Overview: SIRIUS 3SK1 Safety Relay Family
The Siemens SIRIUS 3SK1 series is a modular safety relay platform certified to Safety Integrity Level 3 (SIL 3) per IEC 62061 and EN ISO 13849-1 Performance Level e (PL e). The platform covers the full lifecycle from E-stop and guard-door monitoring to light curtain interfacing, with both standard and advanced base units, plus output expansion modules that scale the switching capacity to 10 A. The product family is documented in the official SIRIUS 3SK Safety Relays product page on the Siemens global site: SIRIUS 3SK Safety Relays.
Two base units are most commonly selected for a Category 4 / PLe / SIL 3 application with three normally-open (3NO) safety contacts plus one normally-closed (1NC) signaling contact:
- 3SK1111-1AB30 — 3SK1 Standard base unit, 24 V DC supply, 3NO enabling contacts, 1NC signaling output.
- 3SK1121-1AB40 — 3SK1 Advanced base unit, 24 V DC supply, 3NO enabling contacts, 1NC signaling output, with extended diagnostic and timing functions.
Both units are documented in detail in the SIRIUS 3SK1 System Manual, available on Siemens Industry Online Support as entry 67585885.
Standards, SIL, and Performance Level Mapping
Safety integrity is quantified by the probability of a dangerous failure per hour (PFH) and the probability of dangerous failure on demand (PFD). For a SIL 3 / PLe 3SK1 application the relevant numbers are:
| Parameter | Value | Standard |
|---|---|---|
| PFH (probability of dangerous failure per hour) | 2.50 × 10⁻⁷ 1/h | EN IEC 62061 |
| PFD (probability of dangerous failure on demand) | 2.13 × 10⁻⁵ | IEC 61511 |
| SIL Claim Limit | SIL CL 3 | EN IEC 62061 |
| SIL (low-demand mode) | SIL 3 | IEC 61511 |
| Performance Level | PLe (Cat 4) | EN ISO 13849-1 |
| Category | Cat 4 | EN 954-1 (legacy) |
| EMC immunity | Industrial | EN 61000-6-2 |
| Low-voltage switchgear | Compliant | EN 60947-5-1 |
The relationship between SIL, PL, and Category is critical during hazard assessment. IEC 62061 defines Safety Integrity Level (SIL) for electrical, electronic, and programmable electronic safety-related systems. ISO 13849-1 defines Performance Level (PL a–e) for mechanical, hydraulic, pneumatic, and electromechanical systems. The two standards are not directly interchangeable; a SIL 3 system is generally considered equivalent to PLe when both architectures (Cat 3 or Cat 4) are realized.
Part Number Selection: 3SK1111-1AB30 vs 3SK1121-1AB40
The 3SK1 base unit is selected on the basis of (1) input circuit configuration (single-channel, dual-channel equivalent, or dual-channel complementary), (2) cross-fault monitoring, (3) cross-circuit monitoring, and (4) timing function. The two flagship part numbers most often discussed for 3NO + 1NC applications are:
| Attribute | 3SK1111-1AB30 (Standard) | 3SK1121-1AB40 (Advanced) |
|---|---|---|
| Supply voltage | 24 V DC | 24 V DC |
| Enabling circuits (NO) | 3 NO | 3 NO |
| Signaling output (NC) | 1 NC | 1 NC |
| Cross-fault monitoring | Yes | Yes |
| Cross-circuit detection | Yes (with sensor feedback loop) | Yes (with sensor feedback loop) |
| On-delay / off-delay timing | No | Yes (selectable) |
| Diagnostic LED count | Power + channel status | Power + channel + fault + timing |
| NO contact rating, DC-13 @ 24 V | 5 A | 5 A |
| NO contact rating, AC-15 @ 230 V | 5 A | 5 A |
| Application tier | E-stop, guard door, light curtain (single-function) | E-stop + monitored guard, muting, time-delayed stop |
The 3SK1121 Advanced base unit adds selectable on-delay or off-delay timing (typically 0.05 s to 30 s, depending on firmware variant), enabling controlled stop category 1 functions where the safety output must remain energized for a defined period after the E-stop is actuated.
Contact Rating and Derating
The base 3SK1111-1AB30 and 3SK1121-1AB40 each deliver 3 NO enabling contacts rated for utilization category DC-13 at 24 V and AC-15 at 230 V, with a switching capacity of 5 A per contact. The utilization categories are defined in IEC 60947-5-1:
- AC-15 — control of electromagnetic loads (> 72 VA) in AC circuits.
