Siemens 3RH2140-1BB40 — SIRIUS 24V DC Relay Buying Guide

By Abdullah Zahid
12 min read
Siemens 3RH2140-1BB40 SIRIUS contactor relay 4NO 24V DC coil DIN rail mounted industrial control panel

Siemens 3RH2140-1BB40 Contactor Relay, 4 Normally Open Contacts, 24 V DC Coil, Frame Size S00, Screw Terminal — Specifications, Wiring, and Selection Guide

When a motor starter interlocking circuit fails or a control panel retrofit is on a tight deadline, the relay selection decision comes down to a few hard constraints: coil voltage, contact configuration, and switching voltage. The Siemens 3RH2140-1BB40 is a SIRIUS family electromechanical contactor relay with a 24 V DC coil, four normally open contacts, a maximum insulation voltage of 690 V, and a mechanical service life of 30 million cycles. It is the direct successor to the 3RH1140-1BB40 and mounts on a standard 35 mm DIN rail in Frame Size S00 — making it one of the most commonly specified auxiliary relays in industrial control panels worldwide.

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

Who Should Buy the 3RH2140-1BB40 — and Who Shouldn't

This relay is the right choice for control engineers and panel builders who need a compact, low-cost electromechanical relay for auxiliary circuit switching, signal routing, or hardwired interlocking in industrial control systems. It is specifically the right fit when all of the following apply:

  • Control system coil voltage is exactly 24 V DC — this relay cannot operate on 24 V AC or any other voltage
  • All four contacts are required in a normally open (4NO) configuration for standard interlocking circuits
  • Switching loads do not exceed 690 V AC or 600 V DC, and current at the rated voltage falls within the datasheet limits (10 A at 230 V AC down to 1 A at 690 V AC)
  • Installation is on a 35 mm DIN rail with at least 57.5 mm of panel height clearance for Frame Size S00
  • Screw-type terminals are acceptable for the permanent installation environment
  • Switching frequency does not exceed 10,000 cycles per hour under no-load conditions

If you need a mixed contact arrangement such as 2NO/2NC, consider the 3RH2140-1AB40. If your application demands switching pilot duty above 10 A at 230 V AC, a larger frame size (S0 or S2) is required. For very high switching frequencies above 1,000 cycles per hour under DC load, the 3RH2141 series offers faster response characteristics.

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What the 3RH2140-1BB40 Actually Does in a Control System

The 3RH2140-1BB40 is not a power contactor — it is a control relay. Its role is to act as an electrically controlled switch within the logic layer of an industrial control system. When 24 V DC is applied across coil terminals A1 (positive) and A2 (negative), the relay closes all four normally open contacts simultaneously, enabling downstream circuits to carry loads up to the rated switching voltage and current. When the coil is de-energized, all four contacts return to open, breaking those downstream circuits.

What makes this relay practical across a wide range of panel designs is its combination of a 690 V insulation voltage rating, finger-safe IP20 front terminals, and a Frame Size S00 footprint of 45 mm wide by 57.5 mm tall by 73 mm deep. It can handle both resistive and pilot duty loads, route discrete 24 V DC PLC output signals to external alarm circuits, and act as a pilot relay driving larger contactor coils in cascade control sequences. The mechanical service life of 30 million unloaded switching cycles means that in typical low-to-moderate frequency applications, this relay will outlast most of the equipment it is installed in.

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

The 3RH2140-1BB40 sits between the logic control layer and the load switching layer — it receives a discrete signal from a PLC output, a hardwired interlock, or a relay cascade, and uses that signal to switch one or more auxiliary circuits downstream.

  • PLC digital output (24 V DC) or hardwired control logic drives the coil at terminals A1 and A2
  • The 3RH2140-1BB40 closes its four NO contacts, connecting downstream circuits
  • Downstream loads include contactor coils, solenoid valves, alarm annunciators, or status indicators
  • External MCB (6 A, C-characteristic recommended for circuits up to 230 V) protects the auxiliary circuit upstream of the relay contacts
  • Optional 3RH2940-series auxiliary contact blocks can be added to the base unit to expand the total number of switched poles up to 8

Applications and Industries That Specify the 3RH2140-1BB40

In motor control centers, the 3RH2140-1BB40 is a standard choice for forward/reverse interlocking circuits. The four NO contacts allow the relay to simultaneously break permissive signals to both forward and reverse contactors, preventing simultaneous closure during directional switching of three-phase motors.

