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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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Figure 15-2 illustrates a Biddle hand-cranked Megger®. Figure 15-3 illustrates a heavy-duty Biddle Megger® in both the hand-cranked and motor-driven types.
Figure 15-2 Biddle hand-cranked Megger® insulation
tester. Courtesy the Biddle Company.
On 480-volt bus and bus air circuit breakers, use the 1000-volt range on the insulation tester and the minimum acceptable test value shall be 100 megohms.
Cables and Conductors
The insulation of cables and cable and conductors will vary inversely with the ambient temperature, or rather the temperature of the insulation. The insulation resistance will also vary with the type
Insulation Testing 175
Figure 15-3 Biddle heavy-duty Megger® insulation tester.
Courtesy the Biddle Company.
of insulation. As the temperature rises, the insulation resistance lowers, and as the temperature lowers, the insulation resistance rises.
Table 15-1 covers several types of insulations. It is one of a number of tables available to you in handbooks. Since temperature affects the insulation resistance, some point in the temperature range must be picked as the zero temperature coefficient. In
Table 15-1 Temperature Coefficient Table
Test Temperature (THW) (RHW) Butyl
Degrees Fahrenheit Thermoplastic Heat Resistant Base
50 0.29 0.73 0.70
52 0.40 0.78 0.75
54 0.55 0.83 0.80
56 0.66 0.88 0.86
58 0.82 0.94 0.92
60 1.00 1.00 1.00
62 1.26 1.07 1.06
64 1.55 1.13 1.13
66 2.00 1.20 1.20
68 2.50 1.28 1.27
70 3.10 1.36 1.37
72 4.00 1.45 1.49
74 5.05 1.55 1.58
76 5.95 1.64 1.69
78 7.05 1.75 1.81
80 8.30 1.86 1.94
176 Chapter 15
Table 15-1, this point is 15.56°C or 60°F. With temperatures other than this zero temperature coefficient, the values in the following table must be used to arrive at a proper insulation resistance value.
In testing the conductors (three-phase, and thus three conductors will be used), tie two conductors to the conduit and/or the equipment-grounding conductor as shown in Figure 15-4, and test from the ungrounded conductor and the other two conductors as shown. Repeat to test the remaining two conductors by grounding other conductors and testing the ungrounded conductor, etc.
A
EARTH ~® ©■
Figure 15-4 Testing feeder conductors.
Take a look at an example using Table 15-1. Assume 600-volt insulation and a 1000-volt insulation tester. The conductor has THW insulation and the ambient temperature is 80°F. We refer to Table 15-1, and find the temperature coefficient to be 8.30. The insulation tester reads the resistance of the insulation at 20 megohms. As stated earlier, a minimum resistance of 100 megohms was required and 20 megohms is well below 100 megohms. But 20 X 8.30 = 166.00 when converted back to a base of 60°F, and this is above the 100-megohms minimum requirement. Had this been RHW insulation under the same conditions and reading, it wouldn’t have passed the minimum requirements, as the table shows a temperature coefficient of 1.86 at 80°F.
Insulation Testing 177
When testing a three-phase AC motor, the author recommends that all three phases should be connected together on the load side of the magnetic starter as shown in Figure 15-5. Table 15-2 is representative of satisfactory tests, when the readings are converted to 60°F, as covered in the formula given below.
Figure 15-5 Motor insulation test.
Table 15-2 Satisfactory Test Results
Insulation Tester Minimum Reading
Equipment Voltage in Megohms
460-volt, 3-phase induction motor 1000 20
208-volt, 3-phase induction motor 500 20
120-volt, 3-phase induction motor 500 5
The corrected reading for rotating equipment and transformers shall be in accordance with the following formula:
log R2 = log Ri +
N
178 Chapter 15
where
Rt = known resistance at temperature Tr in degrees Celsius. Ri is the insulation tester reading in megohms at the time of test and T is the ambient temperature of the winding.
R2 = unknown resistance of the insulation in megohms at temperature T2 in degrees Celsius (15.56°C). R2 is the reading shown in Tables 15-2 and 15-3, and for acceptable reading it should be equal to or greater than the resistance in the tables.
N = 30 for class A insulation
N = 60 for class B insulation
N = 23 for class H insulation
Table 15-3 Acceptable Transformer Readings
Insulation Tester Minimum Reading
Transformer Winding Voltage in Megohms
480-volt winding 1000 45
277-volt winding 500 45
208-volt winding 500 30
As stated before, the insulation resistance varies inversely with temperature. The changes are not linear but are logarithmic in character.
There are three tests that should be made on transformers. These are illustrated in Figure 15-6: Figure 15-6A, high-voltage winding to ground; Figure 15-6B, low-voltage winding to ground; Figure
15-6C, high-voltage winding to secondary winding. Table 15-3 lists acceptable readings for transformers.
A DC voltage supply such as illustrated in Figure 15-7 may be made up or purchased and used with a double-throw switch, as shown in Figure 15-8, and a high-resistance voltmeter. The formula to be used with this type of insulation tester is
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