Siemens SIRIUS Configuration Manual page 14

General information
With or without overload relay
In addition to the combinations comprising a motor starter protector (for motor protection)
and contactor, combinations are also available with motor starter protector (for starter
protection), contactor and overload relay.
In the first case, the motor starter protector assumes the dual function of overload protection
and short-circuit protection, while in the second case, the motor starter protector assumes
only the short-circuit protection function and the overload relay the overload protection
function. The tripping behavior of both solutions under overload and short-circuit conditions
is technically comparable.
For fuseless load feeders with solid-state overload relay, and for higher trip classes, CLASS
20, CLASS 30 and CLASS 40 in particular, a motor starter protector is often used instead of
an MSP for starter combinations. This is due to the following: from the point of view of
thermal destruction limits, MSPs for starter combinations are generally designed for CLASS
10 motor starts. The current measurement of solid-state overload relays usually moves into
saturation upwards of a 10-fold rated current, so that the intrinsic protection of the motor
starter protector is no longer guaranteed for higher trip classes. In order to ensure thermal
intrinsic protection, it is advisable to use a motor starter protector/circuit breaker that protects
itself over the overload release. The motor starter protector/circuit breaker is selected so that
the point at which the characteristic curve of the overload relay intersects with the a-tripping
characteristic of the motor starter protector/circuit breaker is more than 10 x the set current.
This ensures that, in the case of motor faults, such as overload or blocking, the overload
relay always trips and not the motor starter protector.
In this situation, combinations with motor starter protector and contactor offer the most cost-
effective solution. However, combinations with overload relays offer distinct advantages for
certain applications:
● 3RB30 / 3RB31 and 3RB22 / 3RB23 solid-state overload relays or SIMOCODE pro 3UF7
can be used to achieve not only trip CLASS 5 and CLASS 10 but also solutions for heavy
starting, such as CLASS 20, CLASS 30, and CLASS 40.
● Using solid-state overload relays offers a wide setting range of 1:4 or 1:10. This offers
advantages during configuration (e.g. if the exact motor current is not known) and
enables us to reduce the number of variants required.
● Overload and short-circuit protection are carried out separately and can also be signaled
separately. Alternatively, the 3RV2921-1M signaling block can be used for the 3RV motor
starter protector instead of the overload relay. This also supports the separate signaling
of overloads and short-circuits.
● Setting of the overload relay to "Automatic Reset" can also save a walk to the control
cabinet in the case of overload tripping, as a manual reset in the control cabinet is not
required. Alternatively, this function can also be implemented with the "3RV21 motor
starter protector with overload relay function". These devices can be used in the motor
starter protector + contactor tables instead of the 3RV20 motor starter protector.
14
Configuring SIRIUS Innovations
Configuration Manual, 04/2010, A8E56203880102-02