What it is and what it delivers
The Siemens 6ES7405-0RA01-0AA0 is a SIMATIC S7-400 PS 405 power supply that accepts 24 V DC, 48 V DC, or 60 V DC input and provides a regulated 5 V DC output at up to 20 A for the backplane bus. It occupies three slots in the S7-400 rack and draws a typical active power input of 175 W, with a typical power loss of 51 W.
Backplane power and slot budget
The 5 V DC backplane output is rated for 20 A continuous, with a 200 mA base load required to keep the bus regulation stable. An additional 24 V DC backplane output provides up to 1 A and is idling-proof, meaning it can run with no load. The 24 V DC input rating is 7.3 A; at 48 V DC it draws 3.45 A; at 60 V DC it draws 2.75 A. At 75 mm wide and 290 mm high, the module consumes three slot positions in the rack — a key constraint when laying out a dense S7-400 chassis. The depth of 217 mm matches the standard S7-400 form factor.
Ruggedness and compliance
Rated for operation from 0 °C to 60 °C, with FM approval up to 40 °C at T4. Overvoltage category II, equipment protection class I with protective conductor. The supply holds up the output for 20 ms on mains failure and supports NAMUR buffering recommendation. An optional backup battery (2x lithium AA, 3.6 V / 1.9 Ah) maintains the backplane memory during extended power loss.
Connection and protection
The electrical connection uses three 1.5 mm² solid or stranded wires with end sleeves, accepting an external diameter of 3 mm to 9 mm. Short-circuit protection is built in. Inrush current peaks at 56 A with a 1.5 ms half-width — brief enough to avoid nuisance tripping of upstream breakers in most panels.