Siemens 3RH2122-1BB40 — 24 VDC Contactor Relay Buying Guide

By Abdullah Zahid
14 min read
Siemens 3RH2122-1BB40 SIRIUS contactor relay 24 V DC 2NO 2NC S00 DIN rail mount panel control application

Siemens 3RH2122-1BB40 Contactor Relay, 2 NO + 2 NC, 24 V DC, Screw Terminal, Frame Size S00 — Complete Specifications, Selection Guide, and Alternatives

If you have a project spec or control schematic calling out the Siemens 3RH2122-1BB40 by name, you are at the final verification stage: confirming coil voltage, contact configuration, current rating, and safety certification before the purchase order goes through. The 3RH2122-1BB40 is a SIRIUS family contactor relay with a 24 V DC coil, 2 normally open and 2 normally closed contacts rated at 10 A, mounted in an S00 frame on a 35 mm DIN rail. It meets IEC 60947-5-1 positively driven operation requirements and ISO 13849-1 Type 3 classification, making it a viable option for safety-rated interlock and emergency stop circuits at low demand rates. This guide covers everything you need to confirm fit, avoid the most common ordering mistakes, and understand where this relay has real limits.

If you have already confirmed this is the right part, check current pricing and availability at LeadTime.ca — ships worldwide.

Who Should Buy the 3RH2122-1BB40 — and Who Shouldn't

The Siemens 3RH2122-1BB40 is the right part if your application checks all of the following boxes:

  • Your control signal is 24 V DC — not 24 VAC, not 110 VAC, and not 220 VAC
  • You need exactly 2 normally open and 2 normally closed contacts, or are willing to add auxiliary contact blocks for more poles
  • Your total switched load across all contact circuits is 10 A or less at 24 V DC switching voltage (de-rating applies significantly at 110 V DC and above)
  • Your panel uses a standard 35 mm DIN rail for mounting — not a panel-mount chassis or pre-drilled hole pattern
  • You require positively driven contacts per IEC 60947-5-1 for safety-rated interlock circuits, with ISO 13849-1 Type 3 classification
  • You are replacing a 3RH1122-1BB40 and need a current-generation drop-in replacement with the same coil voltage and contact configuration

If you need more than 4 contact poles without auxiliary blocks, require a current rating above 10 A, or are working with an AC control voltage, the correct alternatives are the 3RH2122 with auxiliary contact blocks, a larger frame size SIRIUS relay, or the 3RH2122-1AB40 (24 VAC coil) and 3RH2122-1FA40 (110 VAC coil) respectively — all available through LeadTime.ca.

On this page:

What the Siemens 3RH2122-1BB40 Actually Does in a Control Panel

The 3RH2122-1BB40 is a control relay — not a motor starter, not a power contactor. Its job is to take a 24 V DC signal from a PLC output, a safety controller, or a sensor and translate it into up to four simultaneously switched control circuits. The 2 NO contacts close when the coil energizes; the 2 NC contacts open at the same moment. That combination lets a single triggering signal both enable a downstream process and disable a competing pathway at the same time — a common requirement in batch sequencing, motor start interlocking, and safety chain wiring.

The S00 frame size is the smallest in the SIRIUS contactor relay family. At 45 mm wide and 57.5 mm tall, the 3RH2122-1BB40 fits densely populated control panels where space per relay is limited. Its screw terminals accept wire from 0.5 mm² to 4 mm² per connection, which covers the full range of 24 V DC control wiring practices. The 10,000,000-operation mechanical life and B10 value of 1,000,000 operations at 0.3 x Ie are the two reliability figures that matter most for high-cycle applications like press feed control or conveyor interlocking.

Typical System Architecture: Where This Relay Sits in the Signal Chain

The 3RH2122-1BB40 sits between the PLC or safety controller output layer and the load devices it switches — acting as an electrical isolation and signal multiplication point in the control circuit.

