On 2007-04-23 15:14:16 +0100, said:

On Fri, 20 Apr 2007 23:30:44 +0100, Andy Hall
wrote:

In some
installations, the bath and or shower is sufficiently close to the
floor drain that there is simply a pipe from the appliance waste under
the floor and leading into the floor drain with no trap at the
appliance. A single trap from the floor drain then connects to the
soil pipe.

I'm confused about this. Why doesn't the bath water flood up through
the drain when the plug is pulled out in this scenario?

Because the exit to the soil pipe via the trap in the floor is larger
(typically 75mm) than the pipe from the bath (typically 40mm)