Siemens 3RH2131-1BB40 — 24V DC Control Relay Buying Guide

By Abdullah Zahid
12 min read
Siemens 3RH2131-1BB40 SIRIUS contactor relay 24V DC 3 NO 1 NC Size S00 DIN rail mount industrial control panel

Siemens 3RH2131-1BB40 Contactor Relay, 3 NO + 1 NC, 24 V DC, Size S00, Screw Terminal — Specifications, Pricing and Alternatives Guide

When a controls engineer searches for the Siemens 3RH2131-1BB40 by part number, the decision is usually already close to final. What remains is confirming the coil voltage is exactly 24V DC, that the 3 NO + 1 NC contact configuration matches the interlocking or signaling requirement, and that a compact Size S00 footprint fits the available DIN rail space. This SIRIUS Control Relay delivers all three in a 57.5 x 45 x 73 mm package rated for switching voltages up to 690V AC — making it one of the most consistently specified auxiliary contactors in motor starter and PLC interface applications worldwide.

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

Who Should Buy the 3RH2131-1BB40 — and Who Shouldn't

The Siemens 3RH2131-1BB40 is the correct choice for engineers and procurement teams who need a verified SIRIUS-family control relay with a 24V DC coil and a defined 3 NO + 1 NC contact arrangement. Buy this part if all of the following apply to your application:

  • Your control system outputs exactly 24V DC — not 24V AC and not any other voltage level
  • Your circuit requires precisely 3 normally open and 1 normally closed contacts — no more, no fewer
  • Your switching voltage is at or below 690V AC, with load currents within 10A at 400V or 1A at 690V
  • Your cabinet uses 35mm standard DIN rail and can accommodate the Size S00 form factor
  • Screw terminal connections are acceptable for your wiring infrastructure
  • Mechanical switching life of 30 million cycles meets your duty cycle requirements

If your coil voltage is 110V AC, 230V AC, or any other non-24V DC level, a different suffix variant within the 3RH2 series is required. If you need 4 NO contacts or a 2 NO + 2 NC arrangement, either an auxiliary contact block addition or a different model number is the correct path. If pluggable connectors are preferred over screw terminals, alternate 3RH2 variants with plug-in connections are available.

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What the Siemens 3RH2131-1BB40 Actually Does in a Control Circuit

The Siemens 3RH2131-1BB40 is a contactor relay — a device that occupies the control layer of an automation system, not the power layer. When the 24V DC coil is energized, the three normally open contacts close and the one normally closed contact opens, allowing a low-power logic signal to switch auxiliary circuits rated up to 690V AC. It carries no main motor or load current directly; its role is isolation, amplification, and interlocking between the logic domain and the switched circuit.

Within the SIRIUS Control Relay family from Siemens, this model sits at the compact end of the range. Size S00 means a 45mm wide profile on the DIN rail — roughly 40% less space than larger S1 or modular relay platforms. That size reduction matters when a control panel holds dozens of SIRIUS components side by side. The IP20 protection class keeps the unit finger-safe within an enclosure, and the screw terminals accept conductors from 0.5 to 2.5 mm² with 4 mm² supported in specific terminal combinations.

The relay is compliant with IEC 60947-5-1 T1 for positively driven operation, and its B10d value of 1 million cycles under high demand rates qualifies it for use in safety-related functions — including emergency stop circuits and safety interlocks — when paired with appropriate external monitoring and a documented safety analysis per IEC 61508.

Where the 3RH2131-1BB40 Sits in a Typical Control System

The 3RH2131-1BB40 sits between the logic controller output and the controlled device or interlock circuit — acting as a galvanic break and signal amplifier in the auxiliary circuit chain.

