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Ideal Test & Measurement

Insulation Testing

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Insulation Testing

What is “Good” Insulation?- Most electric wires/conductors are covered with

electrical insulation- They have Relatively high resistance to current- The insulation prevents voltage from escaping to nearby

conductorsInsulation Electrical Leak Copper Wire

VoltageE

CurrentI

ResistanceR

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What Degrades Insulation?

• Mechanical damage/Vibration• Sunlight• Excessive heat or cold• Dirt/Oil• Physical Damage• Corrosive vapors/Chemicals• Moisture

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Measuring Insulation Resistance

• A Meg Ohm insulation tester is essentially a high-range resistance meter (ohmmeter) with a built-in DC generator.

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The Theory of the Test

• The intent of an insulation test is to pressurize a specific conductor and then test to see if that voltage is escaping to any adjacent conductors. If there are just two wires, this is a simple procedure. If there are a number of conductors in a conduit, then you must test between each possible pair combination AND from each conductor to the metal conduit! This can take a fair amount of time to accomplish.

• You may be testing a motor to ensure that current does not leak out to the equipment case, such as a washing machine

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The Theory of the Test

• You may be testing a new piece of equipment or installation or a 30 year old panel. The testing times will vary.

• For new equipment or installations a 30 second or 1 minute test is usually sufficient. This is also suitable if an ongoing PM (Preventative Maintenance) program is in effect and you have a series of test results on an older piece of equipment.

• On older equipment, with no known test history and results to compare against, a 10 minute test is required to understand the true condition of the insulation.

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Types of Insulation Resistance Tests

• For installing new electrical equipment

• Ensures that the insulation is adequate to begin operation

• Provides a baseline reading for future testing

• Also known as a Proof Test

• Based on relative measurements due to factors such as moisture and temperature

• Measurements taken during routine maintenance

• Short time/spot reading, time resistance method, and step voltage test

Installation Tests Maintenance Tests

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Short Time/Spot Reading Test

• Tester is connected across the insulation of the motor windings

• A test voltage is applied for a fixed period (usually 60 seconds)

• Duration must be consistent from test to test

• Most effective use of short time testing is in a series over several months

• Can be affected by temperature and moisture

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Time Resistance Method

• Provides conclusive results without past test measurements

• Successive readings are taken at fixed time intervals

• Very effective when moisture and dirt are present

• Resistance value may start low then increase over time

• This trend will continue for several minutes for healthy insulation – resistance continues to rise

• Decreasing resistance over time indicates a problem

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Dielectric Absorption Ratio

The best way to quantify results of a time resistance test is through a dielectric absorption ratio (DAR) test– a ratio of 2 readings. The second reading should be equal to or higher than the first.

Typical Interval Ratios: 60-second reading / 30-second reading10-minute reading / 1-minute reading

Resulting Value = Polarization Index

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Step Voltage Test

This test involves testing the insulation at two or more voltages and comparing the results

Good insulation = relatively consistent resistance readings regardless of voltage applied

Bad insulation = decreasing resistance levels at applied voltage increasesProcedure: Start with the lowest test voltage and then proceed to the next voltage level. Test duration is usually 60 seconds.

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Maintenance Test Voltages

Equipment AC Rating DC Test Voltage

Up to 100 volts 100 and 250 volts

440 to 550 volts 500 and 1,000 volts

2,400 volts 1,000 to 2,500 volts or higher

4,160 volts and above 1,000 to 5,000 volts, or higher

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Maintenance Program Considerations

• Consistency is key• Test equipment, connections, length of time, and time of

day should all be the same• Monthly, semi-annual and annual intervals are common for

motors and other machinery• Once a year for basic wiring• All installations and systems are different, be prepared to

adjust over time• Test often enough to catch insulation weakness before it

becomes hazardous

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Testing Preparation

• Take equipment out of service• Check what will be included in the test• Discharge capacitance at the end of the test (4 x

test length) – Testing induces a voltage which may be stored in the test article. Be sure to allow for this to discharge at the end of the test!

• Check for current leakage at switches. (Make sure any switches on the circuit are open.)

• Ensure that no voltage is present and ensure that nothing is plugged in. You may destroy a TV left on a branch circuit if you do a test.

