Overview
The SICAM RTU 400 is a modular substation-class Remote Terminal Unit (RTU) from Siemens designed for telecontrol, grid automation, and distributed process I/O in transmission and distribution substations. The platform uses a 19-inch central rack with a backplane that accepts a mix of SIMATIC S7-400 form-factor power supplies, communication processors, central processing units, and SICAM-specific I/O modules such as the SICAM MCP (Measurement and Control Processor) and SICAM DI (Digital Input).
A recurring configuration question when engineering a new SICAM RTU 400 station, or retrofitting an existing one, is whether a dedicated SICAM-branded power supply (PS) is mandatory, or whether a standard SIMATIC S7-400 power supply of narrower slot width can be substituted. The decision has direct consequences for the number of I/O slots available in the central rack, the station battery voltage that can be accepted, and the documentation chain that must be maintained.
Field experience across multiple substation deployments confirms that the standard SIMATIC S7-400 10 A and 20 A power supplies operate correctly in SICAM RTU 400 racks when the slot allocation rules and CPU positioning conventions documented in this reference are followed. The 3-slot SICAM PS variants exist primarily to support 48 Vdc and 60 Vdc station battery input ranges that some telecontrol installations require, ranges that the standard SIMATIC PS family does not cover.
This reference consolidates the slot layout rules, the PS family trade-offs, the CPU slot conventions, and the field-proven commissioning and diagnostic checks that allow an engineer to select and validate the right power supply for a given SICAM RTU 400 deployment.
SICAM RTU 400 Rack Architecture and Slot Numbering
The SICAM RTU 400 station is built around a 19-inch central backplane with numbered slots. The slot allocation rules that govern the backplane are not interchangeable with generic S7-400 layout rules; the SICAM firmware performs a slot-aware boot sequence and validates the presence and position of the CPU and PS before it activates I/O modules.
The left-most slots of the rack are reserved for the power supply area. The number of slots occupied by the PS depends on the module family. A 3-slot SICAM PS occupies slots 1, 2, and 3. A 2-slot standard SIMATIC PS occupies slots 1 and 2 only, leaving slot 3 available for either an interface module or the CPU.
The CPU default position is slot 4. This is a hard rule enforced by the SICAM Manager configuration tool and confirmed by the boot diagnostics. The slot indices to the right of the CPU host the I/O modules, communication processors, and SICAM-specific modules such as the SICAM MCP, SICAM DI, and SICAM AI (Analog Input) modules.
| Slot | Typical Function | Acceptable Modules |
|---|---|---|
| 1-2 | Power Supply (PS) area | SICAM PS (3-slot width spanning slots 1-3) or standard SIMATIC PS (2-slot width, slots 1-2 only) |
| 3 | Reserved / CPU when 2-slot PS is fitted | CPU may be placed here when a 2-slot PS is fitted; otherwise reserved |
| 4 | CPU slot (default) | SICAM CPU / SIMATIC CPU |
| 5+ | I/O and CP slots | SICAM DI, SICAM AI, SICAM MCP, CR, CP, SIMATIC I/O |
Power Supply Module Variants
Two practical families of power supplies are deployed in SICAM RTU 400 racks: the SICAM PS and the standard SIMATIC S7-400 PS. They are mechanically and electrically compatible with the SICAM RTU 400 backplane for the appropriate input voltage scenarios, but they differ in slot width, supported input range, and documentation.
The SICAM PS is a 3-slot width module that is branded and tested by Siemens specifically for the SICAM RTU 400 product line. Variants of the SICAM PS cover 48 Vdc and 60 Vdc station battery input ranges, which are common in transmission substations that standardize on those battery voltages for compatibility with protection relays and other substation peripherals.
The standard SIMATIC PS is a 2-slot width module from the S7-400 family. It is widely stocked, well documented in the SIMATIC literature, and available in 10 A and 20 A output variants. The 2-slot form factor is the principal engineering advantage of the standard SIMATIC PS for SICAM RTU 400 use, because it leaves one additional slot available in the central rack compared with the 3-slot SICAM PS.
- SICAM PS (3-slot width): Branded and tested by Siemens specifically for the SICAM RTU 400 product line. Available in nominal input variants rated for 48 Vdc and 60 Vdc station battery operation. The wider 3-slot form factor limits the available I/O slots in the central rack.
