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WL Circuit Breaker Selection and Application Guide
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,
ETU748, and ETU776 can be retrofit-
ted with a ground fault protection
module.
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 unit ETU776, this setting is
implemented via the display and key
pad or communications.
Ground Fault Protection with I
2
t
Characteristic Curve
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 AT 745-748
Ground fault module GFM A 776
Ground fault module GFM AT 776
WL Breaker
Ground Fault Setting