We keep cross-reference notes for the brands customers most often arrive with, and in panels built around a Siemens PLC that brand is Siemens itself. SIRIUS 3SK turns up in inquiries because it is the relay Siemens ships alongside its own controllers, and because Siemens explicitly positions the 3SK1 and 3SK2 as the replacement for the older 3TK28 series — a narrower design that, per Siemens, can replace nearly every 3TK28 device. So when an engineer asks for a “3SK alternative,” the first job is to find out which 3SK: a basic 3SK1 doing one job, or a configurable 3SK2 doing several. That distinction decides whether a single DA31 is the right answer at all.
Record the complete Siemens order number, hardware configuration and installed schematic. Safety function, input diagnostics, output type/load, reset/feedback, safety data and installation conditions must be satisfied together. Re-termination alone does not prove a safe replacement, and Siemens migration recommendations do not automatically apply to DA31.
What is SIRIUS 3SK, and what is the DAIDISIKE DA31?
A safety relay evaluates the inputs and fault conditions specified for its exact model and mode. Internal force-guided contacts support detection of particular faults; a documented external feedback circuit monitors downstream switching devices. These features and input cross-circuit detection must be verified separately before assigning them to DA31.
Siemens' SIRIUS 3SK is the family that does this in the Siemens world, and it has two distinct halves. The 3SK1 is the basic relay: hard-wired, with one shut-down behaviour — when it trips, it disables all of its fail-safe outputs simultaneously. The 3SK2 is the configurable relay: it can disable its fail-safe outputs individually, it is parameterized in software rather than by wiring links, and supported interface modules connect it to higher-level control for process data and diagnostics. Above both sits the 3RK3 Modular Safety System (MSS), a modular controller offered with Basic, Advanced and ASIsafe central modules for monitoring many functions at once.
DA31 manufacturer data lists dual-channel input, three N.O. safety contacts plus one auxiliary N.C. signalling contact, below-30-ms release response and a 22.6 mm body. Its supporting manual must establish compatibility with the required Siemens function. EDM and monitored-reset equivalence are not established by the cited specification; configurable or modular-system functions require their own suitable solution.
Which 3SK does what? Match the tier before you cross-reference
Identify whether you have a basic 3SK1, a configurable 3SK2, or a 3RK3 system first — only then pick the DAIDISIKE part. This is the single most common place a Siemens-to-third-party swap goes wrong: dropping a hard-wired single relay in where the original was doing software-defined, individually-switchable logic. Here is the honest map.
| Siemens tier | What it is | Anchor order numbers | DAIDISIKE answer |
|---|---|---|---|
| SIRIUS 3SK1 (basic) | Hard-wired safety relay; disables all fail-safe outputs simultaneously | 3SK1112-1BB40, 3SK1121-1AB40, 3SK1121-1CB42, 3SK1121-2AB40, 3SK1121-2CB42 | DA31 candidate only after exact function and diagnostic assessment |
| SIRIUS 3SK2 (configurable) | Software-parameterized; disables fail-safe outputs individually; PROFINET through the supported interface module | 3SK2112-1AA10, 3SK2112-2AA10 | Documented configurable controller assessment; no model equivalence established |
| 3TK28 (legacy) | Older Siemens safety relays; Siemens migrates these to 3SK | 3TK28 series | Separate documented candidate assessment; no automatic DA31 match |
| 3RK3 Modular Safety System (MSS) | Modular controller; Basic / Advanced / ASIsafe central modules; many functions | 3RK3 MSS central modules + expansion | No — configurable safety controller, not one relay |
The family distinction is the first screening step, not a compatibility result. DA31 can be considered for a documented single-function design; neither a 3SK1 label nor a low function count establishes a match. A 3SK2 or 3RK3 system may require independently evaluated outputs, expansion or communication that cannot be retained by one fixed-function relay.
DA31 vs Siemens 3SK1 — the side-by-side
This table is a requirements comparison, not a verified 1:1 match. Use the exact Siemens order number for supply, output type, load and input/reset/feedback functions. DA31 undocumented diagnostics remain unresolved.
| Parameter | DAIDISIKE DA31 | Siemens SIRIUS 3SK1 (basic) |
|---|---|---|
| Safety class | Manufacturer-listed PL e / SIL 3; obtain applicable safety data | Up to PL e / SIL 3, Cat. 4 (per variant) |
| Output contacts | 3 safety NO + 1 auxiliary NC; separate signal transistor | Relay or solid-state outputs; count varies by order number |
| Contacts force-guided | Yes (positive-guided) | Yes (on relay-output variants) |
| Input channels | Dual-channel; cross-circuit detection requires manual evidence | Single / dual-channel |
| EDM (external device monitoring) | Not established in current DA31 specification | Yes |
| Output behaviour | All enabling outputs switch together | All fail-safe outputs disabled simultaneously |
| Response time | < 30 ms | Per Siemens datasheet |
| Supply | 24 V DC | 24 V DC (per variant) |
| Configurability | Hard-wired, single function | Hard-wired (3SK1); configurable is 3SK2 |
| Typical functions | E-STOP, safety gate, light-curtain (OSSD) monitoring | E-STOP, safety gate, light-curtain monitoring |
| Sourcing | Factory-direct quote; confirm quantity and delivery in writing | Siemens distributor channel |
Where the Siemens column reads “per Siemens datasheet,” that is deliberate — we will not put a number in a Siemens column unless it is published, and exact response-time, output-type and contact-rating figures vary by 3SK1 order number. Pull them from your unit's label (e.g. 3SK1121-1AB40 versus 3SK1121-1CB42) and confirm against Siemens' own datasheet. The DA31 column is from DAIDISIKE's published spec.