  • PLC or control system outputs a 24V DC logic signal to the relay coil terminals (A1 and A2)
  • The 3RH2131-1BB40 coil energizes, mechanically switching all four contacts simultaneously
  • The three NO contacts close, completing auxiliary circuits for interlocks, pilot lights, or downstream contactors
  • The one NC contact opens, breaking any interlock or permissive circuit wired in the normally closed path
  • Downstream devices — motor starters, safety circuits, annunciators, or additional contactors — respond to the switched 690V-rated auxiliary circuit

Industries and Applications Where This Relay Gets Specified

Motor starter auxiliary circuits represent the single most common deployment for this relay. When a SIRIUS contactor operates a motor, the 3RH2131-1BB40 provides the auxiliary switching contacts for run-indication pilot lights, interlock permissives, and feedback signals to the PLC — all without wearing the main contactor's contacts on low-level control signals.

PLC output interface is a second major use case. Where a PLC digital output module cannot directly drive a 690V control circuit, the 3RH2131-1BB40 inserts galvanic isolation between the 24V DC logic domain and the higher-voltage switched circuit. This protects the PLC I/O card and provides the current amplification needed for downstream devices.

Emergency stop and safety interlocking circuits use the NC contact specifically. The normally closed path provides a de-energize-to-trip architecture — when the relay coil loses power, the NC contact closes and the NO contacts open, allowing safe-state transitions in dependent circuits. The B10d rating of 1 million cycles supports periodic proof testing requirements in safety function applications.

Reversing motor control circuits and sequential start/stop logic in food processing, automotive assembly, packaging lines, and material handling systems round out the primary application profile. The relay has also established a long track record in water treatment, HVAC control panels, pharmaceutical processing, and renewable energy control systems.

Application Typical Deployment
Motor starter auxiliary circuit Run indication, interlock permissives, feedback to PLC from SIRIUS contactor panel
PLC digital output interface Galvanic isolation between 24V DC PLC output and 690V-rated switched auxiliary circuit
Emergency stop circuit NC contact in de-energize-to-trip architecture for safe-state transitions
Reversing motor control Forward/reverse interlocking with mechanical interlock protection in drive panels
Packaging and material handling lines Sequential start/stop logic control for conveyor and indexing systems
Water treatment and HVAC Pump and fan control interlocking with status feedback in building automation panels

Purchase-Decision Specifications at a Glance

Parameter Rating
Coil Voltage 24V DC
Contact Configuration 3 NO + 1 NC (4 poles total)
Maximum Switching Voltage 690V AC
Contact Rating at 400V AC 10A
Contact Rating at 690V AC 1A
Size Classification S00
Dimensions (H x W x D) 57.5 x 45 x 73 mm
Mounting 35mm standard DIN rail
Operating Temperature -25°C to +60°C
Protection Class IP20

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

3RH2131-1BB40 vs. Other SIRIUS Variants — Which One Do You Actually Need?

Model Coil Voltage Contact Configuration Terminal Type Choose When
3RH2131-1BB40 24V DC 3 NO + 1 NC Screw 24V DC control, standard 3 NO + 1 NC layout, screw terminals required
3RH2161-1BB40 24V DC (series also available in 110V, 230V AC/DC) Different configuration Screw Alternative coil voltages needed (110V AC, 230V AC) or different contact count
3RH2 with plug-in connector Various Various Pluggable Panel design requires plug-in connections for faster replacement or field wiring
Larger frame sizes (S0, S1) Various More than 4 contact positions Screw More than 4 auxiliary contacts required or higher current auxiliary circuits needed

If your architecture requires 24V DC coil control with a 3 NO + 1 NC screw-terminal layout on a 35mm DIN rail, the 3RH2131-1BB40 is the correct choice — check current availability and pricing at LeadTime.ca.

Expert Verdict: Honest Assessment for Controls Engineers

The Siemens 3RH2131-1BB40 earns its position as a default specification for 24V DC auxiliary relay applications in motor control and PLC interface work. The 30 million mechanical cycle rating is not a marketing number — it translates to years of reliable service at even moderately active switching duty. The 57.5 x 45 x 73 mm Size S00 footprint fits neatly into standard SIRIUS motor starter assemblies, and the IEC 60947-5-1 T1 compliance means it qualifies for positively driven operation documentation without special engineering workarounds. For teams already standardized on the SIRIUS ecosystem, there is no compelling reason to step outside it for this contact count and coil voltage.

