Keithley 2461 Quick Start Manual page 12

An interlock circuit is provided on the rear panel of the instrument,
as shown in the following figure.
The interlock is intended for use through a normally open switch,
which may be installed on the lid of a test fixture, on the enclosure of
a semiconductor prober or device handler, or on the door or doors
of a test equipment rack. The switch must be closed to enable the
Model 2461 to produce voltages greater than ± 42 V.