Brochure

SIRIUS
Siemens Industry, Inc. Industrial Control Product Catalog 2017
3/29
3
OVERlOAd RElAYS
Characteristic curves
The tripping characteristics show the relationship between the
tripping time and tripping current as multiples of the set current
I
e
and are given for symmetrical three-pole and two-pole loads
from the cold state.
The smallest current used for tripping is called the minimum trip-
ping current. According to IEC 60947-4-1, this current must be
within specified limits. The limits of the total tripping current for
the 3RB20/3RB21 solid-state overload relays for symmetrical
three-pole loads are between 105 % and 120 % of the set cur-
rent.
The tripping characteristic starts with the minimum tripping
current and continues with higher tripping currents based on the
characteristics of the so-called trip classes (CLASS 10,
CLASS 20 etc.). The trip classes describe time intervals within
which the overload relays have to trip with 7.2 times the set
current I
e
from the cold state for symmetrical three-pole loads.
The tripping times according to IEC 60947-4-1, tolerance band
E, are as follows for:
The tripping characteristic for a three-pole overload relay from
the cold state (see illustration 1) only apply if all three phases are
simultaneously loaded with the same current. In the event of a
phase failure the 3RB20/3RB21 solid-state overload relays
switch off the contactor more quickly in order to minimize heating
of the load in accordance with the tripping characteristic for two-
pole loads from the cold state (see illustration 2). With phase
unbalance the devices switch off depending on the reason for
the unbalance between the two characteristic curves.
Compared with a cold load, a load at operating temperature
obviously has a lower temperature reserve. The tripping time of
the 3RB2/3RB3 solid-state overload relays is reduced there-
fore to about 30 % when loaded with the set current I
e
for an
extended period.
Tripping characteristics for 3-pole loads
Illustration 1
Tripping characteristics for 2-pole loads
Illustration 2
The above illustrations are schematic representations of
characteristic curves.
Trip class Tripping time
CLASS 5 3 … 5 s
CLASS 10 5 … 10 s
CLASS 20
10 … 20 s
CLASS 30 20 … 30 s
NSB0_01525
1
2
4
6
8
10
20
40
60
80
100
200
400
600
800
1000
0,6 1 2 4 6 8 10×
e
s
CLASS 30
CLASS 20
CLASS 10
CLASS 5
Tripping current
Tripping time
NSB0_01526
1
2
4
6
8
10
20
40
60
80
100
200
400
600
800
1000
0,6 1 2 4 6 8 10×
e
s
CLASS 30
CLASS 20
CLASS 10
CLASS 5
Tripping current
Tripping time
Overload Relays
3RB2 / 3RB3 Solid-State Overload Relays
3RB20, 3RB21, 3RB30, 3RB31 up to 630A for standard applications