- Standard SIMATIC PS (2-slot width): 10 A and 20 A output variants of the SIMATIC S7-400 power supply family. Field experience confirms compatibility with SICAM RTU 400 stations when the configuration rules documented in this article are followed.
| Attribute | SICAM PS (3-slot) | Standard SIMATIC PS (2-slot) |
|---|---|---|
| Slot width | 3 slots | 2 slots |
| Output current | Per variant datasheet (verify against order code) | 10 A and 20 A variants available |
| 48/60 Vdc input | Supported (SICAM-specific variants) | Not offered in 48/60 Vdc; 24 Vdc / 120-230 Vac input on standard S7-400 PS |
| CPU slot | Slot 4 (PS occupies 1-3) | Slot 4 or slot 3 (PS occupies 1-2, frees slot 3) |
| Documentation | Documented in SICAM RTU 400 manuals | Not explicitly described as compatible in SICAM manuals; field-validated |
Slot Width Trade-offs and I/O Capacity
The slot width of the PS directly determines the number of I/O slots left in the central rack. A 3-slot SICAM PS reduces the effective I/O capacity of a single central rack by one slot compared with a 2-slot PS. In a typical nine-slot central rack, the count of usable I/O slots is therefore approximately five with a 3-slot PS and six with a 2-slot PS.
For substation bays with a high digital input count, the freed slot from a 2-slot PS can accommodate an additional 32-channel SICAM DI module. This is a non-trivial economic and spatial decision when the I/O count is close to the rack capacity, because adding a single SICAM DI module in a 2-slot PS configuration may avoid the cost of an expansion rack.
The trade-off is the input voltage compatibility. If the station battery is 48 Vdc or 60 Vdc, a 3-slot SICAM PS variant is required because the standard SIMATIC PS does not support this input range. If the station battery is 24 Vdc or 120/230 Vac is available, the standard 2-slot SIMATIC PS is acceptable and preserves one I/O slot.
For densely loaded stations approaching the 10 A budget, the 20 A standard SIMATIC PS is the practical choice. The 10 A PS is appropriate for small stations with a limited I/O count, where the slot savings is more important than the current headroom.
CPU Slot Placement Rules
By default, the SICAM RTU CPU is plugged into slot 4 of the central rack. This rule applies when a 3-slot SICAM PS occupies slots 1 through 3, leaving slot 4 as the first available CPU position. The SICAM Manager project reflects this as a fixed slot index for the CPU, and the configuration is validated against the physical position during boot.
When a 2-slot PS is fitted, slot 3 becomes available. Field experience documents that the CPU can be moved to slot 3 in this configuration without functional problems, provided the SICAM Manager configuration is updated to match the physical CPU slot number. The SICAM RTU firmware does not refuse to boot when the CPU is in slot 3, but the engineering tool must reflect the actual hardware position to avoid diagnostic mismatches.
Voltage Input Range Options and I/O Module Compatibility
The choice between 48 Vdc and 60 Vdc station battery input is dictated by the substation battery plant rather than by the RTU. Many transmission substations are standardized on 48 Vdc or 60 Vdc station batteries for compatibility with protection relays and SCADA peripherals. The SICAM PS family includes dedicated variants for these nominal inputs.
| Station Battery | Compatible PS | Notes |
|---|---|---|
| 24 Vdc | Standard SIMATIC PS (verify variant) | Most S7-400 PS variants accept 24 Vdc nominal |
| 48 Vdc | SICAM PS 48 Vdc variant (3-slot) | SICAM-specific; standard S7-400 PS does not cover 48 Vdc |
| 60 Vdc | SICAM PS 60 Vdc variant (3-slot) | SICAM-specific; standard S7-400 PS does not cover 60 Vdc |
| 120/230 Vac | Standard SIMATIC PS | Widely stocked; common in industrial auxiliaries |
The SICAM I/O modules, including the SICAM MCP and SICAM DI, are powered from the PS via the backplane. The total I/O module load is added to the CPU and CP loads and the resulting total must be within the output current rating of the selected PS. For a 20 A PS, the 5 Vdc and 24 Vdc backplane rails can typically supply a fully populated central rack with margin; for a 10 A PS, the same configuration may approach the rating limit. The exact current budget must be calculated from the I/O module datasheets and summed against the PS rating.
Where a 48 Vdc or 60 Vdc station battery is mandated by the substation design, the 3-slot SICAM PS is the only compliant option. The slot penalty is offset by the input voltage compatibility, which a standard S7-400 PS cannot provide.
Selection Decision Matrix
Use the following matrix to choose the appropriate PS for a new SICAM RTU 400 station or a retrofit of an existing one:
| Condition | Recommended PS |
|---|---|
| 48 Vdc or 60 Vdc station battery | SICAM PS (3-slot, 48/60 Vdc variant) |
| 24 Vdc station battery | Standard SIMATIC PS 10 A or 20 A (2-slot) |
| 120/230 Vac auxiliary available | Standard SIMATIC PS (2-slot) |
| I/O slot density is constrained in central rack | Standard SIMATIC PS (2-slot) to free one slot |
| Site standardization on SICAM-branded modules | SICAM PS (3-slot) |
| Project documentation requires explicit SICAM PS | SICAM PS (3-slot) |
| High I/O count approaching 10 A budget | Standard SIMATIC PS 20 A (2-slot) |
Commissioning and Verification
- Confirm the station battery voltage and select the PS variant that covers the nominal input with the documented tolerance band.
