Specifications

Principles of Operation
4
The Siemens Electronic Trip Unit utilizes a microprocessor to
execute the myriad of numeric and logic functions programmed
in the unit.The adjustments on the trip unit face allow the user
to select the numerical values to be used by the microproces-
sor in performing its protection function. (The numerical values
are stored in the processor’s permanent memory.)
Current data is derived from current sensors mounted in the
Siemens SB Systems Breaker.These sensors are current trans-
formers. As passive devices they provide high reliability with
minimum signal error, while providing power for the trip unit.
The current signals from the transformers are converted to
digital voltages in the trip unit.These digital voltages are stored
in temporary memory.The digital signals are used by the micro-
processor in detecting and processing overcurrent conditions.
The microprocessor reads the temporarily stored digital
voltages and compares these values with the permanently
stored values that correspond to the user-selected adjust-
ment settings.When an overcurrent condition is detected, the
microprocessor’s software begins to process the appropriate
protection function. During the processing of the protection
function, the microprocessor continues to monitor the incoming
current level data. If the overcurrent condition continues until
the processing is completed and the appropriate delay time has
elapsed, a tripping command is issued by the microprocessor.
The tripping command from the microprocessor causes a signal
to be sent from the electronic trip unit to the low energy, high
speed magnetic latch in the circuit breaker.The signal in the trip
unit counteracts the permanent magnet in the latch, allowing
the latch to trip the breaker.
SENSE
DECIDE
ACT
Current Transformers
Current transformers sense the
levels of the phase currents
and provide data and power to
the electronic trip unit.
Electronic Trip Unit
The trip unit monitors the
current levels at an equivalent
sampling rate of 353 samples
per cycle per phase. In the
event of an overcurrent
condition, the trip unit’s micro-
processor determines when
the circuit breaker should trip.
Magnetic Latch
The magnetic latch causes the
circuit breaker to trip when it
receives a trip command from
the electronic trip unit.
Current
Level
Signals
Trip
Command