Siemens SENTRON 3WL10 Manual page 61

Description of the protection functions
Overload protection (LT)
The overload protection of the ETUs of the 3 and 6-series is inverse-time long-time-delayed
and has an I²t characteristic by default. On electronic trip units of the 6-series, the
characteristics can be adapted to characteristics acc. to IEC 60255 to meet increased or
special requirements, for example, to produce a I
discrimination with respect to fuses or upstream protection devices, such as medium-voltage
protection devices, can be achieved.
I denotes the overcurrent setting value and t
r
the overcurrent trip is defined as 6 times the value of the current setting I
point in the same way as the other size devices of the 3WL portfolio. The minimum tripping
time is limited to 500 ms.
A pre-alarm is also output when 90% of the set current setting I
can be modified on all trip units.
Short-time-delayed short-circuit protection (ST)
The ST function of the trip unit can be used to implement time-selective, short-time-delay,
short-circuit tripping in low-voltage systems in which a number of circuit breakers are
installed in series.
The short-time-delayed short-circuit protection responds when a short-circuit current in at
least one of the conductors to be protected exceeds the set tripping current I
duration of the set delay time t
The ST release provides characteristics with inverse-time curve I
definite-time tripping characteristic (t
Instantaneous short-circuit protection (INST)
The instantaneous short-circuit protection (INST) is a protection function that opens the
circuit breaker on a short-circuit immediately, without a time delay. It responds when a short-
circuit current in at least one of the conductors to be protected exceeds the set tripping
current I
It can optionally be disabled in each trip unit to meet increased requirements for
discrimination. Due to this, the short-time-delayed short-circuit protection remains active with
its set delay time up to the I
to this setting that disables the instantaneous short-circuit protection automatically a certain
time after the circuit breaker has been closed.
Neutral protection (N)
Neutral protection is included in all types of ETU as standard and protects the neutral
conductor from overloads and short-circuits. As the trip condition, it uses the same setting
values as the protection functions LT, ST, INST of the three phases, but its sensitivity can be
scaled from 50% to 200% depending on the design of the conductor. In every 4-pole circuit
breaker, the neutral conductor is protected via the current transformer on the fourth pole that
is always incorporated or, in 3-pole circuit breakers, via an externally connected neutral
conductor CT.
It is also possible to disable neutral protection.
3VA27 molded case circuit breakers & 3WL10 air circuit breakers
Manual, 10/2018, L1V30499596002-01
sd
.
i
value of the circuit breaker. The MCR function is an alternative
cw
t characteristic. In this way, improved
4
the associated time delay. The time delay for
r
.
= const.). It can also be disabled.
sd
Description
2.8 Electronic trip unit ETU
at the reference
r
is reached. This pre-alarm
r
for the
sd
t or a time delay with a
2
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