Abstract: The Petri net theory has been used to model many types of
   systems and many net analysis methods have been devised to study their
   behavioural properties. Safeness or boundedness is one such property,
   which, in its usual sense, sets a bound on the maximum number of
   tokens that can be assembled in a place. In a type of standard Petri
   nets with simple tokens, the Elementary net system, to be safe is to
   avoid a phenomenon called a contact situation, in which the pre- and
   postconditions of an event are satisfied at the same time. We define a
   time extension of EN system called Timed EN (TEN) system. In the
   system, by introducing time constraints, one is able to control the
   flow of tokens, and ensure that the system can still be safe even in a
   contact situation. We call this notion - safeness in time, which is
   then applied to a high-level version of TEN system, called Timed
   Object Petri net (TOPnet) system. A simple queueing example specified
   in TOPnet is used as an illustration.