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R = r(V - v)
V = voltage at test terminals
v = voltage with insulation in series with the voltmeter r= resistance of voltmeter in ohms (generally marked on label inside the instrument cover)
R = resistance of insulation in megohms
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(A) High voltage to ground. (B) Low voltage to ground. (C) High voltage to low
Figure 15-6 Transformer tests.
Figure 15-7 Electronic supply for insulation testing.
When drying out wet insulation the resistance will fall rapidly as the temperature is raised during the drying operation. After falling to a minimum for a given temperature, the resistance will gradually rise as the drying progresses and the moisture is expelled from the insulation. See Figure 15-9 for a representative graph, showing a resistance curve and a temperature curve, both plotted against drying time. Take particular note of points A and B. The resistance is leveling off, indicating that the insulation is dried out and, at this point, the heat is shut off. As the temperature of the winding drops, the insulation resistance will rise rapidly.
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Figure 15-8 Switching arrangement for insulation tester.
HOURS'DRYING TIME Figure 15-9 Insulation drying curves.
Insulation Testing 181
Conductors in Parallel
In Section 310.4 of the NEC, conductors in parallel are covered. A number of rules are set out, including the condition that the paralleled conductors shall all have “the same insulation type.” At a recent meeting of the Rocky Mountain chapter of the I.A.E.I., the author was asked to explain why the insulations of parallel conductors had to be the same type. This question is often asked and should be answered.
The entire problem in paralleling conductors is to keep the impedance of each conductor as nearly the same as is possible, so the load carried by each conductor will be a balancer. You will recall that impedance is AC resistance, which includes inductive reactance, capacitive reactance, and plain resistance.
Conductor insulation resistance in megohms varies with temperature and types of insulation.
Table 15-1 (p. 000) gives temperature coefficients for three types of insulation. Since there is a great variation in insulation resistance with coefficient temperatures, the leakage of some types of insulation is higher than other types. Leakage adds up to a loss of current, even though it might be a small loss. Thus, if different types of insulation are used on parallel conductors, it will affect the impedances of the paralleled conductors.
A case in point: The author was in charge of checking Megger® insulation tests of large conductors. After pulling in the conductors, an insulation test was to be made. The first test was made in early spring, early in the morning when the temperature was low. Parallel circuits were run across a roof, and in rigid conduit exposed to sunlight. The specs called for RHH-Use to be run. It came time to energize these circuits and insulation tests were run again before energizing. The insulation resistance tested quite low, so the circuits were not energized.
After considerable research and investigation it was found that THW insulation was installed instead of RHH-Use. The temperature was high. Graphs of comparisons of the two types of insulation were made and from these the answer was quite apparent.
Figure 15-10 is a graph that may be used for comparing insulation resistance against temperature for RHH insulation. Figure
15-11 is a graph for insulation resistance coefficient and ties in with Table 15-1 for THW and temperature. (Note that logarithmic graph paper is used.)
From the above information, it may readily be seen why the NEC requires the same type of insulation to be used on parallel runs of conductors.
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Figure 15-10 RHW insulation corrected to 60°F. Place straightedge on measured Megger® tester value, scale A, and temperature, scale C. Read corrected Megger® value on scale B.
Insulation Testing 183
Temperature Degrees F
1Q00 40 50 60 70 80 90 100 1 10 120 130 150 1 70 190
i L Temperature Correction Factors For THW Insulation