Siemens SENTRON 3WA1 Manual page 54

Description
2.3 Protection system
Phase failure detection
If the current of the phase with the lowest load is 50% less than the current of the phase
with the highest load, this is interpreted as a phase failure and the setting I
automatically reduced to 80%. If the three phase currents do not differ by more than 50%
with respect to each other, the setting I
Phase failure detection can be switched on/off for the ETU600 electronic trip unit.
Thermal memory
The electronic trip unit offers the possibility to continue the internal mathematical
simulation of the thermal processes in downstream systems and loads even if the circuit
breaker is switched off and no external power supply to the electronics is available. This
ensures effective protection against thermal overload, even with frequent closing and
opening operations and fluctuating loads. An earlier, completed overload excitation can
have a time-shortening effect on a pending overload trip.
Functional principle of the thermal memory:
The thermal memory operates in the overload range. Currents below the tripping
threshold are not included in the evaluation. When the operational current exceeds this
threshold, a strictly monotonic thermal evaluation is performed according to the
characteristic. When the operational current falls below this threshold once again,
cooling takes place according to an exponential function with an adjustable time
constant.
• Behavior when thermal memory is switched on:
The thermal history is taken into account. After tripping, the thermal memories of the
phases including the neutral conductor are preset with the thermal equivalent of the
warmest phase reduced to 90%. This makes it possible to reclose the circuit breaker.
Cooling down takes place according to an exponential function with adjustable time
constant and is active for a maximum of 30 minutes after overload tripping.
• Behavior when thermal memory is switched off:
The thermal history is not taken into account.
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applies again.
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Equipment Manual, 02/2023, 92310000002-06
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3WA1 air circuit breaker