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.

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1. Checking Whether the Megger Works ProperlyStep 1 – Open-circuit test

Connect 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 test

Short 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.

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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
  
  

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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
  
  

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Notes

A Megger provides much more reliable insulation testing than a multimeter.
It is commonly used when a multimeter cannot accurately determine motor condition.