9 – 12
9.1.6 Protective functions
9.1.6.1 Basic protective functions
The basic protective functions of the trip unit are ensured without additional auxiliary voltage. The required energy is supplied by the
circuit breaker's internal energy transformers.
To evaluate the currents, the electronic system of the trip unit calculates the r.m.s value.
The individual functions are parameterized according to the types via:
- rotary coding switch (ETU745 - 748)
- electronic data transfer (ETU776) via:
-the BDA
-the PROFIBUS-DP / MODBUS
- the control board (ETU776)
Overload protection – L-tripping
The current setting I
R
defines the maximum continuous current the circuit breaker can carry without tripping. The long-time delay t
R
determines the maximum duration of an overload without tripping.
The tripping characteristic is an I
2
t characteristic. Some trip units can be switched over to an I
4
t characteristic.
(page 9-15)
Short-time delayed short-circuit tripping – S-tripping
On trip units ETU745 - 776, tripping due to the short-circuit current I
sd
can be delayed by the time t
sd
.
This provides selectivity for short-circuit protection in switchgear with several grading levels.
1)
The delay time 0.02 sec. is not an I
2
t function.
The motor protection function is activated in this position.
2)
For settings t
sd
>0.4 sec., the maximum possible setting I
sd
is reduced automatically according to the frame size:
Frame size II : 20 kA
Frame size III : 30 kA
The setting "OFF" for trip units ETU745 and ETU776 is provided to deactivate the short-time delayed short-circuit protection.
If the zone selective interlocking (ZSI) (page 9-15) is used and the ZSI module is set to "S" or "S+G" the adjusted delay time t
sd
is auto-
matically set to 50 ms provided that in the event of short-circuit the circuit breaker does not receive a blocking signal from a downstream
circuit breaker. In this case regardless of the adjusted t
sd
value the circuit breaker will trip after 50 ms.
If a blocking signal exists the adjusted delay time tsd 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. (page 9-16)
Setting values for I
R
ETU745 - 748 I
R
= (0.4 / 0.45 / 0.5 / 0.55 / 0.6 / 0.65 / 0.7 / 0.8 / 0.9 / 1.0) x I
n
ETU776 I
R
= (0.4 - 1.0) x I
n
(given in Amps)
Setting values for t
R
ETU745 - 748 t
R
= 2 / 3.5 / 5.5 / 8 / 10 / 14 / 17 / 21 / 25 / 30 sec. (at 6 x I
R
)
ETU776 t
R
= 2 - 30 sec. (at 6 x I
R
)
Setting values for I
sd
ETU745 - 748 I
sd
= (1.25 / 1.5 / 2 / 2.5 / 3 / 4 / 6 / 8 / 10 / 12) x I
n
ETU776
I
sd
= 1.25 x I
n
- 0.8 x I
CW
(given in Amps)
Setting values for t
sd
ETU745 t
sd
= 0.02(M)
1)
/ 0.1 / 0.2 / 0.3 / 0.4 sec.; OFF
ETU748 t
sd
= 0.02(M)
1)
/ 0.1 / 0.2 / 0.3 / 0.4 sec.
ETU776 t
sd
= 0.02(M)
1)
/ 0.08 - 4 sec.
2)
; OFF
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