Configuration - Siemens SIRIUS 3RT Manual

9.18.2

Configuration

If the current paths of multi-pole switching devices are connected in parallel, the total current
is distributed across the individual current paths in accordance with their ohmic resistance
and their inductive interactions. The ohmic resistance is primarily generated by the contact
resistance at the contacts, the value of which can vary as a result of erosion and oxidation.
This means that the current distribution is neither even nor stable: Individual current paths
may be overloaded and the overload releases or relays will trip too early (tripping error).
Permanent load with parallel connection
Provided that the relevant catalogs do not contain any information to the contrary, the
following applies for a permanent load with parallel connection:
● If three current paths are connected in parallel, 2.5 times the continuous current can be
conducted; if two current paths are connected in parallel, 1.8 times the continuous current
can be conducted. However, it should be noted that the making and breaking capacities
do not increase, since the contacts do not close and open simultaneously, so the
contacts of one current path have to switch the entire inrush or breaking current.
● The cables should be routed such that the same cable lengths are used for each current
path.
● If a short-circuit current arises, it is divided up in accordance with the ratio of the current
path resistances.
Notice: The operating current of electromagnetic instantaneous short-circuit releases
cannot be reached in such cases.
SIRIUS - SIRIUS 3RT Contactors/Contactor assemblies
Manual, 09/2017, A5E03656507120A/RS-AE/006
Accessories
9.18 Parallel switching connectors
351