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Insulation Resistance Testing
©2004 Fluke Corporation
Insulation Resistance Testing
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Resistance 101
• Resistance is the opposition to current flow
• Resistance is used to lower the amount of
current and/or voltage flowing through a circuit
• Unit of resistance is the ohm
• Ohms symbol is omega: 
©2004 Fluke Corporation
Insulation Resistance Testing
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What is megohm testing?
• It’s testing for electrical insulation failure
under controlled conditions
• Used to determine the integrity of
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Cables
Windings in motors and transformers
Switching gear
Electrical installations
• Similar to pressure-testing a water pipe
©2004 Fluke Corporation
Insulation Resistance Testing
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Why different instruments
read different ohms values
• Ohmmeter (DMM)
• Low voltage (~1-2V).
• Low current (~1-10mA) .
• MegOhmMeter
• High voltages to stress the insulation.
• Insulation Resistance Test (IRT).
• Low Ohms Tester
• Relatively high current (>200 mA).
• Greater accuracy for low-resistance
devices or conductors.
©2004 Fluke Corporation
Insulation Resistance Testing
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Leakage current basics
• All insulators leak because all resistances
conduct some current, however small
• High voltages produce currents in insulators
• The amount of current depends on
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Applied voltage (Ohm’s Law)
System capacitance
Total resistance
Temperature of material
©2004 Fluke Corporation
Insulation Resistance Testing
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Components of leakage current
• Polarization absorption leakage (IA)
• Conductive leakage (IL)
• Capacitive charging leakage (Ic)
©2004 Fluke Corporation
Insulation Resistance Testing
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Measuring insulation resistance
• Customary: between two conductors
“insulated” from each other
• Worst case: tie one conductor to the
conduit
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Measuring insulation resistance
Insulation resistance test
©2004 Fluke Corporation
Insulation resistance test
over time for a motor
Insulation Resistance Testing
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Be careful: I bite!
• A 5 KV tester = up to 5200V DC output
• Nonlethal, but potent
• Limited to 2mA
©2004 Fluke Corporation
Insulation Resistance Testing
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Installation testing
• Performed on these conductors
• De-energized current-carrying (live)
• Grounded
• Grounding
• When
• Construction and maintenance
• After initial installation
• Why
• #1 reason: Safety
• Establish baseline
©2004 Fluke Corporation
Insulation Resistance Testing
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Maintenance testing
Reasons for regular monitoring/testing
• Predicts and identifies imminent failure
• Prevents disaster
• Lowers total cost of ownership
Performed on critical conductors
©2004 Fluke Corporation
Insulation Resistance Testing
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Common voltages for testing
Equipment Voltage Rating
Test Voltage
Up to 50 V
50 and 100 V
Up to 100V
100 and 250V
440 to 550V
500 and 1,000V
2,400V
1,000 to 2,500V or higher
4,160V and above
1,000 to 5,000V or higher
©2004 Fluke Corporation
Insulation Resistance Testing
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Testing procedure
To ensure proper installation and integrity:
1. Verify supply power is isolated/disconnected
from system under test.
2. Select the appropriate voltage level.
3. Connect leads.
4. Take reading.
5. Allow discharge of test voltage
6. Test each conductor.
©2004 Fluke Corporation
Insulation Resistance Testing
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Using test results
• Keep records
• Analyze readings
• Resolve issues
• Monitor suspect cables
©2004 Fluke Corporation
Insulation Resistance Testing
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Spot-reading/Short-time test
• Connection time: 60 seconds
• Applies to low-capacitance equipment
• Steady increase in resistance
• One Megohm Rule of Thumb
• Trending rates of decrease
©2004 Fluke Corporation
Insulation Resistance Testing
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Step voltage test
Resistance testing at discrete voltage settings
• Connection time: 60 seconds
• When: After spot test
• Looking for inconsistencies
• Flaws show at steps
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Insulation Resistance Testing
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Step voltage test, good results
©2004 Fluke Corporation
Insulation Resistance Testing
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Step voltage test failure
©2004 Fluke Corporation
Insulation Resistance Testing
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Dielectric-absorption /
time-resistance test
Absorption characteristics: contaminated vs. good.
• During first minute
apply voltage
every 10 seconds
• Apply once a
minute for 9
remaining minutes
©2004 Fluke Corporation
Insulation Resistance Testing
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Generator, motor, or
transformer windings
Check:
• Winding to winding
• Winding to ground
• Phase to phase
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Insulation Resistance Testing
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Generator or motor windings
• Disconnect stator winding and phases
• Raise brushes for DC
©2004 Fluke Corporation
Motor Nameplate
Voltage Rating (v)
Minimum
Acceptable
Resistance (ohms)
0-1000
1M
1000-2400
2M
2400-5000
3M
Insulation Resistance Testing
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Stop the presses!
Newspaper printing press example
• Initially:
>500 M.
• After a day or two:
10 M.
• At about 1 M:
rebuild the motor.
1000
100
10
1
0
©2004 Fluke Corporation
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Insulation Resistance Testing
200
300
400
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Low ohms testing
Connection quality check
• Measure across connection
• Typically less than 1 ohm
• Requires lead compensation
©2004 Fluke Corporation
Insulation Resistance Testing
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Electrical contractors
IR test applications:
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Leakage
Cable insulation integrity
Windings and switches
Connections, fuses,
breakers, and switches
©2004 Fluke Corporation
Insulation Resistance Testing
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Motor technicians
IR test applications:
• Leakage between
windings
• Leakage between
windings and
motor frame
• When to test?
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Insulation Resistance Testing
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Plant maintenance
IR test applications:
• Wiring and cable
• Contacts and connections
• Grounding system integrity
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Insulation Resistance Testing
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Other users
• Electrical Utility Workers
• Low-voltage equipment
• High voltage equipment
• Elevator Technicians
• Motors
• Wiring
• Controls
©2004 Fluke Corporation
Insulation Resistance Testing
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Final tips
• Use the right tester
• Follow the procedures
• Analyze the results
• Use proper trending
• Take corrective action
©2004 Fluke Corporation
Insulation Resistance Testing
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1520/1550 MegOhmMeter
optional hands-on exercises
• Basic checks
• Normal resistance test
• Low resistance test
• High resistance test
©2004 Fluke Corporation
Insulation Resistance Testing
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©2004 Fluke Corporation
Insulation Resistance Testing
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