Application Scenarios
A 2800 mm heavy-plate rolling mill in Western Europe runs its main-drive screwddown, roller-table sequencing, and cooling-bank spray-zone control on a SIMATIC S5-155U rack farm installed in 1994. The central rack is a 6ES5155-3UA11, jumper-set to 230 V AC (mains stable at the MCC), delivering 5 V / 40 A to the backplane for a 946 CPU, two CP 443 communication processors (tying the mill to the plant-level S5-115U master), three FM 455 closed-loop blocks, and twelve SM 321/322 DI/DO cards across the rack’s expansion slots. After 26 years of continuous service — including ambient spikes to 52 °C during summer rolling campaigns and a 2018 VFD-harmonics event that took out a smaller S5-115U rack downstream — the 6ES5155-3UA11‘s integrated PSU started showing rising ripple on the 5 V rail (measured at the CPU’s diagnostic buffer as sporadic “memory checksum” warnings during high-load rolling passes). The plant’s E&I team had a verified 6ES5155-3UA11 spare on the locker shelf (procured three years prior when Siemens confirmed S5-155U EOL and lead times stretched to 22 weeks). During a 14-hour planned overhaul, they unracked the failing unit — all field wiring terminates on the SM cards, not the rack chassis itself, so the swap meant: disconnect 230 V AC inlet, unbolt the four 19″ cage nuts, slide the old rack out, slide the spare in, bolt, reconnect AC, power up. The 946 CPU’s EPROM submodule stayed on the CPU (not on the rack), so no re-download, no re-qualification — the mill was back in automatic by the evening shift. Post-swap, the S5 diagnostic buffer ran clean for 18 months. The plant’s chief automation engineer noted: “The 6ES5155-3UA11 isn’t just a PSU — it’s the backplane, the 5 V 40 A heart, and the 19″ skeleton for the whole 155U farm. When it drifts, the CPU hallucinates. Having a spare rack-assembly on the shelf, not just a bare 6ES5955-3LF12 PSU module, saved us a 3-day outage because we didn’t have to rebuild the backplane wiring.”
Key Parameters
| Main Parameters | Value/Description |
|---|---|
| Product Model | Siemens 6ES5155-3UA11 |
| Manufacturer | Siemens AG |
| Product Category | SIMATIC S5-155U Central Rack with Integrated Power Supply |
| Input Voltage | 115 / 230 V AC selectable (jumper on PSU section) |
| Main Output | 5 V DC / 40 A (S5 backplane bus supply) |
| Compatible CPU | S5-946, S5-947 (S5-155U family) |
| Rack Slots | S5-155U standard slot count (CPU + up to ~20 expansion modules depending on config) |
| Mounting | 19″ cabinet / 35 mm DIN rail compatible (rack chassis format) |
| Operating Temperature | 0 °C to +60 °C (typical S5 envelope) |
| Dimensions (approx.) | 482 mm (19″) × 220 mm × 280 mm (W × H × D, central rack) |
| Weight | Approx. 6.0 kg (assembled with PSU) |
| Lifecycle Status | End of Life (SIMATIC S5 family discontinued) |
Technical Principles and Innovative Values
- Innovation Point 1: Integrated 40 A 5 V Backplane Supply in the Chassis. The Siemens 6ES5155-3UA11 isn’t a bare PSU module — it’s the S5-155U central rack with the PS955-class 5 V / 40 A supply built into the chassis, feeding the S5 backplane bus that powers the CPU, the SM/FM/CP modules, and the memory submodules. 40 A is substantial for an S5 rack: a 946 CPU + a dozen SM cards + 2–3 CPs can draw 18–25 A, leaving headroom for expansion. The 115/230 V AC jumper means the same rack assembly deploys in North American 120 V panels and European 230 V MCCs without swapping PSU hardware.
- Innovation Point 2: 155U Backplane Bus Architecture. The 6ES5155-3UA11 carries the S5-155U central backplane — a parallel/multi-drop bus that supports the 946/947 CPUs’ higher instruction throughput versus the 115U/135U tier, and accommodates FM (function module) and CP (communication processor) cards that need deterministic data exchange with the CPU. This is why the 155U + 6ES5155-3UA11 combo shows up on rolling mills and turbine auxiliaries where an S5-115U would bottleneck on bus cycle time.
- Innovation Point 3: Serviceability Via Bare PSU Interchange. Although the 6ES5155-3UA11 ships as a rack+PSU assembly, the PSU portion is a PS955 module (commonly 6ES5955-3LF12 5 V/40 A in the 3UA11 config, per Siemens S5 system manuals) that can be unclipped and replaced independently if the chassis and backplane are still healthy. A plant with five 6ES5155-3UA11 racks can stock two bare 6ES5955-3LF12 PSUs as spares instead of two whole rack assemblies — cheaper, but the trade-off is you’re swapping a PSU inside a live(ish) rack during outage rather than swapping the whole 19″ unit on the bolts. Either strategy works; the 6ES5155-3UA11 supports both.
- Innovation Point 4: 19″ Form Factor for Legacy MCC/ERC Cabinets. The Siemens 6ES5155-3UA11 chassis is 19″ rack-width standard, meaning it drops into existing S5-era MCC bays, ERC (electrical room cabinet) profiles, and retrofitted S7-upgrade cabinets where the 155U is being kept alive alongside newer S7-400/S7-1500 nodes. This matters for phased modernization: a plant can keep the 6ES5155-3UA11-hosted 155U alive for the rolling-mill screwdowns (too risky to migrate mid-life) while migrating the upper-level coordination to S7, all in the same cabinet row.