In HVAC systems, this relay routes damper actuator control signals and manages status signaling for secondary air handling units. The 24 V DC coil voltage aligns with the low-voltage control standards used in most building automation systems, and the screw terminals provide the permanent, vibration-resistant connections that HVAC panel builders need.

In machine tool and conveyor applications, hardwired interlocking for machine guarding requires reliable contact operation over millions of cycles. With a mechanical life of 30 million cycles on the base unit, the 3RH2140-1BB40 handles high-repetition on/off duty in these environments without requiring scheduled contact replacement.

In pump and compressor stations, the relay provides auxiliary circuit control for pump start and stop sequencing, including load switching applications at 690 V AC where the contact carries 1 A — sufficient for pilot duty coil driving in high-voltage distribution systems.

Application Typical Deployment
Three-phase motor starter interlocking Forward/reverse prevention logic in MCC panel
PLC discrete output signal routing Routing 24 V DC outputs to external alarm or status circuits
HVAC secondary air handling control Damper actuator and fan relay switching in building automation panels
Cascade contactor control Pilot relay driving larger S0/S2 contactor coils in sequence
Pump and compressor station auxiliary control Start/stop sequencing with 690 V AC pilot duty switching
Machine guarding hardwired interlock Safety permissive circuit in conveyor and machine tool panels

Specifications That Drive the Purchase Decision

Specification Value
Coil Voltage 24 V DC only
Contact Configuration 4 Normally Open (4NO)
Insulation Voltage (Pollution Degree 3) 690 V
AC Contact Rating at 230 V (AC-15) 10 A
AC Contact Rating at 400 V (AC-15) 3 A
AC Contact Rating at 690 V (AC-15) 1 A
DC Contact Rating at 24 V (DC-12) 10 A
Surge Voltage Resistance 6 kV
Mechanical Service Life (base unit) 30,000,000 cycles
Operating Temperature Range -25 to +60 °C

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

3RH2140-1BB40 vs. Related Variants: Which One Do You Actually Need?

Model Coil Voltage Contact Configuration Frame Size Best For
3RH2140-1BB40 24 V DC 4NO S00 Standard interlocking, 24 V DC control systems
3RH1140-1BB40 24 V DC 4NO S00 Obsolete predecessor — replace with 3RH2140-1BB40
3RH2140-1AB40 24 V AC 4NO S00 Applications requiring 24 V AC coil operation
3RH2141 series Various Various S00 Higher switching frequency, faster DC response
S0 / S2 frame variants Various Various S0 / S2 Pilot duty above 10 A at 230 V, higher current loads

If your control panel operates on a 24 V DC logic standard and your interlock circuits all require normally open contacts, the 3RH2140-1BB40 is the correct choice — check current availability at LeadTime.ca.

Expert Verdict: Honest Assessment for Controls Engineers

The 3RH2140-1BB40 does exactly what it is designed to do with a level of reliability that makes it a panel builder's default choice for 4NO auxiliary relay applications. The 30 million cycle mechanical life, finger-safe IP20 front terminals, 6 kV surge voltage resistance, and a contact reliability figure of 1 faulty operation per 100 million cycles at 17 V and 1 mA make this a genuinely low-risk component for hardwired interlocking, signal routing, and pilot duty switching in industrial environments. The 690 V insulation voltage gives it versatility across both low-voltage auxiliary circuits and high-voltage industrial distribution applications — as long as engineers cross-reference the current rating at the actual operating voltage, which drops from 10 A at 230 V AC to 1 A at 690 V AC.

Where this relay has real limits: it is not the right choice if you need a mix of normally open and normally closed contacts on the base unit — that requires a different variant such as the 3RH2140-1AB40 for AC coil operation or an alternative model with a mixed contact arrangement. It also should not be used for DC switching at voltages above 440 V without carefully verifying the 0.3 A DC-12 rating at that voltage. Engineers upgrading from the legacy 3RH1140-1BB40 need to know that while the electrical replacement is direct, the old 3RH1911 auxiliary contact blocks and accessories are mechanically and electrically incompatible with the new 3RH2140 series — budget for new auxiliary blocks if your application requires expanded contact poles.

