What this spacer does in an ET 200S line
The 3RK1903-0CD00 is a spacer module for the Siemens ET 200S distributed I/O system. It creates physical separation between modules when the ambient temperature hits 40 °C or higher, or when a starter rated at 6.3 A or above is in the string — conditions where standard module-to-module spacing doesn't shed enough heat.
Why you'd reach for it
If you're packing starters into an ET 200S node and the ambient in the panel is pushing 40 °C, or you've got a 6.3 A starter on the rail, Siemens calls for this spacer between the power module and the first starter. Without it, the heat rise from adjacent high-current loads can trip the thermal overload early or shorten the life of the electronics. The substance prohibitance date of 05/01/2012 marks its RoHS compliance baseline — it meets the EU RoHS deadline for removing lead, mercury, cadmium, and the other restricted substances from electrical equipment.
Sourcing reality for the BOM line
This is a current-production Siemens SIRIUS part, so it's quoted to order against your BOM quantity through authorized distribution.