Section 4: Using the Model 9100: Insulation/Continuity Function (Option 135) 4.18-13
The insulation tester has a high-voltage DC-DC converter which transforms the low
voltage from its batteries up to the high voltage DC required for insulation tests. Usually,
there is a resistance in series with the converter. To calculate the resistance of insulation,
the tester compares the voltage across its terminals with the voltage from its internal
power supply. The ratio of these two voltages corresponds to the ratio of the potential
divider comprised of the internal series resistance and the insulation resistance being
measured.
The high voltage power supply and series resistance of the UUT can usually be switched
to allow different ranges of voltage output and resistance precision and amplitude.
The topology of the Option 135 circuit is essentially the same as that for the Resistance
function, with the following important differences:
1. The output amplifier has to be able to produce voltages in excess of 1000VDC. This
is a unipolar amplifier that can only produce positive output voltages.
2. If you attempt connection to the 9100 with the lead polarity reversed, when you press
the UUT test button you will see a message ‘UUT polarity reversed or no UUT
applied voltage’. This indicates that the 9100 has detected a negative output voltage
on the HI terminals. Reconnect with the correct lead polarity and repeat the test if this
message appears.
3. The high impedances used in insulation resistance measurements mean that 4-wire
connection with a lead compensation bridge are not required. Because the minimum
selectable resistance of Option 135 is 100kΩ, any small series resistance that occurs
within the system can be automatically compensated within the internal calibration of
the 9100. The series resistance of the test leads (about 0.1Ω) is insignificant compared
with the minimum test resistance (100kΩ), and therefore represents a worst case error
of approximately 0.0001%. This is well below the measurement noise, resolution and
absolute accuracy of the test system.
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