From a procurement standpoint, the 3RH2140-1BB40 is a standard catalog item with predictable availability. Obsolescence risk is low given its position in the active SIRIUS product family. If this matches your system requirements, view current stock and pricing at LeadTime.ca. Volume orders and time-critical replacements during unplanned downtime are handled directly by the LeadTime.ca team, who can confirm lead time before you commit.

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

  • Connect 24 V DC positive to coil terminal A1 and negative to coil terminal A2 — polarity must be maintained; reversed polarity will prevent coil energization
  • Load circuits connect to contact terminals using screw-type connections; acceptable conductor cross-sections range from 0.5 mm² to 4 mm² (AWG 20 to AWG 12); finely stranded conductors require core end processing such as crimped ferrules
  • Install an external C-characteristic MCB rated 6 A in each auxiliary circuit up to 230 V — this relay has no internal overcurrent protection
  • Add a protective varistor or RC snubber network across any inductive load coil (solenoid, contactor coil) connected to the relay contacts to suppress back-EMF arcing and extend contact life
  • Mount on 35 mm DIN rail with at least 57.5 mm of height clearance; maintain 6 mm lateral spacing from adjacent grounded or live parts; mounting orientation allows ±180° rotation and ±22.5° tilt on vertical surfaces; maximum installation altitude is 2,000 m above sea level

Compatible Auxiliary Contact Blocks and System Expansion

The 3RH2140-1BB40 base unit supports auxiliary contact block expansion, allowing the total number of switched poles to reach 8. Only 3RH2140-series compatible blocks should be used — the older 3RH1911 auxiliary blocks are mechanically and electrically incompatible and must not be reused from legacy 3RH1 installations.

  • 3RH2940-series auxiliary contact blocks — add up to 4 additional contacts (4NO configuration) in top or side mounting positions on the S00 frame
  • Multiple contact arrangements are available for the expansion blocks: 4NO, 3NO/1NC, 2NO/2NC, 1NO/3NC, and 4NC — select based on the interlocking logic required in the expanded circuit
  • Electronics-compatible auxiliary blocks are available for low-level signal circuits; note that mechanical service life with these blocks is rated at 5 million cycles versus 10 million cycles with standard auxiliary blocks

What Engineers Get Wrong When Ordering This Relay

Before finalizing your purchase requisition or RFQ, work through this checklist. Every item represents a real-world ordering error that causes delays, returns, or in-panel wiring rework.

  1. Confirm control system coil voltage is exactly 24 V DC; this relay cannot work with 24 V AC or other voltages
  2. Verify that all switching loads are at or below 690 V (AC) or 600 V (DC) maximum
  3. Check that the load current at rated voltage does not exceed 10 A at AC-12 (AC contacts) or 10 A at 24 V DC-12
  4. Ensure 3RH1911 auxiliary contact blocks and accessories from older 3RH1 series will not be reused (incompatible mounting)
  5. Confirm DIN rail width is 35 mm and mounting space height is at least 57.5 mm
  6. Verify that external miniature circuit breaker (MCB) protection is installed for auxiliary circuits up to 230 V (recommend 6 A C-characteristic)
  7. Check that ambient operating temperature range is -25 to +60 °C and this matches site conditions
  8. Confirm that finger-safe terminal cover is installed if panel is accessed by untrained personnel

If any item on this checklist raises a question about your specific application, contact the LeadTime.ca team before ordering — we can confirm compatibility and help identify the correct variant.

Frequently Asked Questions

Can the 3RH2140-1BB40 directly replace the 3RH1140-1BB40 without rewiring?

The electrical replacement is direct — coil voltage, contact configuration, and terminal type are the same between the two generations. Physical DIN rail mounting is also compatible. However, auxiliary contact blocks from the older 3RH1911 series will not physically mount on the 3RH2140 frame, and existing varistors, diodes, and RC elements from the 3RH1 generation are also incompatible. If your installation uses 3RH1911 auxiliary blocks, you will need to source new 3RH2140-compatible blocks separately.

