9 – 13
Neutral conductor protection – N-tripping
Trip units ETU745 - 776 also make it possible to protect the neutral conductor against overload. This requires a current transformer for the
neutral conductor, which can be retrofitted if necessary.
(page 9-97)
For tripping, the same long-time delay t
R
applies as for overload tripping.
9.1.5.2 Additional functions
Load monitoring (load restore / load shedding)
Trip units ETU745 - ETU776 offer the possibility of additional load monitoring. Two current values, "load shedding" and "load restore", as
well as a delay time t
x
, can be set.
If the setting value "load restore" is undershot, and the lower limit value for current transmission is exceeded at the same time, a signal is
output by the
CublicleBUS after the set delay time t
x
has elapsed. If the setting value "load shedding" is exceeded, a signal is output by the
CublicleBUS after the set delay time t
x
has elapsed. These signals can be used to connect or disconnect loads, thereby preventing an over-
load tripping of incoming circuit breakers.
Load monitoring can be adjusted via:
- the alphanumeric display (ETU745)
- the graphical display (ETU776)
- TD400 and the software "powerconfig"
- the COM modules with a PC with the software “powerconfig” installed (page 29-2)
Leading signal for L-tripping
Trip units ETU745 - 776 provide a leading signal for "L-tripping", which is transmitted via the CublicleBUS 100 milliseconds before overload
tripping. In this way it is possible e.g. to disconnect thyristor controllers.
Phase loss sensing
When phase loss sensing is active and the load current on the least-loaded phase is at least 50% below that of the highest loaded phase,
the long-time pickup value I
R
is automatically reduced to 80% of its set value. This is designed to prevent overheating conditions in motor
loads when a single phase of voltage is lost and the motor is operating on 2 phases. If the highest and lowest load phase currents return to
differing by less than 50%, I
R
returns to its set value.
In the trip unit ETU776, phase loss sensing can be activated independently from the motor protection t
sd
setting of 20ms.
Setting values for I
N
ETU745
I
N
= (0.5 / 1.0) × I
n
; OFF
ETU776
I
N
= (0.2 - 2.0) × I
n
; OFF
NOTICE
Neutral Conductor Overheating.
Neutral conductor or insulation may be damaged.
Settings I
N
> 1 x I
n
should only be used if the neutral conductor has been properly sized.
Setting values for load monitoring
"Load shedding" and "load restore" 40 A - 1.5 x I
n
Delay time
t
x
= 1 - 15 sec.
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