Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 148)

reset button. The auxiliary contacts then return to their normal position and prepare such for
switching-on the assigned contactor.
As required by IEC and national standards, the motor protection relays are equipped with a free-
trip release, i.e. normal protective tripping occurs even when the reset button is pressed.
In the automatic reset position, the contacts automatically reset the bimetal strips when the
latter have cooled down.
For safety reasons, motor protection relays with automatic reset should only be used in circuits
whose contactors are actuated by momentary contact control (momentary pushbuttons). With
hand reset they may also be used in circuits with maintained contact control of the contactors.
For remote resetting of motor protection relays, reset magnets are available that can be
mounted onto the motor protection relays.
Test function and
О-button
With the test function, the proper operation of the auxiliary contacts and of the connected control
circuit of an overload relay ready for service can be tested. This function simulates the tripping
of the relay.
With the О-button, the NC contact is opened for as long as the button is pressed. By this means
the contactor connected in series and hence the motor can be switched off. This function is
often used for simple starters in small housings.
Short-circuit withstand capacity
See also Section 2.3.4.5 . In accordance with IEC 60947-4-1, motor protection relays must be
protected against short-circuit so that they are either rendered inoperational by the short-circuit
current and have to be replaced (Coordination type 1) or their full operational capability is
retained (Coordination type 2). The thermal motor protection relays must be protected against
damage by short-circuit currents by circuit breakers or fuses. The fuse values for the respective
coordination-type can be obtained from the technical documentation. The short-circuit withstand
capacity of directly heated overload relays is higher than that of indirectly heated ones.
4.2.4.2 Electronic motor protection relays
Electronic motor protection relays include a wide range of devices for the protection and the
optimized operation of motors and plants. Due to this variety, it is only possible below to
consider a few key aspects that are important in the selection and use of these relays. Section
4.1.2.4 shows a selection of functions that are offered by electronic motor protection relays. The
range of devices extends from simple and economical designs that are intended for use instead
of thermal (for example bimetal) motor protection relays up to very complex devices with a
variety of functions, communication links etc.
LVSAM-WP001A-EN-P - April 2009 4-40