“A Stitch in Time...”

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presence of moisture. If the insulation resistance is first tested on the shorttime reading basis (first at one voltage level and then at a higher potential), a lower value of insulation resistance at the higher DC test voltage usually indicates the presence of moisture. The applied voltages should preferably be in the ratio of 1 to 5. Experience has indicated that a change of 25% in the insulation resistance value, with a 1 to 5 ratio in test voltages, is usually due to the presence of an excessive amount of moisture. This method is not based on a dielectric absorption phenomena, but it does relate to the Evershed Effect. As with time-resistance measurements, the multi-voltage method of testing insulation resistance has increased value when made on a periodic or scheduled basis. Figure 22-Time Resistance Test Forms A STITCH IN TIME 47
Step-Voltage Method In this method, you need a multi-voltage Megger instrument to apply two or more voltages in steps, like 500 volts and then 1000 volts. You look for any reduction of insulation resistance at the higher voltage. If the resistance is lower, it’s a sign of an insulation weakness that shows up only at the higher voltage. Fig. 23 shows an example in which, instead of progressively increasing the voltage, you first test at a low voltage (such as 500 volts) and then, after discharging the sample, you test again at a higher voltage (such as 2500 volts). Any difference in the two tests in terms of megohms will show signs of weakness at the higher voltage – a warning to investigate further. As the conditions within the sample deteriorate, the higher-voltage plot, as shown in Fig. 23, will reduce in megohms compared to the lower-voltage one, and its upward slope will be less. Figure 23-Typical curves with the “step-voltage” test. The theory behind the step-voltage technique is a bit complex, but we’ll try to keep it simple. Moisture and dirt in insulation are usually revealed by tests at voltages far below those expected in service. However, effects of aging or mechanical damage in fairly clean and dry insulation may not be revealed at such low stress. 48 A STITCH IN TIME
Page 1 and 2: “A Stitch in Time...” US $9.95
Page 3 and 4: Contents Page WHAT IS “Good” in
Page 5 and 6: Common sense tells us that the more
Page 7 and 8: The generator can be hand-cranked o
Page 9 and 10: FACTORS AFFECTING INSULATION RESIST
Page 11 and 12: With a background now of how time a
Page 13 and 14: Figure 7-Typical behavior of insula
Page 15 and 16: Fig. 9 shows how a 60-second test w
Page 17 and 18: test Voltage vs. Equipment Rating C
Page 19 and 20: In summary, DC test sets are employ
Page 21 and 22: Figure 10-Typical drying curve wher
Page 23 and 24: Note that the resistance is almost
Page 25 and 26: The aforementioned statement assume
Page 27 and 28: SAFETY PRECAUTIONS Observe all rule
Page 29 and 30: 1. AC Motors & Starting Equipment F
Page 31 and 32: Connections at the main power board
Page 33 and 34: 7. Power Transformers Figure 19 Con
Page 35 and 36: Instructions for Use The untagged e
Page 37 and 38: Interpretation-Minimum Values Insul
Page 39 and 40: Minimum Values for Insulation Resis
Page 41 and 42: These standard minimum values (for
Page 43 and 44: Values of C at 20°C 60-Hertz 25-He
Page 45 and 46: 44 A STITCH IN TIME Values of Log 1
Page 47: To better understand the technique
Page 51 and 52: You only need to keep the test volt
Page 53 and 54: There are also cases, such as found
Page 55 and 56: The highest accuracy is desired in
Page 57 and 58: Figure 33-Connections for testing i
Page 59 and 60: Figure 35-Step 1 Figure 36-Step 2 5
Page 61 and 62: Setting up a Maintenance program To
Page 63 and 64: Megger 5 kV & 10 kV Insulation Test
Page 65 and 66: Megger MIT300 Series The MIT300 Ser
Page 67: Megger makes more than just insulat

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