  • PLC digital output card (24 V DC source) or safety relay module energizes the 3RH2122-1BB40 coil at terminals A1/A2
  • The relay's 2 NO contacts connect to downstream loads such as motor starter coils, solenoid valves, or indicator lamps
  • The relay's 2 NC contacts connect to interlocked devices such as dump valves, competing process steps, or fault annunciators
  • All four contact circuits switch independently of the control signal voltage — the loads can operate at voltages up to 690 V AC
  • Optional auxiliary contact blocks attach to expand pole count without requiring a larger relay frame

Industries and Applications That Specify the 3RH2122-1BB40

Automotive assembly and body shop lines use this relay as an intermediate interface between PLC output cards and motor starters or brake solenoids on press feeds and assembly robots. The combination of 2 NO and 2 NC contacts on a single 24 V DC coil allows one PLC output to simultaneously energize a contactor and a brake solenoid while de-energizing a dump valve — a three-action switching sequence from a single output point.

Food and beverage processing facilities specify the 3RH2122-1BB40 for pump motor control, valve switching, and emergency stop chains. The positively driven contact design per IEC 60947-5-1 satisfies the requirements of machine safety auditors who need to verify that contact state mirrors coil state — a requirement that standard relays cannot meet.

Pharmaceutical and chemical batch processing plants use this relay for process interlocking and alarm circuits where one triggering event must simultaneously enable a next process step and disable a competing pathway. The -25 to +60 degrees C operating temperature range covers both ambient cabinet conditions and the higher temperatures found near process equipment.

Retrofit and maintenance applications are a significant volume driver for this model. The 3RH2122-1BB40 is the current-generation replacement for the 3RH1122-1BB40, which has been widely installed in injection molding, packaging, and material handling panels built over the past two decades. Engineers replacing aging relays choose this model to restore MTBF and maintain the same DIN rail footprint and terminal pitch.

Application Typical Deployment
Automotive press feed control PLC output drives 3RH2122-1BB40 coil; NO contacts energize contactor and brake; NC contact de-energizes dump valve
Safety-rated emergency stop circuit Positively driven contacts wired into ISO 13849-1 Type 3 machine guarding circuit; periodic proof test required
Redundant pump control (diagnostics) Two PLC output cards each drive one NO contact; one card failure detected by contact drop-out monitoring
Batch process sequencing 2 NO + 2 NC configuration enables next process step and disables competing pathway from one trigger signal
Legacy panel retrofit (injection molding) Drop-in replacement for 3RH1122-1BB40; same footprint, same coil voltage, improved contact durability
HVAC fan motor soft-start relay 24 V DC control signal from BMS controller; NO contacts on motor starter circuit; NC on bypass indication

Specifications That Matter at the Purchase Stage

Parameter Specification
Coil Voltage 24 V DC (operating range 0.8 to 1.1 x rated: 19.2 to 26.4 V)
Contact Configuration 2 NO (Form A), 2 NC (Form B)
Rated Current (Main Contacts) 10 A at rated value
Switching Voltage (Insulation) 690 V AC (degree of pollution 3)
Frame Size S00 (45 mm wide, 57.5 mm tall, 73 mm deep)
Mounting 35 mm DIN rail, snap-on and screw; ±180 degree rotation, ±22.5 degree tilt
Terminal Type Screw terminals, 0.5–1.5 mm², 0.75–2.5 mm², or 4 mm² per connection
Operating Temperature -25 to +60 degrees C
Protection Class IP20 (front face, finger-safe per EN 50274)
Mechanical Life / B10 (High Demand) 10,000,000 operations mechanical life; B10 = 1,000,000 operations at 0.3 x Ie

Full technical specifications are available on the product page at LeadTime.ca.

Current De-Rating by Switching Voltage — The Spec Most Buyers Miss

The 10 A contact rating applies at 24 V DC switching voltage. At higher switching voltages, the arc energy across the contacts increases and the allowable current drops significantly. Many panel designers overlook this and assume the 10 A rating is flat across all voltages — it is not. Before finalizing your load wiring, verify your total load current against the de-rated limit for your actual switching voltage.

