Siemens WL - Page 48

Electronic Trip Unit
WL Circuit Breaker
1/44
WL Selection and Application Guide • 2005
Ground Fault Protection
When optional ground fault is
selected, the trip unit detects
fault currents that flow
to ground and represent a
fire hazard to the system. The
adjustable time delay allows
selective staggering of
consecutively arranged circuit
breakers.
When setting the parameters of
the trip unit, a selection can be
made between alarm and trip if
the set current value is
exceeded. The cause of the trip
is displayed on an LED when the
query button is pressed.
Modules
The trip unit versions ETU745,
ETU478, ETU755 and ETU776
can be retrofitted with a ground
fault protection module. In the
case of overcurrent release
ETU727, this module is
permanently installed.
Two versions of the optional
ground fault module can be
ordered:
• Trip and Alarm
• Alarm only
Ground Fault Measuring
Methods
Residual sensing of the ground
fault current
The trip unit calculates the
ground fault current by vectorial
current summation of the 3-
phase currents and the neutral
conductor current.
Direct measurement of the
ground fault current
A current transformer with the
transformer ratio 1200A : 1A
is used to measure the ground
fault current. The transformer
can be installed directly in the
grounded star point of a
transformer.
Setting
The ground fault module can be
set depending on the
measuring method (see above):
Measuring method 1:
in position sum I
Measuring method 2:
in position G.
With trip units ETU755 and
ETU776, this setting
is implemented via the
display and key pad or
communications.
Ground Fault Protection with
I
2
t Characteristic Curve
With the exception of the trip
unit ETU727, all versions of
the ground fault modules are
delivered with an I
2
t or fixed
delay.
Modules are available in either
Alarm only or Alarm and Trip
functions.
Residual sensing of the ground fault current
Direct measurement of the ground fault current
Ground fault module GFM A 745-748
Ground fault module GFM A 755-776
Ground fault module GFM AT 745-748
Ground fault module GFM AT 755-776
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