clin:
I have never heard of any real written standard. However, I do latching circuits in plc's all the time. If you are speaking of ladder design; it is laid out just like a standard latching circuit in any motor control diagram or by a set or reset command (which is dictated by the type of plc instruction set available to you).
For contactors or the like, I always use a mechanical interlock. You have the command structure with a failure to open (I.E. stop) and the mechanical interlock to prevent simultaneous operation of both forward and reverse direction.
For process controls, I always use a watchdog timer to do a safty shutdown if there is a come loss or time out. A lot of the process safeguards are dictated by code (NFPA 70 for furnaces as an example.)
Can you elaborate more on what you are looking for?
Carl