What is the interlock feature of the Keysight B2901/02/11/12A?

To prevent shock hazard, the Keysight B2901/02/11/12A will not operate above 42 V unless you connect the interlock terminal. The terminal should be connected to a switch in the fixture lid or prober door. When the switch is closed, the Keysight B2901/02/11/12A works with voltage up to 210 V.
 

Please refer to the Users Guide of the B2901/02/11/12A in detail.