©2004 Fluke Corporation Insulation Resistance Testing 1 Insulation Resistance Testing

©2004 Fluke Corporation Insulation Resistance Testing 2 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 3 What is megohm testing? It’s testing for electrical insulation failure under controlled conditions Used to determine the integrity of Cables Windings in motors and transformers Switching gear Electrical installations Similar to pressure-testing a water pipe

©2004 Fluke Corporation Insulation Resistance Testing 4 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 5 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 Applied voltage (Ohm’s Law) System capacitance Total resistance Temperature of material

©2004 Fluke Corporation Insulation Resistance Testing 6 Components of leakage current Polarization absorption leakage (I A ) Conductive leakage (I L ) Capacitive charging leakage (Ic)

©2004 Fluke Corporation Insulation Resistance Testing 7 Measuring insulation resistance Customary: between two conductors “insulated” from each other Worst case: tie one conductor to the conduit

©2004 Fluke Corporation Insulation Resistance Testing 8 Measuring insulation resistance Insulation resistance test over time for a motor

©2004 Fluke Corporation Insulation Resistance Testing 9 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 10 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 11 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 12 Common voltages for testing Equipment Voltage RatingTest Voltage Up to 50 V50 and 100 V Up to 100V100 and 250V 440 to 550V500 and 1,000V 2,400V1,000 to 2,500V or higher 4,160V and above1,000 to 5,000V or higher

©2004 Fluke Corporation Insulation Resistance Testing 13 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 14 Using test results Keep records Analyze readings Resolve issues Monitor suspect cables

©2004 Fluke Corporation Insulation Resistance Testing 15 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 16 Step voltage test Resistance testing at discrete voltage settings Connection time: 60 seconds When: After spot test Looking for inconsistencies Flaws show at steps

©2004 Fluke Corporation Insulation Resistance Testing 17 Step voltage test, good results

©2004 Fluke Corporation Insulation Resistance Testing 18 Step voltage test failure

©2004 Fluke Corporation Insulation Resistance Testing 19 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 20 Generator, motor, or transformer windings Check: Winding to winding Winding to ground Phase to phase

©2004 Fluke Corporation Insulation Resistance Testing 21 Generator or motor windings Disconnect stator winding and phases Raise brushes for DC Motor Nameplate Voltage Rating (v) Minimum Acceptable Resistance (ohms) M M M

©2004 Fluke Corporation Insulation Resistance Testing 22 Stop the presses! Newspaper printing press example Initially: >500 M . After a day or two: 10 M . At about 1 M  : rebuild the motor.

©2004 Fluke Corporation Insulation Resistance Testing 23 Low ohms testing Connection quality check Measure across connection Typically less than 1 ohm Requires lead compensation

©2004 Fluke Corporation Insulation Resistance Testing 24 Electrical contractors IR test applications: Leakage Cable insulation integrity Windings and switches Connections, fuses, breakers, and switches

©2004 Fluke Corporation Insulation Resistance Testing 25 Motor technicians IR test applications: Leakage between windings Leakage between windings and motor frame When to test?

©2004 Fluke Corporation Insulation Resistance Testing 26 Plant maintenance IR test applications: Wiring and cable Contacts and connections Grounding system integrity

©2004 Fluke Corporation Insulation Resistance Testing 27 Other users Electrical Utility Workers Low-voltage equipment High voltage equipment Elevator Technicians Motors Wiring Controls

©2004 Fluke Corporation Insulation Resistance Testing 28 Final tips Use the right tester Follow the procedures Analyze the results Use proper trending Take corrective action

©2004 Fluke Corporation Insulation Resistance Testing /1550 MegOhmMeter optional hands-on exercises Basic checks Normal resistance test Low resistance test High resistance test

©2004 Fluke Corporation Insulation Resistance Testing 30