Siemens 3RH1921-1DA11 — 1NO-1NC Auxiliary Block Specs & Price

By Abdullah Zahid
12 min read
Siemens 3RH1921-1DA11 SIRIUS 2-pole lateral auxiliary switch block 1NO-1NC snap-on for motor contactor panel

Siemens 3RH1921-1DA11 SIRIUS Auxiliary Switch Block, 2-Pole Lateral, 1NO-1NC Contact Configuration — Specifications, Selection, and Procurement Guide

Controls engineers and panel builders searching for the Siemens 3RH1921-1DA11 are almost always working from a finalized drawing or BOM — they need to confirm mechanical fit, verify the 1NO-1NC contact configuration matches their interlock logic, and lock in availability before committing the build. This auxiliary switch block is a 2-pole, laterally mounted snap-on unit designed for SIRIUS 3RT1x motor contactors, rated at 6A at 230V AC under AC-15 duty, with a mechanical service life of 10 million cycles and full IEC 60947-5-1 compliance.

If you have already confirmed this is the correct part, check current pricing and availability for the 3RH1921-1DA11 at LeadTime.ca — ships worldwide.

Is the 3RH1921-1DA11 the Right Choice for Your Panel?

This block is the correct selection when all of the following conditions apply to your application:

  • Your main contactor is a SIRIUS 3RT1x frame model — specifically 3RT10, 3RT12, 3RT145, 3RT146, or 3RT147 (S0 through S12 sizing)
  • You need exactly one normally open and one normally closed contact pair — no more, no fewer
  • The auxiliary block will mount laterally on the side groove of the contactor, not on the front face
  • Your control signal voltage falls within the rated range: 24V, 110V, 220V, 230V, 400V, or 690V AC/DC
  • Your signal load current stays at or below 6A at 230V AC (or 3A at 400V, 1A at 690V)
  • The installation environment is an enclosed control cabinet where IP20 protection is adequate

If you require more than two contact pairs, the 3RH1922 (2NO + 2NC) or 3RH1924 (4NO) are the appropriate alternatives. If front-mounted vertical orientation is required, a different auxiliary block variant from the 3RH19xx family applies. Confirm the correct variant before ordering to avoid field returns.

On this page:

What the 3RH1921-1DA11 Actually Does in a Motor Control System

The 3RH1921-1DA11 is a snap-on auxiliary contact block that adds independent signaling and monitoring contacts to a SIRIUS motor or power contactor. Its sole function is to mirror the state of the main contactor coil — when the coil energizes and the main power contacts close, the 1NO contact closes and the 1NC contact opens simultaneously. This positively driven mirror-contact design means a controls engineer can route the auxiliary contacts into a PLC digital input, an interlock coil circuit, or a safety relay without touching the main power path at all.

The block provides two distinct contact outputs from a single device: the NO contact is typically wired to monitoring devices or interlock coil circuits to confirm contactor closure, while the NC contact is routed to safety circuits or alarm inputs that must de-energize when the motor is running. IEC 60947-5-1 compliance and a mechanical service life rated at 10 million cycles make this a reliable choice for high-cycle-count applications such as pump controls, conveyor starters, and HVAC fan duty. The insulation voltage of 500V and a surge withstand rating of 6 kV ensure the auxiliary circuit remains isolated from transients on the main power side.

One frequently overlooked requirement: the 3RH1921-1DA11 does not include integrated short-circuit protection. An external gG 10A fuse rated at 500V with a 1kA interrupt capacity must be installed in the auxiliary circuit. This is a datasheet-specified requirement, not optional — omitting it or substituting a different fuse characteristic is the most common commissioning error with this product.

Typical System Architecture for SIRIUS Auxiliary Contact Blocks

The 3RH1921-1DA11 sits at the interface between the main power switching stage and the control or monitoring layer — it translates a mechanical contactor state into an electrical signal usable by logic systems upstream.

  • PLC or DCS controller issues a digital output signal to energize the SIRIUS contactor coil circuit
  • SIRIUS 3RT1x contactor (e.g., 3RT10) closes main power contacts to start the motor load
  • 3RH1921-1DA11, snapped onto the lateral groove of the contactor, transitions its NO and NC contacts simultaneously with coil energization
  • NO contact feeds back a digital input to the PLC to confirm contactor closure before VFD ramp-up or sequencing logic proceeds
  • NC contact drives a safety relay input or annunciator circuit that must register an open state while the motor is running

Where Engineers Deploy the 3RH1921-1DA11

Motor control interlocking is the most common deployment scenario: the NO contact of one contactor's auxiliary block is wired in series with the coil circuit of a second contactor, physically preventing simultaneous energization of both. This is standard practice in reversing starter panels and multi-motor sequencing systems where simultaneous operation would cause mechanical damage or fault conditions.