Where this relay has real limits: the 1A contact rating at 690V is a genuine constraint in applications with higher auxiliary circuit currents at elevated voltages. If your auxiliary circuit demands more than 10A at 400V, you need a larger frame. The screw terminal format, while universal, does add wiring time compared to pluggable-connector variants when panels require frequent relay replacement in the field. And the 73% dangerous failure proportion at high demand rates — cited directly in the SN 31920 data — means this relay is not a drop-in safety component without external diagnostics or redundancy in high-integrity safety functions. A B10d of 1 million cycles is adequate for many safety applications with proof testing every 20 years, but your safety assessment must account for that 73% figure explicitly.

From a procurement standpoint, the 3RH2131-1BB40 benefits from over 25 years of continuous production within the SIRIUS family — meaning multi-source distributor availability, stable pricing, and long-term spare parts support. Volume discounts apply at 10 or more units, and typical in-stock lead times run 1 to 2 weeks through authorized channels in normal market conditions. During supply disruptions, 4 to 12 week lead times have been observed, making it worth confirming stock status before committing a build schedule. 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

  • Mount on a clean, dry 35mm standard DIN rail with a minimum 6mm clearance on each side for both grounded and live parts; the relay supports ±180° rotation and ±22.5° tilt on vertical surfaces for layout flexibility
  • Connect the 24V DC control supply to coil terminals A1 (positive) and A2 (negative return) using 0.5 to 2.5 mm² stranded or solid conductors — confirm voltage at the terminals is within ±10% of 24V DC nominal before applying load
  • Wire the three NO contacts (typically terminals 11-12, 21-22, 31-32) and the NC contact (typically 41-42) to their respective auxiliary circuit loads using conductors within the 0.5 to 2.5 mm² range; 4 mm² conductors are supported in specific terminal positions
  • Install external protection in the auxiliary circuit before commissioning — a miniature circuit breaker (C characteristic, 6A, 0.4 kA rated for up to 230V) or a gL/gG 10A fuse is required; the relay is not self-protected
  • Verify all terminal screws are hand-tightened only; do not use power tools on the screw terminals — a gentle tug test on each conductor confirms secure seating without over-clamping

Engineers requiring full wiring diagrams, terminal torque specifications, and commissioning procedures should consult the Siemens SIRIUS 3RH2 manufacturer documentation directly.

What Engineers Get Wrong When Ordering This Relay

Before placing an order for the Siemens 3RH2131-1BB40, work through this verbatim checklist. Each item represents a documented source of wrong-part orders or post-installation failures:

  1. Confirm coil voltage is 24V DC, not 24V AC or other voltages (different model suffix required)
  2. Verify contact configuration matches requirement (3 NO + 1 NC) - not 4 NO or other combinations
  3. Check switching voltage rating meets application (690V rated; confirm your control voltage is at or below this)
  4. Confirm DIN rail mounting is required; this model is for 35mm standard rail only
  5. Verify screw terminals are acceptable; if pluggable connectors needed, different part number required
  6. Ensure mechanical duty cycle is sufficient (30 million cycles typical; if higher frequency needed, confirm duty class)

If any item on this list raises a question, contact the LeadTime.ca team before ordering — confirming the right part number upfront is faster than managing a return.

Frequently Asked Questions

Can I use 24V AC on the coil instead of 24V DC?

No. The 3RH2131-1BB40 is rated for 24V DC coil operation only. Applying 24V AC will not reliably energize the coil and may damage it. If your control system outputs 24V AC or another AC voltage, a different model number with the correct AC coil suffix from the 3RH2 series is required — the model number suffix encodes the coil voltage, so this is not a field-adjustable parameter.