- DC-13 — control of electromagnets in DC circuits.
When the application requires switching currents above 5 A, fit an output expansion module from the 3SK1 family (e.g., 3SK1211 for contact multiplication). Output expansion modules increase the number of contacts and lift the switching capacity to 10 A at the same utilization categories. The expansion module approach is described in the Siemens FAQ "SIRIUS 3SK1 contact expansion" on entry 76676971.
Output Expansion and Cascading
Two techniques scale the 3SK1 platform beyond a single base unit: contact expansion and cascading.
Contact Expansion
A 3SK1 output expansion (3SK1211-xABxx) is wired downstream of a 3SK1 base unit and is monitored through the base unit's feedback loop. The expansion inherits the SIL 3 / PLe / Cat 4 rating of the upstream base unit provided the feedback loop is closed correctly. This is the recommended way to:
- Add additional NO or NC contacts when the base unit has used all three enabling circuits.
- Switch currents up to 10 A for high-inrush loads (e.g., large contactor coils or motor contactors).
- Provide isolated contact groups for different downstream subsystems.
Cascading for Higher SIL/PL Architectures
When two or more safety functions must trip in a defined sequence — for example, a press where the guard-door interlock must release the main contactor, and a light curtain must trigger a controlled stop — the safety relays are cascaded. Cascading is documented in the Siemens application note "SIRIUS Safety Integrated: Cascading of safety relays to SIL 3 or PL e" on entry 77282496.
Wiring, Fuses, and Contact Protection
The SIRIUS 3SK1 System Manual (entry 67585885) specifies the wire cross-sections, tightening torques, and required backup fuses for each output contact. The fuse sizing rules from the manual (page 215 of the PDF release) are summarized below.
| Load type | Max current | Recommended fuse | Notes |
|---|---|---|---|
| DC-13 contactor coil, 24 V DC | 5 A | 6 A gG (IEC 60269) | Per individual NO contact |
| AC-15 contactor coil, 230 V AC | 5 A | 6 A gG | Per individual NO contact |
| Auxiliary / signaling output (NC) | 2 A | 2 A gG | Signal-grade contact |
| Expansion output 3SK1211 | 10 A | 10 A gG | Higher inrush loads |
When using a 3SK1 safety relay to drive a PLC digital input (e.g., a fail-safe digital input on a SIMATIC S7-1500 F-DI module), no fuse is required between the relay contact and the F-DI input because the input is current-limited and the safety function does not rely on fuse clearing. However, when the safety contact directly energizes a contactor coil that switches motor power, the gG fuse is mandatory.
Interfacing 3SK1 to a Fail-Safe PLC Input
Many architectures place a 3SK1 safety relay between the field sensor (E-stop, guard interlock, light curtain) and a fail-safe PLC input module. The SIMATIC S7-1500 system, for example, supports F-DI modules that achieve SIL 3 / Cat. 3 / PL d in a single-channel configuration and SIL 3 / Cat. 4 / PL e with a two-channel configuration. The wiring and evaluation rules are documented in the F-Modules Manual Collection.
For the 6ES7526-1BH00-0AB0 F-DI 16×24 V DC module, application example "Application 1: Safety mode SIL3 / Cat. 3 / PLd" is documented at TIA Portal cloud documentation. The 3SK1 output contact wires to the F-DI input; the F-DI then evaluates the discrepancy between the two channels and passes the PROFIsafe telegram to the F-CPU.
Cross-Reference: Force-Guided Coupling Relays
Where the 3SK1 base unit must interface with a non-Siemens safety controller, a force-guided coupling relay provides the safe isolation. Siemens launched a family of force-guided coupling relays certified to SIL 3 and PL e, summarized in the press release Siemens force-guided coupling relays. Phoenix Contact also offers SIL 3 coupling relays with ATEX approval and G3 conformal coating for process-industry use; see the Phoenix Contact safe coupling relays product page.
| Manufacturer | Family | SIL | PL | Notes |
|---|---|---|---|---|
| Siemens | SIRIUS 3SK1 | 3 | e (Cat 4) | Modular, expandable, integrated diagnostics |
| Siemens | Force-guided coupling relays | 3 | e | For interfacing with third-party safety controllers |
| Phoenix Contact | PSRmini / PSRclassic | 3 | e | ATEX, G3 coating, up to 70 °C ambient |
Commissioning Procedure
- Verify part numbers and certifications. Confirm the device label shows 3SK1111-1AB30 or 3SK1121-1AB40 and that the TÜV certificate references SIL 3 / PLe / Cat 4.