In PLC-based automation systems, the 1NC contact provides real-time contactor status feedback to a digital input card. The PLC can compare commanded state against actual contactor state and generate a fault if they diverge — a critical diagnostic capability in food and beverage processing lines and chemical dosing systems where undetected contactor dropout has process safety implications.

Variable-frequency drive soft-start applications commonly use the NO contact to confirm main contactor closure before the drive begins its output ramp. Without this confirmation signal, a VFD applying voltage to an open contactor output circuit can generate an overcurrent fault or trip on output phase loss detection.

Annunciator and light-stack circuits use the NC contact to drive a run-indication lamp or audible alarm. Because the NC contact is open when the contactor coil is energized, a de-energized NC output into a lamp circuit signals motor running state without additional relay logic.

Application Typical Deployment
Motor interlock (forward/reverse) NO contact wired in series with opposing contactor coil to prevent simultaneous closure
PLC contactor status feedback NO contact to PLC digital input for real-time run confirmation and fault detection
VFD soft-start confirmation NO contact confirms main contactor closure before drive output ramp initiates
Annunciator and light stack NC contact drives run-indication lamp or audible alarm in operator panel
Safety relay feedback loop NC contact wired to safety relay input for E-stop verification in redundant motor control
HVAC fan and pump control NO contact provides run signal to BMS or SCADA for motor runtime monitoring

Purchase-Decision Specifications at a Glance

Parameter Value
Contact Configuration 1NO + 1NC (2-pole)
Mounting Type Lateral snap-on (side of contactor)
Max Operating Current — 24V AC/DC (AC-15) 6A
Max Operating Current — 230V AC (AC-15) 6A
Max Operating Current — 400V AC (AC-15) 3A
Max Operating Current — 690V AC (AC-15) 1A
Insulation Voltage 500V AC
Mechanical Service Life 10,000,000 cycles
Electrical Endurance (AC-15 @ 230V) 200,000 cycles
Protection Class / Operating Temperature IP20 / -25°C to +60°C

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

3RH1921-1DA11 vs. 3RH1922, 3RH1924, and 3RH1925 — Which Block Do You Need?

Model Contact Configuration Best For When to Choose Instead
3RH1921-1DA11 1NO + 1NC Standard interlock and PLC feedback — one monitoring point required Your baseline choice for single-status applications
3RH1922 2NO + 2NC Applications requiring two independent NO and NC pairs from a single block When circuit design needs dual interlock and dual feedback simultaneously
3RH1924 4NO Parallel signaling applications — multiple NO outputs, no NC needed When four downstream devices each need a closure confirmation signal
3RH1925 Timing (delayed closure) Sequence control where delayed contact transition is required after coil energization When timing logic must be embedded at the contactor rather than in the PLC
Front-mounted variants (3RH19xx) Various Vertical (front-face) mounting orientation When lateral groove is obstructed or front-mount is specified on drawing

If your circuit requires exactly one status feedback point and one interlock contact from a single SIRIUS 3RT1x contactor, the 3RH1921-1DA11 is the correct choice — check current availability at LeadTime.ca.

Expert Verdict: Honest Assessment for Procurement and Engineering

The 3RH1921-1DA11 earns its place on standard BOMs for motor control panels because it solves a narrow problem extremely well: adding a mirrored status contact pair to a SIRIUS contactor with no rewiring, no tools, and no modifications to the main power circuit. The positively driven contact mechanism eliminates the possibility of a stuck-closed contact giving a false run signal to a PLC — which matters in safety interlock applications. A 10-million-cycle mechanical life and IEC 60947-5-1 certification make this a credible choice not just for convenience, but for circuits where contact reliability has consequences. The screw-clamp terminals accepting 0.5–2.5mm² conductors cover virtually all standard control wiring gauges, and the -25°C to +60°C operating range handles most enclosed industrial cabinet environments without derating.

The real limitations are well-defined. The 3RH1921-1DA11 gives you exactly two contacts — if your circuit design evolves to need three or four, you are looking at a block swap rather than an add-on. The IP20 rating means this is strictly a cabinet-interior component; washdown, outdoor unhoused, or harsh-environment applications require a different approach. And the 200,000-cycle electrical endurance at AC-15 is not a weakness for typical motor start-stop duty, but it is a hard constraint for high-frequency switching applications like heating element control or lighting circuits cycling more than a few times per hour. In those cases, solid-state alternatives are the correct engineering decision, not a different variant of this block.

