Configuration
4.3 Load feeders - protection against short circuit
SIRIUS 3RM1 motor starter
62 Manual, 11/2013, A5E0345285095020A/RS-AB/002
Configuration of a load feeder with 3RM1 motor starter in compliance with UL
4.3.4.1
Function according to UL 508
The 3RM1 motor starters can be combined individually with a circuit breaker (motor starter
protector) or fuses. A group installation is also possible according to UL.
Configuration of individual load feeders
A circuit breaker or appropriate fuses is/are assigned to each motor starter.
Configuring a group of load feeders (group installation)
A "group installation" is a combination of several 3RM1 motor starters with one common
short-circuit protection device (branch circuit protection device). This common branch circuit
protection device can be a circuit breaker according to UL489, e.g. a 3RV27 or fuses
according to UL248.
The 3RM19 infeed system is suitable for a group installation according to UL because it may
be loaded up to a summation current of 16 A. As an alternative, distribution of the main
circuit can be realized with individual cables that have an AWG 12 cross-section.
SCCR short circuit current ratings
SCCR short circuit current ratings
The SCCR (short circuit current rating) indicates the maximum permissible short-circuit
current for a switching device. A higher value can only be achieved in combination with
protection devices such as fuses or circuit breakers.
This rating is required for mounting accessories in control cabinets (control panels)
conforming to UL 508a. The switching device or switchgear assembly with the lowest value
specifies the value for the entire control panel.
A low value makes engineering the supply voltage for the control panel difficult.
According to UL 508a, the 3RM1 motor starters have a standard short circuit current rating of
1 kA for the versions up to 2 A rated current and 5 kA for the version with 7 A.
Tests were also carried out with a short circuit rating of 100 kA (high capacity short circuit
current ratings). The UL reports
(http://support.automation.siemens.com/WW/view/en/66427510) include details of possible
combinations with Class J fuses, as well as with a number of circuit breakers.
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