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2 definitions found
 for scram switch
From 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.
  

From 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)
  

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