Transformer Testing and Records
Megger Test and Insulation Resistance Records
A controlled DC test voltage is applied across the specified insulation system, and the instrument measures resistance for the selected test arrangement.
A megger test commonly refers to an insulation resistance test using a megohmmeter. A controlled DC test voltage is applied across the specified insulation system, and the instrument measures resistance for the selected test arrangement.
Define the test before connecting
The equipment manufacturer and approved test procedure establish the connection, voltage, duration and acceptance criteria. Qualified personnel isolate the equipment, verify the test boundary and arrange discharge and grounding. Record the winding or component identification and instrument details.
Use the approved equipment procedure
Do not infer a universal test voltage or acceptance limit. Qualified personnel follow the manufacturer’s procedure and project-approved test boundary, including isolation, discharge and grounding.
Record conditions with the reading
Capture applied voltage, elapsed time, insulation resistance and equipment temperature. Note moisture, surface condition and the test connection. Readings taken under comparable conditions are more useful for trending than isolated values without a documented test setup.
| Item or condition | What to compare | Input or result |
|---|---|---|
| Before testing | Asset ID, connection and procedure | Approved voltage and duration |
| During testing | Resistance, elapsed time and temperature | Comparable readings |
| After testing | Review, discharge and release | Completed test record |
Review the result in context
Compare results with the applicable procedure, previous records and equipment condition. Where the procedure calls for a polarization index, calculate the ten-minute to one-minute resistance ratio. Keep follow-up actions, discharge verification and release authorization in the same report.
Trend consistently
Compare readings made with documented, comparable test conditions.
Apply the procedure
Use project or manufacturer criteria; acceptance depends on the equipment and method.
Close out safely
Record discharge verification, follow-up actions and release authorization.
Frequently Asked Questions
Is there one resistance limit for every transformer?
Acceptance criteria depend on the equipment, test method and applicable manufacturer or project procedure.
How does insulation resistance differ from hipot testing?
Insulation resistance measures resistance under a defined DC condition. Hipot testing assesses dielectric withstand using a separately specified procedure.
Prepare a transformer test report
Organize equipment identification, approved test method, conditions, readings, review, discharge verification and release authorization in one record.