Keithley SourceMeter 2657A Quick Start Manual page 12

If there are multiple points of operator access in the test system, each
point should have a separate relay wired in series with the interlock.
Using a series connection ensures operator safety if an attempt is
made to access the circuit while hazardous voltages are present.
Connect the enclosure of all metal test fixtures to protective earth
(safety ground). Nonconductive test fixtures must be rated to double
the maximum voltage in the test circuit. Failure to attach the ground
wires to a known protective earth may result in electric shock.
The Model 2657A is provided with an interlock circuit that must
be positively activated in order for the high voltage output to be
enabled. The interlock helps facilitate safe operation of the
equipment in a test system. Bypassing the interlock could
expose the operator to hazardous voltages that could result in
personal injury or death.