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7/27/2019 Prezentare Fluke 1550C

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©2004 Fluke Corporation Insulation Resistance Testing 1

Insulation Resistance Testing




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:




What is megohm testing?

• It’s testing for electrical insulation failureunder 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




Why different instrumentsread 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-resistancedevices or conductors.




Leakage current basics

• All insulators leak because all resistancesconduct 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




Components of leakage current

• Polarization absorption leakage (I A

)• Conductive leakage (IL)

• Capacitive charging leakage (Ic)




Measuring insulation resistance

• Customary: between two conductors“insulated” from each other

• Worst case: tie one conductor to theconduit




Measuring insulation resistance

Insulation resistance testInsulation resistance testover time for a motor




Be careful: I bite!

• A 5 KV tester = up to 5200V DC output• Nonlethal, but potent

• Limited to 2mA




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




Maintenance testing

Reasons for regular monitoring/testing• Predicts and identifies imminent failure

• Prevents disaster

• Lowers total cost of ownership

Performed on critical conductors




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




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.




Using test results

• Keep records• Analyze readings

• Resolve issues

• Monitor suspect cables




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




Step voltage test

Resistance testing at discrete voltage settings• Connection time: 60 seconds

• When: After spot test

• Looking for inconsistencies

• Flaws show at steps




Step voltage test, good results




Step voltage test failure




Dielectric-absorption /time-resistance test

Absorption characteristics: contaminated vs. good.

• During first minuteapply voltage

every 10 seconds• Apply once a

minute for 9remaining minutes




Generator, motor, or transformer windings

Check:

• Winding to winding

• Winding to ground

• Phase to phase




Generator or motor windings

• Disconnect stator winding and phases• Raise brushes for DC

Motor Nameplate

Voltage Rating (v)

Minimum

Acceptable

Resistance (ohms)0-1000 1 M

1000-2400 2 M

2400-5000 3 M




1

10

100

1000

0 100 200 300 400

Stop the presses!

Newspaper printing press example

• Initially:>500 M.

• After a day or two:10 M.

• At about 1 M:rebuild the motor.




Low ohms testing

Connection quality check

• Measure across connection

• Typically less than 1 ohm

• Requires lead compensation




Electrical contractors

IR test applications:

• Leakage

• Cable insulation integrity

• Windings and switches

• Connections, fuses,breakers, and switches




Motor technicians


• Leakage betweenwindings

• Leakage betweenwindings and

motor frame

• When to test?




Plant maintenance


• Wiring and cable

• Contacts and connections

• Grounding system integrity




Other users

• Electrical Utility Workers

• Low-voltage equipment

• High voltage equipment

• Elevator Technicians

• Motors

• Wiring

• Controls




Final tips

• Use the right tester

• Follow the procedures

• Analyze the results

• Use proper trending

• Take corrective action




1520/1550 MegOhmMeter optional hands-on exercises

• Basic checks

• Normal resistance test

• Low resistance test

• High resistance test

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