Electrical Tool: How to Use a Megger (Insulation Resistance Tester)
A Megger produces high voltage and very low current, so safety is important.Never touch both test terminals with your hands during operation.1. Checking Whether the Megger Works ProperlyStep 1 – Open-circuit testConnect the Megger leads to a multimeter (voltage measurement).
Rotate the handle — the voltage indication should rise steadily as the generated voltage increases.On the Megger scale, the pointer will move toward infinity (∞).Step 2 – Short-circuit testShort the Megger output terminals together and rotate the handle.
The pointer should swing toward 0 Ω.If both behaviors are normal, the Megger is working correctly.2. Testing a MotorA. Insulation between winding and motor frame (check leakage)Do NOT remove the copper links on the motor terminals
(horizontal links = star connection, vertical links = delta connection — either is fine for this test)
[*]Connect one Megger lead to any phase terminal
[*]Connect the other lead to the motor housing
[*]Rotate the handle
Result:
[*]High resistance (near ∞) → Motor insulation is good
[*]Low resistance → Motor has leakage to ground
B. Insulation between phases
[*]Remove the copper links
[*]Connect leads between U and V, rotate handle
[*]Then test U–W and V–W
Result:
[*]High resistance → Normal winding insulation
[*]Low resistance → Winding insulation damaged
NotesA Megger provides much more reliable insulation testing than a multimeter.
It is commonly used when a multimeter cannot accurately determine motor condition.
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