Abstract: Demonstrator apparatus utilizes a display to portray in log-log coordinate scale the trip-time curve of a solid state (static) trip circuit breaker. A simulated static trip unit programmer is selectively adjustable to vary the shape and position of the displayed trip-time curve. An overlay bearing the trip time-current curve for a prospective upstream or downstream circuit protective device is superimposed on the display and the programmer adjusted to demonstrate selective trip coordination. Alternatively, a pair of simulated trip unit programmers are connected to the display to jointly portray thereon a pair of trip-time curves. Also disclosed is the utilization of a computer to generate on a display plural time-current response curves of a series of circuit protective devices selected on the basis of specific distribution system parameters to provide requisite system protection and response coordination.