Test Voltage; Spot Testing - AEMC 6501 User Manual

4.3.1
No published standard tells which voltage to choose for any given
winding. However, published recommendations could be summa-
rized as follows:
4.3.2
As a general rule in spot testing, test voltage should be applied until
no variation in reading is noted for 15 seconds or applied for a fixed
60 seconds. On capacitance circuits, cranking for up to a minute or
more may be necessary to fully charge the sample. What minimum
value should be measured? The IEEE standard N° 43-2000 states
that it is impossible to specify the value of insulation resistance at
which a winding will fail electrically, but on motors, minimum read-
ings generally stated are:
There is no fixed figure for determining what is good and bad in
resistance readings, but a good guide would be 1MΩ for every one
hundred applied operating volts, as a minimum figure. This applies
to motors and transformers. When the insulation resistance gets
this low, an electrical break-down can be expected at any time and
rewinding or replacing should be considered.
It is not unusual for a winding to be 10 to 100 times the recom-
mended minimum value (IEEE standard 43-2000: Recommended
Practice for Testing Insulation Resistance of Rotating Machinery),
but this varies with temperature and humidity.
Megohmmeter Models 6501/6503
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Test Voltage

Rated Voltage of Motor
Below 115
115
230
460

Spot Testing

Rated Voltage
250 or less
460
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Test Voltage
250V
250 or 500V
500V
500 or 1000V
R minimum
2 MΩ
2 MΩ
13
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