9 – 12
Motor protection function
When the short-time delay is set to 20ms (t
sd
= 0.02 sec.), a special motor protective function is enabled which prevents short-time
tripping during the turn-on inrush for motors. At the same time, a phase loss sensing function is enabled (page 9-13) and the thermal time
constant used for long-time protection is changed from one suitable for bus protection to one suitable for motor protection.
Instantaneous short-circuit tripping – I-tripping
If the current setting I
i
is exceeded, the circuit breaker is tripped instantaneously.
Ground-fault tripping – G-tripping
If the trip unit is equipped with a ground-fault protection module, loads can be protected against impermissibly high ground-fault currents.
Trip units ETU745 - 776 can be equipped with it optionally. Ground-fault protection modules (page 9-44)
The response value I
g
, together with the setting for the delay time t
g
, determines the shutdown of ground-fault currents.
If the zone selective interlocking (ZSI) (page 9-14) is used and the ZSI module is set to "S" or "S+G" the adjusted delay time t
g
is auto-
matically set to 100 ms provided that in the event of ground-fault the circuit breaker does not receive a blocking signal from a downstream
circuit breaker. In this case regardless of the adjusted t
g
value the circuit breaker will trip after 100 ms.
If a blocking signal exists the adjusted delay time t
g
is valid. For safety reasons after 3 s the blocking signal is terminated. Some trip units
can be switched over to an I
2
t-characteristic.
Setting values for I
i
ETU745 OFF: I
i
= I
CW
I
i
= (1.5 / 2.2 / 3 / 4 / 6 / 8 / 10 / 12 × I
n
MAX = 0.8 x I
CW
ETU776 I
i
= 1.5 x I
n
- 0.8 x I
cs
; OFF : I
i
= I
CW
Setting values for I
g
Frame size
I & II III
A 100 A 400 A
B 300 A 600 A
C 600 A 800 A
D 900 A 1000 A
E 1200 A 1200 A
Setting values for t
g
ETU745 t
g
= 0.1 / 0.2 / 0.3 / 0.4 / 0.5 sec.
ETU776 t
g
= 0.1 - 2.0 sec.
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