From a procurement standpoint, the 3RH1921-1DA11 is a standard Siemens SIRIUS catalog item with predictable availability. Stock levels at major distributors typically run 5–20 units, with same-day or next-business-day shipping on in-stock orders. Backorder lead times when stock is depleted run 2–4 weeks under normal conditions — longer during supply chain disruptions, which makes keeping a small buffer stock on hand a reasonable practice for high-uptime facilities. Volume orders above 10 units typically attract 10–20% pricing improvement. If you are specifying this part into a larger system package, OEM bulk pricing is worth requesting directly. View current stock and pricing for the 3RH1921-1DA11 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 3RH1921-1DA11 snaps onto the lateral groove on the side of the 3RT1x contactor frame — confirm the groove is clear and unobstructed before attempting to seat the block; it should seat with an audible click and no visible gaps at the corners
  • Screw-clamp terminals accept solid conductors from 0.5–2.5mm² and stranded conductors from 0.5–1.5mm² without ferrules, or 0.75–2.5mm² with ferrules; finely stranded wire requires a ferrule to prevent fraying under the screw
  • An external gG 10A fuse rated at 500V with 1kA interrupt capacity is mandatory in the auxiliary circuit — this is a datasheet requirement, not a recommendation; do not substitute C-characteristic or fast-blow fuse types
  • NO contact terminals are typically labeled 13–14 and NC contact terminals 11–12 per IEC 60947 convention; verify labeling on the block face before wiring to avoid contact polarity errors in interlock logic
  • After initial commissioning, allow the system to run through normal on-off cycles for 24 hours, then retighten all screw terminals — thermal cycling can cause initial settling in screw-clamp connections

Compatible SIRIUS Contactor Models

The 3RH1921-1DA11 is verified compatible with the following SIRIUS contactor series. Confirm your contactor nameplate shows a 3RT1x designation before ordering — the 3RT2x series uses a different auxiliary block design and is not mechanically interchangeable.

  • 3RT10 — S0 to S3 frame, small-frame motor contactors up to approximately 7 kW
  • 3RT12 — S0 to S2 frame, compact-frame contactors
  • 3RT145 — S0 to S4 frame, medium-frame contactors
  • 3RT146 — S0 to S4 frame, medium-frame contactors
  • 3RT147 — S0 to S5 frame, medium-large frame contactors up to approximately 30 kW

Wrong-Part Prevention Checklist Before You Order

Before placing an order for the 3RH1921-1DA11, work through this checklist item by item. These checks eliminate the most common causes of field returns and commissioning delays with SIRIUS auxiliary switch blocks.

  1. Confirm target contactor is SIRIUS 3RT1x frame (not 3RT2x, which uses different block design)
  2. Verify lateral snap-on attachment is mechanically compatible with existing contactor (measure mounting groove on contactor frame)
  3. Check control signal voltage and current: ensure auxiliary contact rating (e.g., 6A @ 230V) exceeds signal load by at least 20%
  4. Confirm IP20 protection is adequate for cabinet environment (no washdown or outdoor unhoused use)
  5. Verify operating temperature range (-25 to +60°C) matches installation environment
  6. Ensure external fuse protection (gG 10A, 500V) will be installed in auxiliary circuit per datasheet requirement
  7. Confirm screw-type terminals (0.5-2.5mm² wire) are compatible with control wiring gauge

If any item on this checklist fails to confirm, contact the LeadTime.ca team before ordering — we can help verify the correct variant for your application and confirm compatibility with your existing contactor platform.

Frequently Asked Questions

Can the 3RH1921-1DA11 be used with 3RT2x series contactors?

No. The 3RH1921-1DA11 is designed for SIRIUS 3RT1x frame contactors — specifically 3RT10, 3RT12, 3RT145, 3RT146, and 3RT147. The 3RT2x series uses a different auxiliary block mechanical interface and is not compatible with this block. Attempting to force-fit the block onto a 3RT2x contactor will result in improper seating and unreliable contact operation. Check the contactor nameplate before ordering.

Is an external fuse really mandatory, or is it just a recommendation?

It is mandatory per the 3RH1921-1DA11 datasheet. The block has no integrated short-circuit protection. The specified external protection is a gG 10A fuse rated at 500V with a 1kA interrupt capacity — or alternatively a miniature circuit breaker with C characteristic rated at 10A and 0.4kA. Omitting this protection leaves the auxiliary wiring and the block itself unprotected during a short-circuit fault on the auxiliary circuit. Substituting a different fuse type, such as a fast-blow or D-characteristic fuse, will cause nuisance trips under normal switching transients.

