Siemens SENTRON 3VL series, System Manual

Page: 138 / 360

Parameter assignment/addressing
10.1 Setting the parameters
3VL molded-case circuit breakers
138
System Manual, 03/2009, 110 0110 - 02 DS 01
I
2
t waveform:
A I
2
t waveform of the characteristic curve can be switched in (depending on the ETU), the
delay time t
sd
is based on the reference point 8 x I
R
. Two different procedures are used to
form the characteristic curve. As well as a fixed time delay for all currents in the
characteristic curve section, the I
2
t characteristic can also be used. The tripping time falls
continuously as the current increases, and the product of squared current and time remains
constant.
,
W
,tWZDYHIRUP
)L[HGWLPH
Figure 10-1 I
2
t
I instantaneous short-circuit protection I
i
:
On some trip units, the instantaneous short-circuit release I
i
can also be set. This refers to
the rated current I
n
of the circuit breaker. It must always be noted that either the
instantaneous short-circuit release (I
sd
) or the delayed short-circuit release (I
i
) handles the
protection of personnel. The tripping current of the short-circuit release of the circuit breaker
is set to a value that is at least 20% (tolerance of the trip unit) lower than the lowest short-
circuit current at the installation location and simultaneously higher than the maximum
operating c20%. This guarantees that the circuit breaker will trip within the required
time even with the smallest short-circuit current, and that correct currents will not result in
unwanted trips.
G ground fault protection I
g
:
The tripping value of the of the ground fault release I
g
is fixed to the rated breaker current on
the ETU12 and ETU22. The tripping current of the ground fault release can be set to
between 0.6 and 1 x I
n
, and the delay time t
g
can be set between 0.3 s and 0.6 s. The
measuring methods for the ground fault protection are specified on the representation of the
trip unit. On the ETU42, the tripping current of the ground fault release can be set to between
0.4 and 1 x In, and the delay time can be set between 0.1 s and 0.5 s.
Note
Ground-fault protection
It must be noted that the ground fault protection is not an r.c.b. circuit breaker (FI or RCD in
the building installation). Fault currents to ground therefore cannot be detected, only ground
"short"-circuits. However, an RCD module can be used for 3VL.