On Thu, 05 Apr 2018 22:14:01 GMT, im2oldBob
m wrote:

replying to hrhofmann, im2oldBob wrote:
In a textbook, Electric Wiring Residential 15th edition based on 2005 National
Electrical Code by Ray C Mullen. ISBN1-4018-5019-. Mr. Mullen addresses the
subject. Mr. Mullen' advocates, that while not a code requirement that the
ground ( U blade) should be up for the reason that conductive items falling
would encounter the ground first, or at least fall across either one of the
conductors and the U blade if the conductor was positive then a shunt trip
would occur. Note; this is not an exact quote.

The only "book" that counts is the National Electrical Code and it is
silent on the issue.
If you look long enough you will find people arguing the opposite
case. When a plug loosens and starts falling out, ground down assures
the ground connection is the last to break.
Neither are a significant enough reason to drive a code change.