Device Description - Siemens sirius System Manual

3RU11, 3RB10, and 3RB12 overload relays

4.2 Device description

Models
4-4
Overload relays are used to protect electrical equipment such as three-
phase induction motors and transformers from overheating. Overheating
can be caused by overload, asymmetric current consumption, loss of a
phase in the main supply conductor, or a blocked rotor.
There are 3 overload relay models available:
• 3RU11 thermal overload relays
The 3RU11 thermal overload relays up to 100 A are designed for the cur-
rent-dependent protection of loads with normal starting (tripping class 10)
against impermissible overheating.
Impermissible overheating as a result of the above-mentioned causes
leads to an increase in the motor current beyond the set rated current for
the motor. This increase in current heats up the bimetal strips inside the
device by means of heating elements. The strips are deflected and ope-
rate the auxiliary contact elements by means of a tripping mechanism.
The auxiliary contact elements switch the load off by means of a contac-
tor.
• 3RB10 electronic overload relays
The 3RB10 self-supplying electronic overload relays up to 100 A are desig-
ned for the current-dependent protection of loads with normal and heavy
starting (tripping classes 10 and 20) against impermissible overheating.
Impermissible overheating as a result of the above-mentioned causes
leads to an increase in the motor current beyond the set rated current for
the motor. This increase in current is detected by the current transfor-
mers integrated in the devices and evaluated by an appropriate electronic
circuit, which then sends a pulse to the auxiliary contact elements. These
switch the load off by means of a contactor.
• 3RB12 electronic overload relays
The 3RB12 externally supplied electronic overload relays up to 820 A are
designed for the current-dependent protection of loads with normal to
heavy starting (tripping classes 5, 10, 15, 20, 25, and 30, which can be set
on the device) against impermissible overheating. Impermissible overhea-
ting as a result of the above-mentioned causes leads to an increase in the
motor current beyond the set rated current for the motor. This increase in
current is detected by the current transformers integrated in the devices
and evaluated by an appropriate electronic circuit, which then sends a
pulse to the auxiliary contact elements. These switch the load off by
means of a contactor. In addition to the current-dependent protection of
the loads against impermissible overheating, the 3RB12 electronic over-
load relay allows the temperature of the motor winding to be monitored
by connecting a PTC thermistor detector circuit in order to protect the
load against overtemperature. This can be caused indirectly, for example,
if the flow of the coolant is hindered and cannot be detected.
SIRIUS System Manual
GWA 4NEB 430 0999-02b