance measurement delivers meaningful reference data for transformer condition assessment. Skipping safety steps or ignoring wiring details often leads to misleading figures and puts field staff at risk.
As a seasoned manufacturer, Zhejiang Hengli Electrical carries out strict insulation performance checks for every batch of power transformer, distribution transformer and dry type transformer before products leave our factory. Insulation resistance testing is one of our mandatory factory-acceptance items. Even with rigorous quality control during production, we still advise end-users and maintenance teams to perform periodic on-site insulation resistance measurements throughout the equipment service life. Environmental humidity, dust buildup and long-term operational stress can gradually degrade internal insulation materials over years of running. Regular testing helps catch early-stage insulation problems, preventing unexpected breakdowns and extending transformer service life. If you have technical questions about transformer testing or product selection, our engineering team is ready to offer professional support for your projects.