Switching Voltage 1 Contact Path (A) 2 Paths in Series (A) 3 Paths in Series (A)
24 V DC 10 10 10
60 V DC 10 10 10
110 V DC 3 4 10
220 V DC 1 2 3.6
440 V DC 0.3 1.3 2.5
600 V DC 0.15 0.65 1.8

Note: The values above are for DC-12 contact utilization category. If your application uses DC-13 (switching DC electromagnets), verify the current limits in the latest Siemens datasheet for the 3RH2122-1BB40, as ratings differ by utilization category. At 440 V DC with a single contact path, the rated current is only 0.3 A — a common source of contact erosion when designers apply the flat 10 A figure without checking voltage-specific de-rating.

3RH2122-1BB40 vs. Other SIRIUS Variants: Which One Do You Actually Need?

Model Contact Config Coil Voltage Frame Size When to Use This Instead
3RH2122-1BB40 2 NO, 2 NC 24 V DC S00 Standard choice for 24 V DC control
3RH2125-1BB40 4 NO 24 V DC S00 All 4 contacts must be normally open
3RH2122-1AB40 2 NO, 2 NC 24 VAC S00 AC control signal; same contact config
3RH2122-1FA40 2 NO, 2 NC 110 VAC S00 110 VAC control voltage required
3RH2122-1KA40 2 NO, 2 NC 220 VAC S00 Higher AC control voltage application
3RH2122-1BB40 with auxiliary blocks 4 to 8 poles 24 V DC S00 More contact pairs needed without stepping up frame size

If your control architecture uses a 24 V DC PLC output and requires 2 NO plus 2 NC contact pairs, the 3RH2122-1BB40 is the correct selection — check current availability at LeadTime.ca.

Expert Verdict: Honest Assessment for the Panel Builder and Procurement Specialist

The Siemens 3RH2122-1BB40 is a well-proven SIRIUS control relay that earns its place in the panel because it does two things unusually well: it delivers positively driven contacts for safety-rated circuits without requiring a dedicated safety relay module in low-demand-rate applications, and it provides a genuine current-generation replacement for the widely installed 3RH1122-1BB40 at the same DIN rail footprint and terminal pitch. The 10,000,000-operation mechanical life and -25 to +60 degrees C ambient range make it a reliable choice for automotive and food processing environments where thermal cycling and vibration are real operating conditions, not datasheet footnotes. The 10 A contact rating at 24 V DC is more than adequate for the control circuit loads this relay is designed to handle.

Where this relay has real limits: the current de-rating curve above 60 V DC is steep, and specifying this device to switch 440 V DC loads at anything above 0.3 A per contact path will result in accelerated contact erosion. For high-demand-rate safety applications — where the safety function activates more than once per 1,000 hours — the proportion of dangerous failures at 73% (high demand, per IEC 61508) means you need redundant architecture or a higher-rated safety device; the 3RH2122-1BB40 alone is not sufficient for those scenarios without careful system-level design review. And if you are replacing a 3RH1122-1BB40, note clearly that the old 3RH1911 series auxiliary contact blocks, varistor modules, and RC elements will not physically mount to the 3RH2122 frame — order new auxiliaries matched to the 3RH2 generation.

From a procurement standpoint, the 3RH2122-1BB40 is an active catalog item in the current SIRIUS generation, not a legacy or phased-out part. Lead times through authorized distribution channels are generally predictable for standard quantities. Volume orders and project-level bundling with other SIRIUS components — contactors, overload relays, and circuit breakers — can be handled through LeadTime.ca for consolidated sourcing. If this matches your system requirements, view current stock and pricing at LeadTime.ca.

For volume pricing or to confirm lead time before committing to a build, contact the LeadTime.ca team directly — we ship worldwide.

Wiring and Installation Overview

The following points cover the key requirements and verifications for installing and wiring the 3RH2122-1BB40. Engineers requiring full step-by-step procedures should refer to the official Siemens installation documentation for the SIRIUS 3RH2 series.