- Wire the sensor circuit. For dual-channel equivalent (cross-fault monitored), wire both sensor contacts in series into IN1 and IN2. For dual-channel complementary, wire NO and NC into the two inputs.
- Wire the feedback loop (EDM). Connect an NC contact of each downstream contactor to the EDM terminal of the 3SK1 base unit. The base unit will not restart if a contactor is welded.
- Apply 24 V DC supply. Power LED illuminates; channel LEDs indicate sensor state.
- Actuate the sensor and verify dropout. All three NO enabling contacts must open within the response time. Verify with a continuity tester on each contact pair.
- Reset and verify re-energization. With the reset circuit wired (manual or auto reset), the relay must re-energize only after the sensor is in the safe state and the feedback loop is closed.
- Test the NC signaling output. The signaling output must change state when the relay trips. This output is for indication only, not for the safety function.
- Document the safety function. Record PFH, PFD, test interval, and proof-test interval in the safety file per IEC 61511.
Verification Checks
| Test | Expected result | Pass criterion |
|---|---|---|
| Sensor actuation dropout time | All 3 NO contacts open | < 20 ms (typical) |
| Cross-fault test (short between channels) | Relay latches off, fault LED on | No restart without power cycle |
| EDM test (welded contactor) | Relay refuses to start | Power LED flashes, channel LED off |
| Signaling contact (NC) | Changes state on trip | Continuity loss verified |
| Supply voltage tolerance | Operates from 20.4 V to 28.8 V DC | Per datasheet |
Troubleshooting Matrix
| Symptom | Likely root cause | Remedy |
|---|---|---|
| Relay does not start, power LED off | 24 V DC supply missing or reversed | Verify supply polarity and voltage at terminals A1/A2 |
| Relay starts, then drops out | EDM loop open (welded contactor or wiring fault) | Inspect contactor NC feedback wiring |
| Channel LED on, but load does not drop | Welded NO contact in 3SK1 or downstream contactor | Replace relay; verify fuse integrity |
| Fault LED on, latch condition | Cross-fault detected or sensor short circuit | Inspect sensor wiring, cycle power |
| Relay trips but does not re-start | Auto-reset not configured, manual reset not pressed | Apply reset pulse; check reset wiring |
| Signaling NC contact does not change | Mis-wired to load instead of indicator | Move wiring to indicator input; signal contact is rated 2 A only |
| Discrepancy fault on F-DI when relay contact is shared | Bounce on single contact shared between two F-DI channels | Use two independent NO contacts from 3SK1 for the two F-DI channels |
Frequently Asked Questions
What is the difference between SIL 3, PLe, and Category 4?
SIL 3 is defined by IEC 62061 with PFH between 10⁻⁸ and 10⁻⁷ 1/h. PLe is defined by ISO 13849-1 as the probability of dangerous failure per hour between 10⁻⁷ and 10⁻⁶ 1/h, with Category 4 architecture requiring redundancy, monitoring, and well-tried components. A Category 4 / PLe system generally satisfies SIL 3, but the two standards are not interchangeable; the controlling standard depends on the application sector.
Can a 3SK1111-1AB30 switch 6 A on each NO contact?
No. The rated switching capacity is 5 A per NO contact at DC-13 24 V and AC-15 230 V. For loads above 5 A, add a 3SK1211 output expansion module rated to 10 A, or interpose an auxiliary contactor with the correct utilization category rating.
Do I need a fuse on the safety output contact?
Yes, when the safety contact drives an inductive load such as a contactor coil. Use a 6 A gG fuse per NO contact for 5 A loads, or 10 A gG for the 3SK1211 expansion. For signaling-only wiring to a fail-safe input module (e.g., SIMATIC F-DI), the input is current-limited and a fuse is not required for the safety function.
Can I cascade 3SK1 safety relays and still meet SIL 3 / PLe?
Yes. Cascading two or more 3SK1 base units through their feedback loops preserves the SIL 3 / PLe / Cat 4 rating for the combined system, provided the EDM loop of the downstream relay is closed back to the upstream base unit. See Siemens application note 77282496 for the validated topology.
Which 3SK1 part number has an off-delay timing function?
The 3SK1121-1AB40 Advanced base unit includes selectable on-delay and off-delay timing. The 3SK1111-1AB30 Standard base unit is a non-timed device. For controlled stop category 1 applications, select 3SK1121 or add a 3SK1211 timed output expansion.