Siemens SENTRON 3VL series, System Manual

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Functions
5.1 Current protection
3VL molded-case circuit breakers
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System Manual, 03/2009, 110 0110 - 02 DS 01
5.1.8 Ground-fault protection
Ground fault trip "G" (ground fault overcurrent protection) captures fault currents escaping to
ground that can cause fires in the plant. Several circuit breakers connected in series can be
assigned time-graded discrimination by means of the adjustable delay time.
Measurement method 1: Vectorial summation current formation
Ground fault detection in balanced systems
The three phase currents are evaluated using vectorial summation current formation.
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Figure 5-4 Circuit breaker in balanced system
Ground fault detection in unbalanced systems
The neutral conductor current is measured directly. Only the ground-fault current is
evaluated for the 3-pole circuit breakers. In the case of the 4-pole circuit breakers, the
neutral conductor overload protection is also evaluated.
The overcurrent trip unit calculates the ground-fault current using the vectorial summation of
the three phase currents and the neutral conductor current.
The 4th current converter of the neutral conductor is installed internally in the case of 4-pole
circuit breakers.
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Figure 5-5 3-pole circuit breaker, current converter in neutral conductor current