Source : Free On-Line Dictionary of Computing

scram switch
     
         (From the nuclear power industry) An emergency
        power-off switch (see {Big Red Switch}), especially one
        positioned to be easily hit by evacuating personnel.  In
        general, this is *not* something you {frob} lightly; these
        often initiate expensive events (such as Halon dumps) and are
        installed in a {dinosaur pen} for use in case of electrical
        fire or in case some luckless {field servoid} should put 120
        volts across himself while {Easter egging}.
     
        SCRAM stands for Safety Control Rod Ax Man.  In the early days
        of nuclear power, boron moderator rods were raised and lowered
        on ropes.  In the event of a runaway chain reaction, a man
        with an axe would chop the rope and drop the rods into the
        nuclear pile to stop the reaction.
     
        See also {molly-guard}, {TMRC}.
     
        [{Jargon File}]
     
        (2003-05-17)