On Mon, 28 Sep 2009 15:44:00 -0500, Alfred K. wrote:

Consider an extreme situation: a fully opened faucet and a completely
closed pop-up bathroom sink drain. Should a properly installed overflow
opening actually prevent the sink overflowing? Or it is supposed to
just help the main [open] drain by lowering the about to overflow water
level down faster?



I would NOT expect the overflow to keep up with a fully opened faucet.
Certainly not a good idea to run a faucet full blast and leave the
room.