  • Mount on a 35 mm standard DIN rail; the relay supports ±180 degree rotation and ±22.5 degree tilt for vertical surface mounting, giving flexibility in cable routing direction
  • Coil terminals (A1/A2) accept 0.5–1.5 mm², 0.75–2.5 mm², or 4 mm² wire per connection — use 0.75–1.5 mm² multi-stranded wire for 24 V DC coil circuits to minimize voltage drop that could prevent pull-in at the lower end of the 19.2 V operating range
  • Before energizing loads, verify NO contacts read 0 to 1 ohm (closed) and NC contacts read open circuit (infinite resistance) when 24 V DC is applied to the coil, then confirm contacts return to initial state when coil power is removed
  • If replacing a 3RH1122-1BB40, confirm that no 3RH1911 series auxiliary blocks, diode modules, or RC elements are being reused — these will not mount to the 3RH2122 frame and must be replaced with 3RH2 generation accessories
  • The relay is finger-safe per EN 50274 at the front face (IP20), but the installation enclosure must also provide IP20 or higher protection — do not install in open or unprotected locations

Is the 3RH2122-1BB40 Suitable for Safety Functions?

Yes — under specific conditions. The 3RH2122-1BB40 meets IEC 60947-5-1 positively driven operation requirements and is classified as a Type 3 device per ISO 13849-1 and a Type A device per IEC 61508-2. This means it can be used in machine guarding and emergency stop circuits, but the suitability depends directly on your demand rate category.

At low demand rates — where the safety function is activated less than once per 1,000 hours and tested at least annually — the proportion of dangerous failures is 40% and the failure rate is 100 FIT per SN 31920, with a maximum service life of 20 years. These figures allow the 3RH2122-1BB40 to participate in ISO 13849-1 Category 3 / PLd architectures when used with a redundant second channel.

At high demand rates — where the safety function activates more than once per 1,000 hours — the proportion of dangerous failures rises to 73%. The B10 value of 1,000,000 operations at 0.3 x Ie applies here. For high-demand applications, you must either design a redundant dual-channel architecture that can tolerate this dangerous failure proportion or select a device rated specifically for high-demand safety duty. If you are unsure of your demand rate classification, define it before specifying this relay in any safety function and consult IEC 61508 and ISO 13849-1 guidance documentation.

Wrong-Part Prevention: Verify Before You Order

Before placing your order for the Siemens 3RH2122-1BB40, confirm every item on this checklist. These are the most common sources of ordering errors and installation delays on this part:

  1. Is your control signal output 24 VDC? (Not 24 VAC, not 110 VAC, not 220 VAC)
  2. Do you need at least 2 normally open and 2 normally closed contacts? (Not more; if yes, add auxiliary blocks)
  3. Is your total switched load (sum of all contacts) 10 A or less at the highest voltage you'll switch?
  4. Does your panel have a 35 mm DIN rail? (Not pre-drilled holes; not panel-mount chassis style)
  5. Is screw terminal connection acceptable? (Not spring terminal, not push-in connector)
  6. Are you replacing a 3RH1122-1BB40 or starting fresh? (If replacing, note that old 3RH1911 auxiliaries will NOT mount to this model)
  7. Do you need safety certification for emergency stop or machine guard circuits? (3RH2122-1BB40 is rated; confirm ISO 13849-1 requirement)

If any item on this checklist raises a question, contact the LeadTime.ca team before ordering — confirming the right part number takes minutes and avoids delays that can hold up a panel build.

Frequently Asked Questions

Will the 3RH2122-1BB40 accept auxiliary contact blocks from the older 3RH1122-1BB40?

No. The 3RH2122 frame has a different mechanical interface than the 3RH1 generation. Auxiliary contact blocks, varistor modules, RC elements, and diodes designed for the 3RH1911 accessory series will not physically mount to the 3RH2122-1BB40. If you are retrofitting a panel with 3RH1122-1BB40 relays, you must order new 3RH2 generation accessories to match. This is the single most common compatibility error on this part number.

What is the actual operating voltage range of the 24 V DC coil?

The 3RH2122-1BB40 coil operates reliably from 0.8 to 1.1 times rated voltage, which is 19.2 V to 26.4 V DC. When calculating wire gauge for the coil circuit, ensure that the voltage drop from the PLC output card to the relay coil terminals does not reduce the supply below 19.2 V under worst-case load current — this is especially relevant in long cable runs in large panels.

