Joseph Meehan wrote:
surf wrote:
...AC ... pluged into a 3 prong wall socket
...assumed the wall socket was grounded ...can determine this ?

The unit resets itself after a couple of days, that is it shuts off,
the green light on the plug turns off and you have to hit the test and
reset switch ...on the plug that goes into the wall socket. It seems to
trip when it's not running. ...

That sounds like your AC is plugged into a GFI. Normally I would not
suggest doing that, in part due to exactly what you are seeing.

Where is the A/C located? Is it in a kitchen or bath?
....

Agree it sounds like it's in an external GFCI unit. To OP, reason
asked about the kitchen bath is that GFCI is req'd by new code for
them. An A/C is heavy load and GFCIs are very sensitive so I'm
guessing it's tripping it occasionally when it tries to restart before
the compressor has been off long or when something else on the circuit
may be on at the same time.

To check the outlet, remove the socket from the wall (carefully or
better turn the breaker off first) and see if the third wire is
connected to the ground terminal of the plug. One could also go to the
service panel and remove the cover and see if there's a ground wire on
corresponding to that breaker (again, of course, turn the whole panel
off first).

There are some plug-in-the-wall circuit testers but I have to honestly
say I've never used one and don't know just how reliable they are for
testing for the ground connection.

In general, though, if the house is mid-60s or later, chances are good
(and better the newer, pretty rapidly) it was wired w/ ground
initially. 50s or earlier, chances diminish rapidly the other way
unless it has been upgraded.