What is the difference between the 1NO-1NC configuration and the 2NO+2NC offered by 3RH1922?

The 3RH1921-1DA11 provides one normally open contact and one normally closed contact — two total switching elements from a single block. The 3RH1922 provides two NO and two NC contacts, giving four total switching elements for applications that need to simultaneously drive two interlock circuits and two monitoring circuits from the same contactor. If your circuit design requires only one feedback point and one interlock contact, the 3RH1921-1DA11 is the correct and more cost-effective choice. If your circuit has grown beyond two contact points, specify 3RH1922 rather than stacking two auxiliary blocks.

What does the 200,000-cycle electrical endurance mean in practice for a typical motor application?

The 200,000-cycle electrical endurance at AC-15 duty at 230V represents the expected contact life under rated load switching. For a motor that starts and stops 20 times per day, 200,000 cycles translates to approximately 27 years of service — well beyond typical equipment replacement cycles. For higher-frequency applications such as heating element or lighting control cycling dozens of times per hour, this endurance figure becomes a real constraint, and the 3RH1921-1DA11 is not the appropriate technology. The 10-million-cycle mechanical service life reflects the physical snap mechanism and applies to low-energy or dry-circuit signal switching.

Can the 3RH1921-1DA11 be retrofitted onto a contactor already installed in a live panel?

The snap-on design is tool-free and takes approximately 2–3 minutes for mechanical installation, which makes retrofit practical during a scheduled maintenance window. However, the contactor coil circuit must be de-energized and lockout/tagout procedures followed before touching the contactor or wiring the auxiliary terminals. The block itself does not require the main power circuit to be de-energized to physically attach, but auxiliary circuit wiring must never be made live. Confirm the lateral mounting groove on the installed contactor is accessible before scheduling the retrofit — some cabinet layouts restrict side access.

What wire size should I use for the auxiliary circuit on the 3RH1921-1DA11?

The screw-clamp terminals accept solid conductors from 0.5–2.5mm² and stranded conductors from 0.5–1.5mm² without ferrules. If using finely stranded wire, ferrules are required and the accepted range is 0.75–2.5mm². In North American AWG equivalents, this corresponds to approximately 20–16 AWG solid and 18–14 AWG stranded. For most PLC digital input circuits operating at 24V DC with signal currents well below 1A, 1.0mm² or 1.5mm² stranded wire with ferrules is a practical standard choice that fits comfortably within the terminal range.

Why Source the 3RH1921-1DA11 from LeadTime.ca

  • LeadTime.ca ships worldwide — no regional restriction on orders regardless of project location
  • Direct access to current stock levels and lead time confirmation before you commit to a build schedule
  • Volume pricing available for quantities above 10 units — contact the team directly for OEM and project-level pricing
  • Hard-to-find and short-lead-time Siemens SIRIUS parts sourced for controls engineers and procurement specialists who cannot wait on standard distributor backorder queues
  • Technical questions answered before purchase — confirm variant compatibility and fuse requirements with the team prior to order placement

At-a-Glance Summary

  • Model: Siemens 3RH1921-1DA11 — SIRIUS Auxiliary Switch Block, 2-Pole Lateral, 1NO-1NC Contact Configuration
  • Contact configuration: 1NO + 1NC; positively driven mirror-contact design per IEC 60947-5-1
  • Maximum current: 6A at 24V and 230V AC (AC-15); 3A at 400V; 1A at 690V
  • Insulation voltage: 500V AC; surge withstand: 6kV
  • Mechanical service life: 10,000,000 cycles; electrical endurance: 200,000 cycles at AC-15 / 230V
  • Dimensions: 80mm (H) x 10mm (W) x 71mm (D); weight approximately 0.05 kg
  • Operating temperature: -25°C to +60°C; protection class: IP20
  • Compatible contactors: 3RT10, 3RT12, 3RT145, 3RT146, 3RT147 (S0–S12 frames); not compatible with 3RT2x
  • External fuse required: gG 10A, 500V, 1kA — mandatory, not optional
  • Terminal wire range: 0.5–2.5mm² solid; 0.75–2.5mm² stranded with ferrule
  • Certifications: IEC 60947-5-1, CE, UL 508, CSA, CCC, EAC
  • Typical lead time from stock: same-day to next business day; backorder: 2–4 weeks