What it is and what it does
The Siemens 6ES7405-0RA00-0AA0 is a SIMATIC S7-400 PS 405 power supply that takes a 24V DC input and delivers 5V DC at 20A to the S7-400 backplane. It's the module that powers the rack — without it, the CPU and I/O slices don't wake up. Rated for 20A on the 5V rail, this supply handles a fully loaded S7-400 rack with multiple high-current I/O modules and a fast CPU. The 24V DC input side means it's fed from a plant DC bus or a dedicated 24V power supply, not directly off line voltage.
Lifecycle and sourcing reality
This power supply carries a lifecycle stage of mature, meaning it's not in active new-production but remains available through the independent distribution channel. For a BOM line that's frozen on the S7-400 platform, this is the part you source against an RFQ — not off the shelf from Siemens. The documentation carries a date of 25.06.2015 with a copyright notice from Siemens AG 2015. That's the revision date of the manual, not a production date. The part itself has been in the field for years, and the mature status reflects that.
What the ratings mean for fit
The 5V/20A DC output is the critical number for a controls integrator. That 20A must cover the total 5V draw of every module in the rack — CPU, interface modules, I/O slices, and any special-function cards. If your rack's total 5V load exceeds 20A, this supply won't hold up; you'd need to redistribute modules across multiple racks or step up to the higher-current PS 407 variant. The 24V DC input is standard for industrial control panels where a 24V DC bus is already present. It's not a line-voltage supply — don't feed it 120V or 230V AC. That input rail also means the supply itself draws from the same 24V source that powers field devices, so the upstream 24V DC supply must be sized to include this load.