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.
I normally only load the wire up till needle settles down - about 5 to 10 seconds, if it is just for my own information
Facts about Insulation Testing:
1. The commonly used DC test voltages for AC rated equipment are:
up to 100 Volts 100 or 250 Volts DC
440-550 Volts 500 or 1000 Volts DC
2400 Volts 1000-2500 Volts DC
4160 Volts 1000-5000 Volts DC
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.
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