Can this relay be used in an emergency stop circuit without a separate safety relay module?

Yes, in low-demand-rate applications. The 3RH2122-1BB40 meets IEC 60947-5-1 positively driven operation requirements and is classified ISO 13849-1 Type 3 and IEC 61508 Type A. At low demand rates (safety function activated less than once per 1,000 hours), the failure rate is 100 FIT with 40% proportion of dangerous failures — sufficient for Category 3 / PLd architectures with proper redundant channel design. For high-demand-rate applications, the dangerous failure proportion rises to 73% and additional system architecture measures are required.

What is the maximum switching frequency for the 3RH2122-1BB40?

The maximum no-load switching frequency is 10,000 operations per hour for DC coil operation. The mechanical life rating is 10,000,000 operations. For high-cycle applications such as press feeds or packaging line controls, verify that your expected daily cycle count does not approach the switching frequency limit, which could cause thermal buildup in the coil over extended operating periods.

Why does the current rating drop so sharply at 440 V DC switching voltage?

At higher DC switching voltages, the arc energy at contact break is significantly higher, increasing contact erosion and reducing allowable current to protect contact life and prevent welding. At 440 V DC with a single contact path, the rated current is 0.3 A — compared to 10 A at 24 V DC. Using 2 or 3 contact paths in series raises the effective limit to 1.3 A or 2.5 A respectively at 440 V DC. Always check the de-rating table in the Siemens datasheet for your specific switching voltage before finalizing load assignments.

Is the 3RH2122-1BB40 a direct replacement for the 3RH1122-1BB40?

It is a functional replacement with the same coil voltage (24 V DC), contact configuration (2 NO + 2 NC), current rating (10 A), and S00 DIN rail footprint. However, it is not a mechanical accessory-compatible replacement — the 3RH1911 series auxiliary contact blocks and add-on modules for the 3RH1 generation will not fit the 3RH2122 frame. If your retrofit project requires auxiliary contact expansion, order 3RH2 generation accessories alongside the 3RH2122-1BB40.

Why Source the 3RH2122-1BB40 Through LeadTime.ca

  • Ships worldwide — no geographic restriction on orders; international procurement handled with direct quotes
  • Authorized distribution for Siemens SIRIUS product family — genuine parts, not gray market stock
  • Volume pricing available for project quantities and panel builder blanket orders — contact team for line-item quotes
  • Technical team can confirm part number compatibility and cross-reference 3RH1 to 3RH2 generation transitions before order placement
  • Hard-to-source SIRIUS accessories and auxiliary contact blocks can be consolidated with the 3RH2122-1BB40 into a single shipment

At-a-Glance Summary

  • Model: Siemens 3RH2122-1BB40 — SIRIUS contactor relay, 2 NO + 2 NC contacts, 24 V DC coil, frame size S00
  • Rated current: 10 A at 24 V DC switching voltage; de-rates to 0.3 A per contact path at 440 V DC
  • Coil operating range: 19.2 V to 26.4 V DC (0.8 to 1.1 x rated voltage)
  • Mounting: 35 mm DIN rail, snap-on and screw, with ±180 degree rotation and ±22.5 degree tilt flexibility
  • Dimensions: 45 mm wide x 57.5 mm tall x 73 mm deep; weight 0.271 kg
  • Operating temperature: -25 to +60 degrees C; storage -55 to +80 degrees C
  • Safety certifications: IEC 60947-5-1 positively driven, ISO 13849-1 Type 3, IEC 61508 Type A
  • Mechanical life: 10,000,000 operations; B10 (high demand) = 1,000,000 operations at 0.3 x Ie
  • Failure rate (low demand): 100 FIT; dangerous failure proportion 40% (low demand), 73% (high demand)
  • Terminal type: Screw terminals, 0.5–1.5 mm², 0.75–2.5 mm², or 4 mm² per connection
  • Key compatibility note: 3RH1911 series auxiliary blocks from the 3RH1122-1BB40 generation will NOT fit this model
  • Current-generation drop-in functional replacement for 3RH1122-1BB40 with same footprint and coil voltage