Product Overview

The SCHLEICHER UST21 is a high-integrity,programmable safety timer relay engineered for use in safety-related control circuits where precise time-delayed responses are essential to protect personnel and machinery.As part of Schleicher’s trusted UST(Universal Safety Timer)family,the SCHLEICHER UST21 delivers certified performance up to SIL 3(IEC 61508)and Performance Level e(PL e per ISO 13849-1),making it suitable for integration into emergency stop systems,access guarding interlocks,light curtain muting sequences,and controlled stopping functions on industrial equipment such as presses,packaging machines,and robotic cells.

Designed for both standalone hardwired safety circuits and hybrid architectures interfacing with safety PLCs,the SCHLEICHER UST21 features dual redundant signal paths with force-guided(positively driven)relay contacts that comply with EN 50205,ensuring mechanical linkage between NO and NC contacts for reliable fault detection.Its timing function—adjustable via front-panel rotary switch or internal DIP settings—supports multiple modes including ON-delay,OFF-delay,and interval timing,with a broad range from 0.1 seconds to 30 minutes.Housed in a compact 22.5 mm wide DIN-rail enclosure with clear LED status indicators for power,output,and fault conditions,the SCHLEICHER UST21 combines functional flexibility with space efficiency,enabling seamless installation in crowded control cabinets without compromising safety integrity.

Technical Specifications

Parameter Name Parameter Value

Product Model UST21

Manufacturer Schleicher GmbH&Co.KG

Product Type Programmable Safety Timer Relay

Safety Rating SIL 3(IEC 61508),PL e/Category 4(ISO 13849-1)

Supply Voltage 24 VDC(±10%)

Timing Range 0.1 s to 30 min(selectable via rotary switch or DIP)

Output Contacts 2 x force-guided changeover relays(3A 250 VAC)

Input Channels Dual-channel monitored safety inputs(for 24 VDC sensors or switches)

Response Time<20 ms(typical)

Mounting DIN rail(EN 60715,35 mm)

Degree of Protection IP20(front),for use inside enclosures

Operating Temperature-20°C to+60°C

Certifications CE,TÜV,cULus(pending/variant-dependent)

Main Features and Advantages

Certified safety with flexible timing logic:The UST21 isn’t just a delay relay—it’s a safety-certified control element that actively contributes to risk reduction in machinery.Its dual-channel monitoring continuously checks for input signal consistency,contact welding,and internal faults,automatically de-energizing outputs if a dangerous failure is detected.This self-diagnostic capability,combined with force-guided contacts,ensures compliance with stringent machinery safety standards.Users can select from multiple timing functions(e.g.,“delayed start after guard closure”or“controlled coast-down before restart”)without needing external logic,simplifying circuit design and reducing component count.

Compact yet rugged for real-world integration:Despite its small footprint(only 22.5 mm wide),the UST21 is built for industrial durability,featuring robust terminal blocks,wide temperature tolerance,and immunity to electrical noise common near VFDs or servo drives.The clearly labeled front panel includes three LEDs—green for power,yellow for timing active,red for fault—enabling instant visual verification during commissioning or maintenance.The timing scale is intuitively marked,and the sealed rotary switch prevents accidental adjustment in operational environments,enhancing long-term reliability.

Seamless compatibility across automation architectures:Whether used in a traditional hardwired safety chain with emergency stops and door switches,or as a dedicated timing block within a larger safety PLC system(e.g.,from Pilz,Siemens,or Rockwell),the UST21 integrates effortlessly.It requires no software tools for basic setup—just wire,set the time,and operate—making it ideal for OEMs,system integrators,and maintenance teams managing legacy or mixed-vendor equipment.For retrofit projects replacing obsolete timers(e.g.,from Moeller or Telemecanique),the UST21 offers a modern,certified drop-in alternative that future-proofs safety compliance without redesigning the entire control panel.

Application Field

The UST21 excels in applications where a verified time delay is integral to safe machine operation.In metal forming and stamping,it ensures that press brakes remain de-energized for a defined period after an emergency stop,allowing full mechanical release before reset—preventing inadvertent cycling.Within robotic workcells,the UST21 manages safe entry protocols:when a safety gate opens,it triggers a controlled stop and enforces a minimum wait time before enabling restart,ensuring robots are fully stationary before human access.In packaging and bottling lines,it coordinates light curtain muting sequences during product passage,using precise timing to maintain throughput while preserving safety integrity.

It is also widely used in material handling systems,such as palletizers or conveyor gates,where delayed restart prevents jams or collisions after a safety interruption.In test benches and laboratory automation,the UST21 provides repeatable,fail-safe timing for cyclic safety checks or cooldown periods.Educational institutions and training centers utilize the UST21 to demonstrate real-world implementation of ISO 13849 principles due to its transparent operation and certification clarity.Across all these scenarios,the UST21 bridges the gap between simplicity and safety rigor—delivering dependable,standards-compliant timing without complexity.

Related Products

UST22:Enhanced version with extended timing range or additional output contacts

RSM24:Schleicher safety relay module for basic emergency stop circuits(non-timing)

UST21-230:Variant rated for 230 VAC supply voltage instead of 24 VDC

MGB2:Modular safety controller that can replace multiple UST21 units in complex systems

SLP2:Safety logic processor for configurable safety networks,compatible with UST21 as a sub-component

RSE24:Dual-channel safety relay with manual reset,often used alongside UST21 in guard locking

UST11:Simpler,non-programmable safety timer with fixed delay(e.g.,1s or 5s)

AS-i Safety Modules:Alternative distributed safety I/O that may integrate timing functions digitally

PNOZsigma PNOZ m B1:Competitor safety timer from Pilz with similar functionality

Schleicher SLP Configurator Software:Tool for advanced safety system design involving UST21-based architectures

Installation and Maintenance

Pre-installation preparation:Before installing the UST21,confirm that the control circuit voltage matches the 24 VDC rating and that all connected safety devices(e.g.,door switches,E-stops)provide clean,bounce-free contacts suitable for dual-channel monitoring.Wire inputs and outputs according to the safety circuit diagram,ensuring separation from high-power cables to avoid induced voltages.Verify that the DIN rail is securely mounted and grounded,and that ambient temperature remains within-20°C to+60°C.Set the desired timing function and duration using the front rotary switch before energizing—note that some variants may require internal DIP configuration for specific modes.

Maintenance recommendations:The UST21 is maintenance-free under normal conditions but should be visually inspected periodically for signs of overheating,terminal corrosion,or LED anomalies.Test the safety function regularly per ISO 13849 requirements—typically by simulating an input fault or triggering the timing sequence and verifying correct output behavior.If the red fault LED illuminates persistently,check field wiring for short circuits,open channels,or welded contacts in upstream devices.Avoid exposing the UST21 to condensation or conductive dust,which could compromise internal isolation.When replacing an older timer,ensure the new UST21 is configured identically to maintain functional safety equivalence.Always document timing settings and test results for audit compliance.

Product Guarantee

Every UST21 we supply is sourced through authorized distribution channels or factory-refurbished to original Schleicher specifications,with full functional validation of timing accuracy,contact integrity,and safety diagnostics.We guarantee compliance with stated SIL 3/PL e certifications and provide complete technical documentation upon request.Each UST21 unit comes with a 24-month warranty against defects in materials and workmanship,backed by in-house engineers specializing in functional safety systems.Should your UST21 exhibit any deviation from expected performance,we offer immediate technical support and rapid replacement—because in machinery safety,there is no room for compromise,only confidence in every component.

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