Safety Guidelines For Hazardous Voltages - National Instruments 9221 Getting Started Manual

for measurements of voltages from specially protected secondary
circuits. Such voltage measurements include signal levels, special
equipment, limited-energy parts of equipment, circuits powered
by regulated low-voltage sources, and electronics.
Caution
signals or use for measurements within Measurement
Categories III or IV.
Note
equivalent. These test and measurement circuits are not
intended for direct connection to the MAINS building
installations of Measurement Categories CAT II,
CAT III, or CAT IV.

Safety Guidelines for Hazardous Voltages

You can connect hazardous voltages only to the NI 9221 with
screw terminal and the NI 9221 with spring terminal. Do not
connect hazardous voltages to the NI 9221 with DSUB.
If hazardous voltages are connected to the device, take the
following precautions. A hazardous voltage is a voltage greater
than 42.4 Vpk voltage or 60 VDC to earth ground.
Do not connect the NI 9221 with DSUB to
Measurement Categories CAT I and CAT O are
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