- Insert the PS in slots 1-2 (standard SIMATIC) or slots 1-3 (SICAM PS) and torque the front-panel screws per the rack mechanical specification.
- Position the CPU in slot 4 for the default 3-slot PS, or slot 3 when a 2-slot PS is fitted.
- Load the SICAM Manager configuration and verify that the slot indices in the project match the physical modules. The slot for the CPU must be set to the actual physical slot.
- Download the configuration to the RTU and observe the diagnostic LEDs. The CPU RUN and PS OK indicators must transition to a steady state within the documented boot interval.
- Read the diagnostic buffer in SICAM Manager and confirm no slot-allocation errors are reported. Any "module not responding" entry should be investigated against the slot table before placing the station in service.
- Document the PS variant and slot position on the rack label and in the as-built documentation, including the order code and the input voltage rating.
- Calculate the backplane load budget by summing the 5 Vdc and 24 Vdc current draws of all installed modules and comparing to the PS output rating with the recommended derating margin.
- Power-cycle test the station and verify that the boot sequence completes within the documented time and that all configured modules report healthy state in the diagnostic buffer.
Diagnostics and Troubleshooting Matrix
The following matrix maps common SICAM RTU 400 PS-related symptoms to probable root causes and corrective actions.
| Symptom | Probable Root Cause | Corrective Action |
|---|---|---|
| PS OK LED off after power-up | Input voltage outside PS tolerance; PS fault | Measure station battery voltage at PS input terminals; verify against PS input range; replace PS if input is correct and LED remains off |
| CPU does not enter RUN | CPU in wrong slot; configuration mismatch | Verify physical CPU slot (slot 4 default, slot 3 if 2-slot PS); update SICAM Manager project to match; re-download configuration |
| "Module not responding" for an I/O slot | Slot index in project does not match physical slot | Open SICAM Manager hardware view; reconcile slot indices with rack label |
| PS reports overload on 5 Vdc rail | Total module current exceeds PS 10 A rating | Sum all module 5 Vdc draws; if > 10 A, switch to 20 A PS or redistribute I/O across expansion rack |
| Intermittent I/O module faults | Insufficient 24 Vdc rail current; undersized PS | Check 24 Vdc rail load; upgrade to 20 A PS or split load between central and expansion racks |
| Configuration download rejected | Firmware version mismatch between SICAM Manager and RTU | Verify SICAM Manager version against RTU firmware matrix in release notes; update either side to a compatible pair |
| Slot 3 free but project insists on slot 4 CPU | Old project file with 3-slot PS assumption | Open project, change PS type to 2-slot SIMATIC variant, move CPU index to slot 3, save, and re-download |
| I/O module diagnostic indicates wrong module type | Slot index points to physical location of a different module family | Cross-check slot table; verify each I/O module order code against SICAM Manager module catalog |
Grounding, Wiring, and EMC Practice
The PS module is the reference point for the backplane protective earth and the 24 Vdc logic ground. A low-impedance station ground bond at the PS input terminals is mandatory to limit touch voltage and to provide a stable reference for the analog input modules. Cable cross-section for the station battery feed should be sized for the worst-case PS input current with a voltage drop budget that keeps the PS input above its undervoltage threshold at full load.
Twisted-pair cable is preferred for the station battery feeders to reduce common-mode noise. The cable shield, if used, should be bonded to ground at one end only, typically at the PS terminal, to avoid ground loops that inject noise into the analog inputs. The PE conductor should be sized to local substation grounding standards and should not be used as a current-carrying conductor.
For substation installations with 48 Vdc or 60 Vdc station batteries, the PS-to-battery cable run should be kept short and routed away from high-voltage apparatus and CT/VT secondary wiring. The SICAM PS 48/60 Vdc variants are designed for the elevated input voltage and the resulting higher touch voltage, but the wiring practice still must follow the local utility's grounding and bonding rules.
Separation of the SICAM RTU 400 cabinet cable runs from power and signal classes should follow IEC 61918 / IEC 61784 cable separation guidance applicable to substation LANs, with power feeders on one side of the cabinet and signal wiring on the other. Where crossings are unavoidable, crossings should be at 90 degrees to minimize capacitive coupling.
Firmware and Configuration Compatibility
SICAM RTU 400 firmware revisions have introduced progressive checks on slot allocation, CPU position, and PS identification. A configuration that booted cleanly on a legacy firmware revision may surface as a diagnostic warning on a newer revision. The pragmatic mitigation is to maintain a single revision of the SICAM Manager configuration tool that is qualified against the deployed RTU firmware, and to validate every configuration change against the actual hardware before placing the station in service.
