The static logic box receives the signal from the primary current transformer.
It monitors the signal, senses overloads or faults, and executes the required
action in accordance with preselected settings. The tripping actuator receives
the output signal from the static logic box and in turn causes the circuit
breaker contacts to open.
Manufacturers have made great strides in the development and application
of solid-state trip devices over the past 10 years or so. The motivation for this
effort has been to bring protective devices to market that offer improved
features over the original direct-acting designs yet preserve the totally selfcontained
concept exhibited by their forerunners. When the static trips were
fi rst introduced, manufacturers struggled to provide reliable and repeatable
devices that would prove to be lower maintenance items than the ones they
replaced. Their diffi culties were not easy to overcome. The main problem has
always been related to a consistent and accurate method to both derive
operational power from the signals resulting from current fl owing through
the breaker, and to accurately measure the current at the same time. The
developments came one by one. For some time, most manufacturers still had
to rely on a magnetic device to get an INST function. The designs of that time
did not allow an electronic device to build up suffi cient power to trip the
actuator fast enough to be called INST.