Why does the contact current rating drop from 10 A at 230 V to 1 A at 690 V?

The current rating is not a fixed value — it is a function of the arc energy that must be interrupted at contact opening. Higher voltages sustain longer arcs, requiring the contact to interrupt more energy per switching cycle. At 690 V AC-15, the maximum safe switching current is 1 A to preserve contact integrity over the rated service life. Always cross-reference the specific operating voltage against the datasheet current table — specifying based on the 230 V rating when the load runs at 690 V is one of the most common selection errors with this relay type.

Does the 3RH2140-1BB40 require an external circuit breaker, or is internal protection sufficient?

External protection is required. The relay has no internal overcurrent protection. For auxiliary circuits up to 230 V, a C-characteristic MCB rated at 6 A is the standard recommendation. Inductive loads also require a protective varistor or RC snubber network across the load coil to suppress back-EMF arcing on contact opening. Operating this relay without coordinated external protection accelerates contact wear and reduces service life.

What happens if the 24 V DC coil supply drops below the rated operating voltage?

If coil voltage falls below the minimum pick-up threshold, the relay will fail to energize or will chatter — rapidly opening and closing in an unstable state. Chattering causes accelerated contact wear and can produce erratic behavior in the controlled circuit. Check the power supply regulation, verify that terminal screws are tight (loose connections create resistance that drops the coil voltage), and confirm the MCB protecting the control circuit is correctly rated and not causing a voltage sag on close.

Is the 3RH2140-1BB40 suitable for safety interlock applications requiring SIL rating?

The relay is designed to IEC 61508 functional safety standards and supports safety integrity level compliance with appropriate auxiliary blocks and monitoring architecture. The B10 value at high demand rate is 1,000,000 cycles per SN 31920, with a proportion of dangerous failures at high demand rated at 73%. For SIL-rated applications, the full safety calculation must be performed using the relay's published PFD and B10 values in combination with the system architecture — this relay alone does not constitute a SIL-rated safety function.

What is the lead time for the 3RH2140-1BB40, and can it be sourced outside of Canada?

As a standard catalog item in the active SIRIUS product family, lead time from stocked distributors is typically 1 to 3 weeks for standard orders. Large quantity orders may require 4 to 8 weeks depending on regional inventory. LeadTime.ca ships worldwide — customers in any region can request a quote or check current availability directly through the product page or by contacting the team.

Why Order the 3RH2140-1BB40 From LeadTime.ca

  • Ships worldwide — no geographic restriction on order origin
  • Access to current stock levels and confirmed lead times before you commit to a build schedule
  • Volume pricing available for multi-unit orders — contact the team directly for RFQ on quantities of 5 or more
  • Sourcing support for hard-to-find variants and compatible auxiliary contact blocks in the SIRIUS family
  • Direct response for time-critical replacement orders during unplanned downtime situations

At-a-Glance Summary

  • Coil voltage: 24 V DC only — no AC operation, no other DC voltages
  • Contact configuration: 4 Normally Open (4NO) on the base unit
  • Maximum switching voltage: 690 V (insulation voltage, Pollution Degree 3)
  • AC contact rating at 230 V (AC-15): 10 A; at 400 V: 3 A; at 690 V: 1 A
  • DC contact rating at 24 V (DC-12): 10 A; at 110 V: 3 A; at 220 V: 1 A; at 440 V: 0.3 A
  • Surge voltage resistance: 6 kV
  • Mechanical service life (base unit, unloaded): 30,000,000 cycles
  • Frame size S00: 45 mm wide × 57.5 mm tall × 73 mm deep; mounts on 35 mm DIN rail
  • Operating temperature: -25 to +60 °C; storage: -55 to +80 °C; max altitude: 2,000 m
  • Direct electrical and physical replacement for the obsolete 3RH1140-1BB40
  • 3RH1911 auxiliary blocks are NOT compatible — source 3RH2140-series auxiliary blocks only
  • External 6 A C-characteristic MCB required for auxiliary circuits up to 230 V
  • Finger-safe IP20 front terminals; UL listed and CE marked