Apple iMac (21.5-inch and 27-inch, Mid 2010) - Manual (Page 63)

63 Chapter 5 Last, but Not Least
Unplug the power cord (by pulling the plug, not the cord) and disconnect the ethernet
cable or phone cord if any of the following conditions exist:
 You want to add memory
 The power cord or plug becomes frayed or otherwise damaged
 You spill something into the case
 Your iMac is exposed to rain or excess moisture
 Your iMac has been dropped or the case has been damaged
 You suspect that your iMac needs service or repair
 You want to clean the case (use only the recommended procedure described later)
Important: The only way to turn off power completely is to unplug the power cord.
Make sure at least one end of the power cord is within easy reach so that you can
unplug your iMac when you need to.
WARNING: Your AC cord has a three-wire grounding plug (a plug that has a third,
grounding pin). This plug fits only a grounded AC outlet. If you are unable to
insert the plug into an outlet because the outlet isn’t grounded, contact a licensed
electrician to replace the outlet with a properly grounded outlet. Do not defeat the
purpose of the grounding plug.