Siemens 3VA1 100 A, Manual

Page: 76 / 532

Description
2.6 Protection system
3VA molded case circuit breakers
76
Manual, 04/2015, A5E03603177010-02
2.6.4 Overload protection (L)
The ID letter for overload protection is L (stands for "Long-time delay").
The trip unit is inverse-time delayed and exhibits the following characteristics depending on
the trip unit type:
● Bimetal characteristic with thermal-magnetic trip units
● I
2
t characteristic with electronic trip units
The letters I
r
refer to the current setting value, and t
r
to the associated time delay.
The time delay with the device-specific reference point defines the current-dependent curve
shape of the setpoint characteristic. With 3VA molded case circuit breakers, this trip unit
reference point corresponds to 6 times the current setting value I
r
. The following
mathematical equation determines the remaining curve of the setpoint characteristic:
I
2
t = constant
Thermal image
The electronic trip units feature a thermal image function that can be deactivated on some
electronic trip units.
Thermal image activated (ON)
If the thermal image is activated, the previous thermal history is taken into account. After
tripping, the thermal memories of the phases are set to a value corresponding to 85 % of the
thermal equivalent of the warmest phase. This setting allows reclosure of the contacts of the
molded case circuit breaker. This is followed by a cool-down period according to an
exponential function with a time constant T
au
(τ).
Thermal image deactivated (OFF)
The thermal image for ETU 5-series and 8-series can be deactivated to allow use of the
TD500 test device for characteristic curve tests. The thermal image must remain activated in
normal operation. When the image is deactivated, the thermal prehistory of the circuit
breaker, the cables/busbars, other components of the lower-level power distribution system
or an end consumer is ignored.
It is the responsibility of the commissioning engineer/operator to provide additional thermal
overload protection for the lower-level power distribution system while the thermal image is
deactivated.