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The One-Megohm Rule

• One megohm = lower limit for ordinary industrial equipment rated up to 1,000 volts

• For equipment rated above 1,000 volts, use one megohm per thousand volts

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Connection Diagrams

When testing AC motors and starters, connect the Tester in parallel to the circuit and turn the starter switch to the on position. If a weakness exists, disconnect and test each component of the system separately.

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Connection Diagrams

By connecting a communication cable, a single wire can be tested against all other wires and the cable jacket at the same time.

Caution! Data cable is delicate. Testing with 1000V will likely destroy it. Choose your test voltage, typically 50 volts cautiously!

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Safety

• Apparatus under test must not be live• Because there is a possibility of induced voltage

reaching the equipment, it is important to disconnect the apparatus from the exposed line rather than removing a workman’s ground in order to complete a test

• Always use proper gloves when connecting test leads and performing an insulation test

• Make sure that any neutral or ground connection is not carrying current when you disconnect it

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Safety

• Observe the voltage rating of your insulation tester. Large equipment can store up enough test voltage to be dangerous. Remember to discharge capacitance before and after every test.

• Never use an insulation tester in an explosive atmosphere.

• Slight sparking may be encountered when attaching test leads, during a test of faulty insulation, or following a test when capacitance is discharged. You can minimize sparking by making connections in a safe atmosphere, and by not disconnecting test leads for at least one minute following a test.

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Questions to Ask

• The customer may ask for any of the following; They want an insulation tester, Meg-Ohm Meter, or Megger. Megger is a trade mark and we have insulation testers, but Megger is a specific brand. We have many of the exact same functions and features. It is exactly like asking for a WireNut.

• What is the nominal voltage of the item they want to test? We can test up to 1000 volts, so if they have a 480 volt motor, then we can test that.

• Maintenance electricians, new installers, service technicians all require these testers to troubleshoot problems and to test for damage during installation.

61-795 Insulation Tester

• Simple timed and spot testing.

• 250, 500 and 1000 VDC tests

• Tests to values of 2000 meg ohms or 2 gig ohms.

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61-797 Insulation Tester

• 5 Voltage Tests, 50, 100, 250, 500 and 1000 VDC

• Automated PI and DAR tests.

• Remote probe• Memory recall• Tests to 20 gig ohms.

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Test Questions (True/False)

• Exposure to water or moisture can affect insulation effectiveness?• An air gap between two conductors will show a high value reading, even when

insulation is scraped away?• The applied test voltage should be twice that of the nominal voltage found on the

conductor?• The length of the test depends on the age of the installation?• The length of the conductor has no impact on the length of the test?• Testing a live circuit should not present any problems?• When faced with a bundle of 20 conductors in metal conduit in an old building,

testing several pairs should be sufficient?• When making a PI or DAR (Ratio tests) the second reading should be equal to or

higher than the first?• Keeping records is not necessary as you can always test to find out the true

condition of the insulation?

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Answers• Exposure to water or moisture can affect insulation effectiveness? TRUE

Insulation submersed in water can actually absorb moisture and its resistive properties will change. Insulation can sometimes be dried out and returned to service after flooding.

• An air gap between two conductors will show a high value reading, even when insulation is scraped away? TRUE High tension million volt lines do not have insulation on them. They rely on the glass insulators and air, (distance) to not arc to adjacent conductors.

• The applied test voltage should be twice that of the nominal voltage found on the conductor? TRUE Simply double the nominal voltage and test at that level up to 2400 Volts.

• The length of the test depends on the age of the installation? TRUE, but if you have a history of tests to refer, then you can possibly shorten the test.

• The length of the conductor has no impact on the length of the test? FALSE Longer cable lengths will require more time to build up a charge before they start to ‘leak’ to an adjacent conductor.

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Answers

• Testing a live circuit should not present any problems? FALSE Always de-energize a circuit and unplug any loads for safety reasons and to prevent damage to loads. Most modern testers have defeat circuitry so as not to test a live circuit.

• When faced with a bundle of 20 conductors in metal conduit in an old building, testing several pairs should be sufficient? FALSE You must pressurize each conductor and test to see if current is leaking to any other conductor and the metal conduit!

• When making a PI or DAR (Ratio tests) the second reading should be equal to or higher than the first? TRUE This shows that the insulation is good.

• Keeping records is not necessary as you can always test to find out the true condition of the insulation? TRUE But it will cause unnecessarily lengthy tests to be certain each and every time of the results.

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