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Insulation Resistance Testing For Maintenance & Safety

Insulation failure can cause electrical shocks, creating a real hazard to personnel and machinery. A regular program of testing insulation resistance is strongly recommended to prevent this danger, as well as to allow timely maintenance and repair work to take place before catastrophic failure. All new equipment, motors, transformers, switch gears, and wiring should be tested before being put into service. This test record will be useful for future comparisons in regular maintenance testing.

Some of the more common causes of insulation failure include excessive heat or cold, moisture, aging, corrosive atmospheres and vibration. Insulation values are in ohms, and insulation values change according to temperature. Take all of your readings at 20 deg C or at the same temperature, or correct for 20 deg C. A general rule is to take 1/2 the resistance reading for every 10 deg C increase, and double the resistance for every 10 deg C decrease. For instance, if you measure 10 megohms at 30 deg C, a 10 deg increase, your true reading is 5 megohms.

Definitions:

Corona: Partial discharge in a high-voltage field that does not bridge the gap between the high voltage and ground, usually due to a breakdown of a small area of insulation. This discharge can often be seen and heard as a hissing sound or glowing around high-voltage cable and insulation during wet weather.

Dielectric: All materials can be considered conductors of electricity; some are better conductors than others, and the difference between an insulator and a conductor is the dielectric or "resistance", i.e. the amount of force required to move free electrons.

Dielectric Test Set: Usually DC medium voltage insulation resistance testing up to 7.5 kV with fast charging of high capacitance samples. Often used to compare the absorption characteristics of humid or otherwise contaminated insulation. Test voltages are applied and varied over extended periods, drawing a curve showing the resistance value versus time.

DLRO: Digital Low Resistance Tester to 0.10 micro-ohm for measuring resistance of switch and circuit breaker contacts, winding resistance of transformers and motors, busbar and other joint resistance, bonding material, splices, welds and fuses, graphite electrodes and cracks in squirrel cage rotors.

Earth Tester (Ground Resistance Tester): Measurement of Resistance of Ground Connections to Earth and for Determining Earth Resistivity effectiveness and integrity. Used to test grounds of utility poles, distribution transformers, telephone and CATV systems, Computers, Machinery, Antennas, Portable Generators. Usually operated with accessory kits containing Ground Probes & Test Leads. Having a good ground is critical for some equipment types; not having a good ground is a safety issue. Four Terminal Earth testers can also be used to prospect for geophysical properties; to locate ore bodies, clays and water bearing gravel beneath the soil, as well as the depth of bedrock and thickness.

Megger: While there are many brands of insulation testers, there is only one "MEGGER"; it is a registered trademark of AVO Biddle.

Megohmmeter: Insulation Testing Device, usually 500 or 1000 V DC Non-Destructive Testing

One Megohm Rule: Suggested based upon equipment rating:

                                         < 1000 V = 1 megohm minimum

                                         > 1000 V = 1 megohm/1000 Volts

High-Pot Tester (High Potential): AC and AC/DC types. AC Charging current is extremely large with minor leakage current. Sometimes this is called destructive testing in that voltage is increased to some specific point to see if the insulation can withstand that particular voltage. It is a Go/No Go type of test, and can cause deterioration of the insulation, as opposed to the DC non-destructive test megohmmeters. AC versions are often used to test the dielectric strength of electrical insulation and ground circuit continuity of 3-wire appliances , tools, lighting fixtures and lamps, heating elements.

Such tests are required by UL; such Hipot Testers should carry safety interlocks because they are often used by non-technical personnel.

DC High-Pot testing is superior to medium voltage testing because it stresses the insulation at or above the working level, providing more information about the condition of the insulation and helping you to predict problems before a breakdown occurs. The tests are more complicated and the instruments more expensive, but the condition of the apparatus in question is better documented and tested and less likely to fail without warning.

Test Procedures:

1. Proof Test: Simple Go-No Go procedure; if the insulation is good, it will not fail.

2. Insulation Resistance Test: Regular testing to determine gradual decrease in insulation resistance. This provides a means for predicting future insulation failure.

3. Polarization Index Test: For testing high capacitance systems such as large motors, generators, or apparatus with complex insulation systems. Steady readings are only possible after the time-dependent currents have almost died down. Test measures the ratio between insulation resistance readings taken at one minute and ten minutes after the application of voltage to judge the rate of disappearance of capacitance and absorption currents. A ratio known as the polarization index can be obtained by dividing the value from the 10 minute reading by the value from the one-minute reading. A low polarization index usually indicates excessive moisture and contamination. On large motors or generators, values as high as 10 are commonly expected.

4. Step Voltage Test: Provides more information about the insulation than any other test; measures the current at several different levels of voltage to provide a trend or curve to predict need for repair or replacement. Step increases with two or more voltages @ 1:5 ratio, each step lasting 60 seconds.

Facts about Insulation Testing:

1. The commonly used DC test voltages for AC rated equipment are:

Readings will never be the same unless the specimen is discharged (4-5 times test period). 500 VDC is the most commonly required voltage since it is used to test all circuits except low voltage circuits with a nominal voltage up to and including 500 Volts. A 250 VDC test capability is necessary to test low voltage circuits supplied by an isolation transformer. A 50 Volt range will allow testing delicate components and equipment with up to 55 V of electrical interference, or cross talk.

2. Test or Proof voltages for equipment is considerably higher than used for routine maintenance testing. The rule of thumb:

DC maximums can be determined by the following formula:

Equipment before being put into service

Factory AC Test = 2 x Nameplate + 1000 Volts

DC proof test or insulation 0.8 x nameplate x 1.6

DC proof test after service 0.6 x nameplate x 1.6

Example:

Motor with 2400 Volt AC nameplate
Factory AC test= 2 (2400) + 1000 = 5800 VAC
Maximum DC test of Insulation: 0.8 (5800) 1.6 = 7242 VDC
Max DC test after service: 0.6 (5800) 1.6 = 5568 VDC

3. There are 3 types of current that appear in insulation testing:

A. Capacitance Charging Current
      This is the current which is like a condenser, which starts out high and tapers off rapidly to zero.

B. Absorption Current

This is due to the polarization of the insulating materials. It takes longer for absorption current to reach a static point than charging current and, likewise, takes a much longer time to bleed off. On large or long cables it is important to short out the cable after test to eliminate the possibility of shock to the person conducting the test.

C. Leakage Current

This is the current we are really concerned about. It is a steady current leakage through or over the insulation due to moisture, dirt, or other reasons. This test must be continued for one minute or until the reading holds steady for 15 seconds. This assures us that the capacitative and absorption currents have reached a static point. This will vary with the equipment under test. Motors and transformers will take longer than average conductors.

4. Megger Insulation Testers come in three basic types: Hand crank, Line or Motor operated, or Battery (or in combination). Battery or AC is preferred for tests one minute or longer in duration.

5. Danger:

A. All equipment under test MUST be disconnected and isolated.

B. Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.

C. Never use an Insulation Tester in an explosive atmosphere.

D. Make sure all switches are blocked out and cable ends marked properly for safety.

6. Manuals on Electrical Insulation Testing for the Practical Man are available from us through AVO Biddle:

A. A Stitch in Time-The Complete Guide to Electrical Insulation Testing

B. The Lowdown on High-Voltage DC Testing

C. Getting Down to Earth