Siemens 3RB20 Panduan Referensi - Halaman 16

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Siemens 3RB20 Panduan Referensi
Overload Relays
3RU1 Thermal Overload Relays
3RU11 for standard applications
Characteristic curves
The tripping characteristics show the relationship between the
tripping time and tripping current as multiples of the current
setting I
and are given for symmetrical three-pole and two-pole
e
loads from the cold state.
The smallest current used for tripping is called the minimum
tripping current. According to IEC 60947-4-1, this current must be
within specified limits. The limits of the minimum tripping
current for the 3RU11 thermal overload relays for symmetrical
three-pole loads are between 105 and 120 % of the current
setting.
The tripping characteristic starts with the minimum tripping cur-
rent and continues with higher tripping currents based on the cha-
racteristics 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 current setting I
from the cold state for symmetrical three-pole loads.
The tripping times are as follows for:
Trip class
CLASS 10A
CLASS 10
CLASS 20
CLASS 30
Tripping time
10 000
100
5000
min
60
s
40
2000
1000
10
500
5
200
2
100
1
50
20
2-pole
loading
10
5
2
1
0,6 0,8 1
2
3
This is the schematic representation of a characteristic curve. The
characteristic curves of the individual 3RU11 thermal overload re-
lays can be ordered from "Technical Assistance":
• Either by e-mail to:
• Or on the Internet at:
http://www.siemens.com/automation/service&support
14
Siemens · 2011
Tripping times
2 s ... 10 s
4 s ... 10 s
6 s ... 20 s
9 s ... 30 s
NSB0_00288
3-pole
loading
60 80 x 1
4
6
8 10
15
20
30 40
A
Current
The tripping characteristic for a three-pole 3RU11 thermal over-
load relay (see characteristic curve for symmetrical three-pole
loads from the cold state) only applies if all three bimetal strips are
simultaneously loaded with the same current. If only two bimetal
strips are heated due to a phase failure, these two strips alone
must generate the necessary force to trigger the tripping mecha-
nism which would result in a longer tripping time or require a
higher current. If these higher currents are applied over a longer
period, they usually cause damage to the load. To avoid damage,
the 3RU11 thermal overload relays are fitted with phase failure
sensitivity which ensures faster tripping in accordance with the
characteristic curve for 2-pole loads from the cold state by means
of a suitable mechanical mechanism.
Compared with a cold load, a load at operating temperature obvi-
ously has a lower temperature reserve. This is taken into account
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by the 3RU11 thermal overload relays by reducing the tripping
time to about 25 % when loaded with the current setting I
extended period.
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for an
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