National Instruments NI 9401, Manual

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Safety Voltages
Connect only voltages that are within the following limits:
Maximum voltage
[1]
Channel-to-COM ±30 V maximum on one channel at a time,
Measurement Category I
Isolation voltages
Channel-to-channel
None
Channel-to-earth ground
Continuous
60 VDC, Measurement Category I
Withstand 1,000 Vrms, verified by a 5 s dielectric withstand
test
Measurement Category I is for measurements performed on circuits not directly
connected to the electrical distribution system referred to as MAINS voltage. MAINS
is a hazardous live electrical supply system that powers equipment. This category is
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
Do not connect the NI 9401 to signals or use for measurements
within Measurement Categories II, III, or IV.
Note
Measurement Categories CAT I and CAT O are 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.
1
The maximum voltage that can be applied or output between any channel and
COM without damaging the module or other devices.
Safety Guidelines for Hazardous Locations
ni.com
4
NI-9401