The time delay bands are accomplished by the action of the solenoid’s pull
against springs, pneumatic, or hydraulic devices. Since these elements are
purely mechanical, different characteristics cannot be supplied in a single trip
device. Although some calibration points and some effects on the time can be
changed by adjustments, completely different delay bands can be selected only
by physically changing out the trip devices with others of the desired type.
Direct-acting trips are still used on some applications, and are still required on
power circuit breakers used on DC unit substations.