The order code and variant designation of the installed PS should be recorded in the as-built configuration database. When retrofitting a station with a different PS family, the new PS variant must be reflected in the SICAM Manager project and a full configuration download must be performed; partial configuration updates may leave stale PS type information that triggers diagnostic warnings at the next boot.
When upgrading firmware across a major revision boundary, review the release notes for any change to the slot validation rules, the PS identification protocol, or the CPU position check. A change in any of these areas will require a re-validation pass on every SICAM RTU 400 station in the affected fleet, not just the pilot station.
Redundancy, Spare Strategy, and Lifecycle
For transmission-class substations where the RTU availability directly affects grid visibility, a redundant PS configuration is often specified. The SICAM RTU 400 architecture supports a redundant PS pair wired in parallel-or-ORing-diode configuration depending on the rack variant. When designing a redundant PS arrangement, ensure both PS modules share the same input voltage class and output current rating, and that the SICAM Manager project is configured to report a PS fault as a station-level alarm rather than a per-module alarm.
The spare strategy for PS modules should distinguish between the two families. The standard SIMATIC PS is widely stocked in substation stores and can be ordered against the S7-400 catalog. The SICAM PS, being a product-line-specific module, may have a longer lead time and should be stocked in proportion to the fleet size and the supplier's delivery commitment. A recommended baseline is one spare PS per family per ten deployed stations, adjusted upward for remote sites with limited logistics access.
Lifecycle-wise, both the SICAM PS and the standard SIMATIC PS are subject to vendor discontinuation notices. Track the active order codes against the Siemens product lifecycle portal and plan replacements or fleet-wide retrofits ahead of the announced end of production. Retrofitting a 3-slot SICAM PS to a 2-slot standard SIMATIC PS in a station that no longer needs 48/60 Vdc input is a valid lifecycle mitigation that also frees an I/O slot in the process.
Related A8000 Series Power Supplies
The successor SICAM A8000 / CP-8000 platform uses the PS-8620 and PS-8622 power supply modules. The CP-8021 and CP-8022 communication processors are documented in the A8000 / CP-8000 manual, which is the appropriate reference for new A8000-based stations rather than legacy RTU 400 deployments. The slot, voltage, and grounding concepts are similar between the RTU 400 and A8000 generations, but the modules are not interchangeable. Refer to the official manual for the A8000 series when designing new stations:
SICAM A8000 / CP-8000 CP-8021 / CP-8022 Manual (PDF)
Frequently Asked Questions
Is the SICAM PS mandatory in an SICAM RTU 400 station?
No. Field experience documents that standard SIMATIC S7-400 10 A and 20 A power supplies (2-slot width) operate correctly in SICAM RTU 400 racks. The SICAM PS (3-slot) is required only when the station battery is 48 Vdc or 60 Vdc, because the standard SIMATIC PS does not cover these input ranges.
Can the CPU be installed in slot 3 instead of slot 4?
Yes, when a 2-slot PS is fitted. The SICAM RTU does not refuse to boot with the CPU in slot 3, but the SICAM Manager project must be updated so that the configured CPU slot matches the physical position.
What is the difference between a SICAM PS and a standard SIMATIC PS?
The SICAM PS is a 3-slot module explicitly documented for the RTU 400 line, with 48 Vdc and 60 Vdc input variants. The standard SIMATIC PS is a 2-slot S7-400 module with 10 A or 20 A output and 24 Vdc / 120-230 Vac inputs. The two are mechanically and electrically compatible with the SICAM RTU 400 backplane for 24 Vdc and AC input scenarios.
Does using a 2-slot PS free an I/O slot in the central rack?
Yes. A 2-slot PS occupies slots 1-2 and leaves slot 3 available, while a 3-slot SICAM PS occupies slots 1-3 and forces the CPU into slot 4. In a 9-slot central rack this means up to 6 I/O slots with a 2-slot PS versus 5 I/O slots with a 3-slot PS.
When should the 20 A PS be selected over the 10 A PS?
Select the 20 A PS when the summed 5 Vdc and 24 Vdc backplane current of the CPU, CPs, and I/O modules approaches the 10 A budget. As a rule of thumb, if the station is fully populated with SICAM DI and SICAM AI modules, the 20 A PS is the practical choice to maintain derating margin.
Where can I find the official SICAM RTU 400 power supply documentation?
Refer to the SICAM RTU 400 system manual and the SICAM A8000 / CP-8000 manual for the legacy and current power supply modules. Order codes and variant-specific datasheets should be cross-checked against the Siemens Industry Online Support portal for the exact variant ordered.