What happens if I need more than 3 NO + 1 NC contacts?

The 3RH2131-1BB40 provides exactly 4 poles in a 3 NO + 1 NC arrangement — this is fixed by the model number. If your application requires 4 NO contacts, a 2 NO + 2 NC arrangement, or additional contact positions, you either need an auxiliary contact block added to the relay or a different model number. Confirm the full contact requirement before ordering to avoid rework.

Is external circuit protection really required, or can I rely on upstream fusing?

The Siemens datasheet specifies dedicated external protection for the auxiliary circuit. A miniature circuit breaker with C characteristic rated 6A at 0.4 kA (for circuits up to 230V) or a gL/gG 10A fuse is the documented requirement. The relay itself has no internal overcurrent protection. Upstream fusing on the panel supply does not substitute for protection sized specifically to the auxiliary circuit.

What is the maximum switching frequency, and what happens if I exceed it?

The operating frequency limit is 10,000 switching cycles per hour for both AC and DC operation. Operating consistently at or above this threshold accelerates contact wear and reduces mechanical service life below the 30 million cycle rating. If your application requires switching frequencies approaching this limit, verify that the duty class is appropriate and consider whether a solid-state output device is a more suitable solution for the high-frequency portion of the circuit.

Can the 3RH2131-1BB40 be used in a safety-related application such as an emergency stop circuit?

Yes, with qualification. The relay complies with IEC 60947-5-1 T1 for positively driven operation and carries a B10d value of 1 million cycles per SN 31920, supporting safety function use with periodic proof testing every 20 years under the applicable demand rate assumptions. However, the proportion of dangerous failures at high demand rate is 73%, which requires explicit treatment in your safety function analysis — external diagnostics or redundancy may be needed depending on the required Performance Level or SIL. Always complete a formal safety assessment before relying on this relay in a safety function.

How does the Size S00 form factor affect panel layout compared to larger SIRIUS sizes?

At 45mm wide, the S00 footprint occupies approximately 40% less DIN rail space than larger S1 or modular relay platforms. This directly affects how many relays can be installed in a given panel width and reduces overall assembly labor in space-constrained industrial control panels. The ±180° rotation and ±22.5° tilt flexibility further support tight layout arrangements without requiring special mounting hardware.

Why Order the 3RH2131-1BB40 from LeadTime.ca

  • LeadTime.ca ships the Siemens 3RH2131-1BB40 worldwide — no regional restrictions on sourcing
  • Stock status and current pricing are confirmed at order time, with lead time transparency before you commit to a build schedule
  • Volume pricing is available for orders of 10 or more units — contact the team directly to discuss quantity breaks
  • Hard-to-source or short-lead-time SIRIUS components can be expedited through established distribution channels
  • The team can assist with part number verification, cross-reference confirmation, and variant selection before an order is placed

At-a-Glance Summary

  • Model: Siemens 3RH2131-1BB40 — SIRIUS Control Relay, Size S00
  • Coil voltage: 24V DC — not suitable for AC control systems without a different model suffix
  • Contact configuration: 3 NO + 1 NC, 4 poles total, screw terminals accepting 0.5–2.5 mm² conductors
  • Maximum switching voltage: 690V AC; contact rating is 10A at 400V and 1A at 690V
  • Dimensions: 57.5 x 45 x 73 mm — approximately 40% less DIN rail space than larger S1-frame relays
  • Mechanical service life: 30 million cycles (contactor only); 10 million cycles with auxiliary switch block
  • Operating temperature: -25°C to +60°C; storage down to -55°C
  • Protection class: IP20; compliant with IEC 60947-5-1 T1 and IEC 61508
  • B10d value: 1 million cycles at high demand rate (SN 31920) — qualifies for safety function use with appropriate analysis
  • External protection required: C6A miniature circuit breaker or gL/gG 10A fuse in the auxiliary circuit
  • Supply stability: over 25 years of continuous SIRIUS family production; multi-source distributor availability worldwide