The Jargon File (version 4.4.7, 29 Dec 2003):

scram switch
 n.

    [from the nuclear power industry] An emergency-power-off switch (see Big
    Red Switch), esp. 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. (See
    also molly-guard, TMRC.)

    ?Scram? was in origin a backronym for ?Safety Cut Rope Axe Man? coined by
    Enrico Fermi himself. The story goes that in the earliest nuclear power
    experiments the engineers recognized the possibility that the reactor
    wouldn't behave exactly as predicted by their mathematical models.
    Accordingly, they made sure that they had mechanisms in place that would
    rapidly drop the control rods back into the reactor. One mechanism took the
    form of ?scram technicians?. These individuals stood next to the ropes or
    cables that raised and lowered the control rods. Equipped with axes or
    cable-cutters, these technicians stood ready for the (literal) ?scram?
    command. If necessary, they would cut the cables, and gravity would
    expeditiously return the control rods to the reactor, thereby averting yet
    another kind of core dump.

    Modern reactor control rods are held in place with claw-like devices, held
    closed by current. SCRAM switches are circuit breakers that immediately
    open the circuit to the rod arms, resulting in the rapid insertion and
    subsequent bottoming of the control rods.


The Free On-line Dictionary of Computing (30 December 2018):

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)