National Instruments GPIB-120A User Manual page 5

Danger
The GPIB-120A does not generate high voltages; however, you must use extreme caution
if your system or application can cause high voltages on the shield or logic ground of the
GPIB cable you attach to the electrically isolated connector (GPIB Port B) located on the
rear panel of the GPIB-120A. Many GPIB cables use a metal shell to enclose the GPIB
piggyback connectors. This metal shell is normally connected to the cable shield.
Therefore, if the shield is at a high potential with respect to the chassis ground of the
GPIB-120A, the high voltage is also present, in most cases, on the cable connector
housing.
The chassis, back panel, and front panel of the GPIB-120A are electrically connected to
the earth ground pin of the AC power connector on the back panel. If the AC wiring in
your building is installed correctly, you can safely operate the GPIB-120A. Because the
EMI shield and jack screws of rear panel connector 'GPIB A' are electrically shorted to
the rear panel, a GPIB cable attached to GPIB Port A has its shield and connector
housing connected to earth ground, as well, and no danger should exist associated with
GPIB Port A. There may be, however, a hazardous voltage between the cable connector
housings of the GPIB cable attached to GPIB Port A and the GPIB cable attached to
GPIB Port B.
Do not touch the GPIB-120A case, power cord, or any GPIB cable connected to the
GPIB-120A unless you are absolutely sure that there are no hazardous voltages present.
In addition, never open the GPIB-120A case unless you have removed the power cord
and all GPIB cables from the back panel.
The GPIB-120A is not recommended for use when ground potential differences in excess
of 30 V are present between the two rear panel GPIB connectors and associated cables,
unless sufficient precautions are taken to ensure human safety.