Biddle MEGGER - Page 56

Describing Figure 45—a 500-volt instrument has a ballast re-
sistance of approximately 100,000 ohms; if the resistance under
test happens to be 100,000 ohms, the rated instrument voltage of
500 volts will obviously be divided equally between these two
resistances, and the voltage at the Earth and Line terminals will
be 250. When testing an external resistance of one megohm, the
terminal voltage will be about 455 volts (see formula on page 53).
This example shows that, knowing the internal ballast resistance
of a Megger Insulation Tester, the terminal voltage characteristics
can be plotted, for example—the Cat. No. 21159 “Major” Megger
Insulation Tester. If the instrument reads 2 megohms on the 500
volt range, the voltage across the test specimen will be about 475
volts.
1100
Βρες coger πας [TT
ως ee MULTIPLY READING BY 2 E
maw |
|_| | | “is
ο
3 w l T =
55
Ξ η! Επι
O 600 ;
A --
«τ 500 —— -- ES ee 6౬...
z ΠΡ. MULTIPLY READING BY 1 Bk
č 400 -
Ww
E BAA η
300
pL a 250-VOLT_RANGE ||
— జా
DIVIDE READING BY 5 ae
50 100
500 1000 %
FIGURE 48. Typical Voltage Characteristics for Cat. Νο. 21159 "Major" Megger
Insulation Tester.
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