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.

