Wiktionary explains the origin of the term molly guard:

Originally a Plexiglas cover improvised for the Big Red Switch on an IBM 4341 mainframe after a programmer's toddler daughter (named Molly) tripped it twice in one day. Later generalised to covers over stop/reset switches on disk drives and networking equipment.

What specifically did this switch do? Which parts of the computer were powered down or disabled? Was there any direct danger to people (e.g. electrocution or moving parts), or did it merely protect the equipment from fire or other damage? Was there any particular reason to put it at toddler height?

(If available, a picture of this specific button and its guard would also be nice.)

This question discusses the general properties of Big Red Buttons, but does not address specifics to the IBM 4341, which is notable for causing the invention of the molly guard.

IBM 4131 advertisement

  • 3
    The large-scale systems museum’s 4341 setup shows a couple of big red switches; there is also a power-down pushbutton on the operator console (which initiates a controlled shutdown). I don’t know which of these ended up being guarded. Mar 7, 2022 at 5:56
  • 1
    Emergency Off (EMO) or Emergency Power Off (EPO) buttons have a long history (see en.wikipedia.org/wiki/Kill_switch). Currently (pun intended) they are covered under NFPA 79 in the US, and equivalent requirements for industrial equipment in Europe. Court cases around the danger from the lack of an EMO date to the 60's and 70's, much like most of the workplace safety regulations.
    – Jon Custer
    Mar 7, 2022 at 15:55
  • The boxes IBM built to interface the (real) shuttle flight computers (also IBM) to the rest of the shuttle simulator, had these EPO red buttons. The units were designed in the late 70s. i.imgur.com/7geT6N5.jpg Mar 8, 2022 at 1:04
  • I recall one of the machines at the CHM, perhaps while still in Boston, had a unique Big Red Button that not only killed power but also physically popped open the front of the machine to allow airflow to cool it down. I think it may have been an NEC or NCR? Mar 21, 2022 at 16:23

1 Answer 1


The Big Red Switch (which took a bunch of forms) on the IBM System/370s, including the 4341, initiated a power-down of the entire system. There were Emergency Power Off (EPO) cables that were strung from the central machine (the CPU, or its master console) to all the various peripheral controllers (intermediary machines). Likewise, from the controllers to the peripherals themselves. Thus, in a fully-connected machine room, you could throw one switch and initiate a full-system power-down - the disk drives, the tape drives, printers, card readers, you name it. Some of these things didn't take well to having the power yanked in certain circumstances, so using the Big Red Switch was generally frowned upon. On certain S/360 and S/370 models, it was described as the Emergency Power Pull, and it latched in the off position if you pulled it.

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