A compatible safety input feeds documented safety logic, whose safety outputs control the selected final switching devices. Where the design requires EDM, their prescribed feedback returns to a controller documented to evaluate it and inhibit restart on a detected fault.
This is not terminal wiring. DA31 EDM and monitored-reset support remain unconfirmed; no terminal names or competitor functions transfer to it.
When should you use a configurable controller instead of a DA31?
When the Siemens unit is a 3SK2 or a 3RK3, the original is doing more than one relay's job — match it with a configurable controller, not a single DA31. The tell-tales are concrete: the 3SK2 can disable its fail-safe outputs individually (so different zones stop independently), it is set up in software rather than by wiring links, and it reports diagnostics to a higher-level controller through the supported PROFINET interface module. The 3RK3 Modular Safety System goes further — its Basic, Advanced and ASIsafe central modules monitor many functions across a machine. None of that is reproducible with one hard-wired relay, and we will not claim it is.
A configurable-system replacement needs its own evidence. The previously mentioned DADISICK LS-2A4S is not established here as a DAIDISIKE product, a Siemens-approved equivalent or a replacement for 3SK2/3RK3 functionality. Submit the full I/O list, output dependencies, program, restart zones and communication needs for assessment against a documented safety controller; channel counts alone are insufficient.
Evaluate the selected safeguard, documented safety logic, final switching devices and machine response as one function. Required architecture, reliability, diagnostic coverage, common-cause and systematic-failure measures all contribute to the achieved integrity.
A component rating does not approve the whole machine. Verify each required diagnostic and restart function; do not assume DA31 external-device monitoring.
What to match before you swap any 3SK for a DA31
Six requirement groups start the assessment; collect the full installed-unit manual and schematic. This is the same discipline we apply to every brand cross-reference.
- Safety function & tier — one function on a basic 3SK1 (DA31 territory), or several with individual outputs on a 3SK2 / 3RK3 (controller territory).
- Input channels — single-channel, or the architecture and fault diagnostics required by the risk-assessed function. Two channels alone do not establish the required integrity.
- Output type, count & rating — relay (NO/NC) versus solid-state, how many enabling outputs, and switching current. DA31 = three safety NO and one auxiliary NC, plus a separate signal transistor.
- EDM / reset — is external device monitoring used, and is reset automatic or monitored-manual?
- PL / SIL target — from your risk assessment (DA31 is PL e / SIL 3).
- DIN width & supply — panel space (DA31 is 22.6 mm) and 24 V DC supply.
Send those six off your installed safety relay for a documented DA31 assessment; unresolved requirements may mean it does not fit — for instance, if you actually need a configurable controller, a two-hand module, or a higher switching current than a single DA31 provides. The wider method and the other brand cross-references (such as our Pilz PNOZ alternatives and our Allen-Bradley Guardmaster alternatives) live in the safety relay manufacturer guide.
Is naming Siemens, SIRIUS and 3SK legal, and how do you keep this honest?
Siemens, SIRIUS and the order numbers identify the comparison targets; no affiliation, endorsement or approval is implied. This is not a legal opinion or a compatibility declaration. Use Siemens primary documentation for exact configurations and separate DAIDISIKE evidence for DA31. Missing features cannot be filled in from a competitor brochure.
Sources & specifications cited
- Siemens SIRIUS 3SK family overview — positioned by Siemens as the replacement for the 3TK28 series; disables all fail-safe outputs simultaneously. Public Siemens / distributor data (order numbers 3SK1112-1BB40, 3SK1121-1AB40, 3SK1121-1CB42, 3SK1121-2AB40, 3SK1121-2CB42).
- Siemens SIRIUS 3SK2 configurable safety relays — can disable fail-safe outputs individually, parameterized by software; supported interface modules provide PROFINET diagnostics/process data. Public Siemens / distributor data (order numbers 3SK2112-1AA10, 3SK2112-2AA10).
- Siemens 3RK3 Modular Safety System (MSS) — Basic, Advanced and ASIsafe central modules for multi-function safety. Public Siemens data.
- Configurable replacement: no LS-2A4S-to-Siemens equivalence established here; request separate manufacturer documentation.
- DAIDISIKE DA31 safety relay datasheet — PL e / SIL 3, 3NO+1NC, three safety N.O. contacts plus one auxiliary N.C.; <30 ms; 22.6 mm body; 24 V DC; EDM unconfirmed.