The core question of a safety circuit is: when an emergency stop or door interlock is triggered, how do you make sure the machine actually stops, rather than a broken signal wire or poor contact being misread as “safe”? A safety monitoring relay is designed to solve exactly this — it continuously checks the integrity of the input circuit with redundant circuitry, and if it detects a fault (including broken wires or welded contacts), it immediately cuts its outputs so the machine cannot keep running in an unsafe state.
The Siemens SIRIUS 3TK2822-1CB30 is the basic safety relay of the SIRIUS 3TK28 series (2-channel input, auto start, 2 normally open safety contacts), used to monitor safety input devices such as emergency stop buttons and safety door switches.
Redundant detection: even if a single input contact welds or breaks, the relay still detects the fault and cuts its outputs.
Specifications
| Item | Details |
|---|---|
| Model | 3TK2822-1CB30 |
| Series | SIRIUS 3TK28, basic safety relay (auto start) |
| Input voltage | 24 V AC/DC (AC 50/60 Hz) |
| Output contacts | 2NO safety contacts (13-14, 23-24), no signaling contact |
| Delay time | None (safety contacts open immediately, stop category 0) |
| Dimensions | 22.5 mm wide, DIN rail mounting |
| Protection | Enclosure IP40 / terminals IP20 |
| Operating temperature | −25 to +60 °C |
| Safety rating | SIL 3 (IEC 62061), PL e (EN ISO 13849-1) — per Siemens catalog |
| Terminals | Screw terminals |
What does “auto start” mean?
The 3TK2822 is an “auto start” device: as soon as both input channels close (for example, when the safety door is shut), the 2 safety contacts close, with no separate reset button needed. It has no delay — as soon as an input opens, it cuts off immediately (stop category 0). To handle coast-down time (large rotating machines and heavy conveyors keep turning for several seconds after power is cut), add a door lock or a delayed safety device; for circuits where “the machine must not restart until the reset button is pressed after the emergency stop is released”, the corresponding Siemens model is the monitored-start 3TK2823.
Use with STO (Safe Torque Off)
For drives that support STO (such as OMRON R88M-1L series and Siemens SINAMICS S120), the safety monitoring relay can act as the monitoring layer of the external safety circuit: emergency stop triggered → 3TK2822 detects the input fault → 2 safety contacts open → drive STO signal cut → torque output switched off. This forms a double confirmation and improves the reliability of the whole safety circuit.
- LED indicators: three LEDs on the front, POWER, CHANNEL 1 and CHANNEL 2, show the power and the state of both channels directly.
- Feedback to the PLC: this unit has no signaling contact; to let the PLC know the safety circuit state, use an auxiliary contact of the controlled contactor, or choose the 3TK2821 with 1 NC signaling contact.
- Control supply: 24 V AC/DC, not wide-range; panels with a 110 V / 220 V control supply need a separate 24 V power supply.
Trademarks: Siemens, SIRIUS, SINAMICS, OMRON and the model names on this page are trademarks or model names of their owners, used here only to identify the products. JIAHON is not the authorized distributor of these brands in Taiwan.
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