What this power supply does in an S7-400 rack
The Siemens 6ES7405-0DA02-0AA1 is a PS 405 4A XTR power supply for S7-400 racks and SIMATIC PCS 7 systems. It accepts a wide-range DC input — 24 V DC, 48 V DC, or 60 V DC — and delivers a regulated 5 V DC output at up to 4 A to the backplane bus, with no base load required. A secondary 24 V DC bus output supplies up to 0.5 A for modules that need it, and the supply is idling-proof so it won't drop out under light load. This is the XTR variant, meaning it's built for extended temperature range (0 °C to 70 °C) and includes conformal coating on the PCB — a must for environments with condensation, corrosive gases, or high humidity. The conformal coating protects against ISA-S71.04 severity levels G1, G2, and G3, which covers most industrial chemical exposure short of direct immersion.
The headline number is the 5 V DC backplane bus output rated at 4 A. That's the current available to power the CPU and all I/O modules in the rack. For a typical S7-400 configuration, 4 A covers a CPU 41x-2 plus a handful of digital and analog modules, but you'll want to sum the module current draws from their datasheets to confirm headroom. The 24 V DC bus output adds another 0.5 A for modules that draw from that rail. Inrush current hits 18 A peak with a 20 ms half-width — that's a brief surge on power-up. The mains buffering holds the output for 20 ms on a voltage dip, which meets NAMUR recommendations for process plants. That 20 ms hold-up is enough to ride through most line sags without dropping the rack. Typical power loss is 16 W, with an active power input of 48 W. That's moderate for an S7-400 supply; the heat dissipation is manageable in a ventilated cabinet but worth noting if the enclosure is sealed and tightly packed. Short-circuit protection is built in, so a fault on the backplane won't take out the supply permanently.
The XTR variant (extended temperature + conformal coating) is a specific build that isn't interchangeable with the standard PS 405 4A (6ES7405-0DA00-0AA1). If your application doesn't need the wide temp range or coating, the standard version costs less; if it does, this is the one that keeps the machine running in a steam-and-chemicals environment. Approvals include BIS (India), CCC (China), and FM with T4 temperature class (Ta 0 °C to 70 °C). The FM approval means it's certified for use in hazardous locations where the surface temperature must stay below 135 °C — relevant for oil and gas or chemical process areas. Overvoltage category II applies, so it's suitable for fixed installations downstream of a main distribution panel.
Integration into an S7-400 rack
The supply snaps into a standard S7-400 rack (UR1, UR2, or ER1/ER2) in a single slot. Width is 25 mm, so it leaves plenty of room for the CPU and I/O modules. The 290 mm height and 217 mm depth match the rack form factor — no overhang. Connection is via screw terminals on the front face, accepting 1.5 mm² wire. An optional backup battery (1x lithium AA, 3.6 V / 2.2 Ah) can be installed to maintain the real-time clock and retentive data during a power loss. The battery is not included with the supply; order separately if your application needs memory retention beyond the 20 ms hold-up.