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Function
Basic functions
The 3RB20/3RB21 solid-state overload relays are designed for:
• Inverse-time delayed protection of loads from overloading
• Inverse-time delayed protection of loads from phase
unbalance
• Inverse-time delayed protection of loads from phase failure
• Protection of loads from high-resistance short-circuits (internal
ground-fault detection only with 3RB21).
Control circuit
The 3RB20/3RB21 solid-state overload relays have an internal
power supply, i. e. no additional supply voltage is required.
Short-circuit protection
Fuses or motor starter protectors must be used for short-circuit
protection.
For assignments of the corresponding short-circuit protection
devices to the 3RB20/3RB21 solid-state overload relays with/wi-
thout contactor see "Technical specifications" and
Catalog LV 1, "Selection and ordering
Trip classes
The 3RB20 solid-state overload relays are available for normal
starting conditions with trip CLASS 10 or for heavy starting con-
ditions with trip CLASS 20 (fixed setting in each case).
The 3RB21 solid-state overload relays are suitable for normal
and heavy starting conditions. The required trip class (CLASS 5,
10, 20 or 30) can be adjusted by means of a rotary switch de-
pending on the current start-up condition.
For details of the trip classes see "Characteristic
Phase failure protection
The 3RB20/3RB21 solid-state overload relays are fitted with
phase failure protection
(see "Characteristic Curves")
minimize temperature rises of the load during single-phase ope-
ration.
Phase failure protection is not effective for loads with star-con-
nection and a grounded neutral point or a neutral point which is
connected to a neutral conductor.
Setting
The 3RB20/3RB21 solid-state overload relays are set to the rated
motor current by means of a rotary knob. The scale of the rotary
knob is shown in ampere.
With the 3RB21 solid-state overload relay it is also possible to se-
lect the trip class (CLASS 5, 10, 20 or 30) using a second rotary
knob and to switch the internal ground-fault detection on and off.
Manual and automatic reset
In the case of the 3RB20/3RB21 solid-state overload relays, a
slide switch can be used to choose between automatic and ma-
nual resetting.
If manual reset is set, a reset can be carried out directly on the
device after a trip by pressing the blue RESET button. Resetting
is possible in combination with mechanical and mechanical re-
set options from the range of accessories
"Accessories"). As an alternative to the mechanical RESET op-
tions, the 3RB21 solid-state overload relays can be equipped
with electrical remote RESET by applying a voltage of 24 V DC
to the terminals A3 and A4.
If the slide switch is set to automatic RESET, the relay is reset
automatically.
The time between tripping and resetting is determined by the
recovery time.
Recovery time
With the 3RB20/3RB21 solid-state overload relays the recovery
time after inverse-time delayed tripping is 3 minutes when auto-
matic RESET is set. This recovery time allows the load to cool
down.
If the button is set to manual RESET and automatic RESET, the
3RB20/3RB21 devices can be reset immediately after tripping.
TEST function
With motor current flowing, the TEST button can be used to
check whether the relay is working correctly (device/solid-state
test). Current measurement, motor model and trip unit are tes-
ted. If these components are OK, the device is tripped accor-
ding to the table below. If there is an error, no tripping takes
place.
data".
Note: The TEST button must be kept pressed throughout the test.
In this case the motor current must be equal to more than 80 %
of the current setting I
current setting.
Testing of the auxiliary contacts and the control current wiring is
possible with the switch position indicator slide. Actuating the
Curves".
slide simulates tripping of the relay. During this simulation the
NC contact (95-96) is opened and the NO contact (97-98) is
closed. This tests whether the auxiliary circuit has been correctly
wired.
in order to
After a test trip the relay is reset by pressing the RESET button.
Self-monitoring
The 3RB20/3RB21 solid-state overload relays have a self-moni-
toring feature, i. e. the devices constantly monitor their own ba-
sic functions and trip if an internal fault is detected.
Display of operating state
The respective operating state of the 3RB20/3RB21 solid-state
overload relays is displayed by means of the position of the mar-
king on the switch position indicator slide. After tripping due to
overload, phase failure, phase unbalance or ground fault
(ground-fault detection possible only with 3RB21) the marking
on the slide is to left on the "O" mark, otherwise it is on the "I"
mark.
Auxiliary contacts
The 3RB20/3RB21 solid-state overload relays are fitted with an
NO contact for the "tripped" signal, and an NC contact for swit-
ching off the contactor.
(see Catalog LV 1,
Overload Relays
3RB2 Solid-State Overload Relays
3RB20, 3RB21 for standard applications
Trip clas-
Required loading with the
ses
rated current prior to
pressing the TEST button
CLASS 5
3 min
CLASS 10
5 min
CLASS 20
10 min
CLASS 30
15 min
and have at least the value of the lower
e
Tripping within
30 s
1 min
2 min
3 min
Siemens · 2011
19
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