Coupling relays and signal converters

When standard controllers with their standard inputs/outputs reach their limits, coupling relays and signal converters come into their own. A wide selection of coupling relays and signal converters is available for the electrical isolation of I/Os and PLC inputs and outputs, signal amplification or reproduction, as well as conversion to different voltage levels or signal forms.

Coupling relays and signal converters portfolio

3RQ1 Coupling Relays

Force-guided coupling relays up to SIL 3 / PL e

SIRIUS 3RQ1 coupling relays force-guided with functional safety up to SIL 3 / PL e

With the 3RQ1 coupling relays fail-safe controls can be reliably coupled. They are available in widths of 17.5 or 22.5 mm. Because the relays are force-guided in accordance with IEC 60947-5-1 (IEC 61810-3), the mechanical connection between the contacts ensures the NO and NC contacts are never closed simultaneously. This means failures to open can be reliably recognized in order to ensure maximum safety. This makes them suitable for use in rail, signal systems, and elevators, for example.


These coupling relays are also an integral component of the 3SK safety relay system, and serve as an output expansion up to SIL 3.


  • Wide voltage ranges from 24 to 240 V AC/DC with a mounting depth of 90 mm available for all variants
  • Permanent wiring due to removable terminals in screw and spring-type technology (push-in)
  • Force-guided contacts used to read back relay status values and provide reliable diagnostics or signaling
  • All variants with genuine load contacts, including in NC circuit
  • Usable as an output expansion for SIRIUS 3SK safety relays via a device connector
  • Safety certification based on functional safety up to SIL 3 / PL e (IEC 61508 / ISO 13849)

3RQ2 coupling relays

Universal with wide voltage range

SIRIUS 3RQ2 coupling relays with wide voltage range for universal use

The SIRIUS 3RQ2 coupling relays in high-quality titanium gray industrial enclosure ensure a convenient connection system with removable terminals. Outstanding features also include a wide range supply voltage of 24 to 240 V AC/DC for global use.


  • One product for all voltages (24 to 240 V AC/DC) with 1 to 3 changeover contacts
  • Version with hard gold-plated contacts for maximum contact reliability, even at low currents
  • International standards and certifications include CE, UL/CSA, EAC, CCC and confirmations for railway applications

3RQ3 coupling relays

Compact with relay and semiconductor output

SIRIUS 3RQ3 coupling relays for space-optimized use in the control cabinet

The SIRIUS 3RQ3 coupling relays with a width of just 6.2 mm and low overall depth/height are ideal for narrow tier spacing or in flat switchboxes. They are available with either relay or semiconductor output.


  • Permanently soldered relays for enhanced contact reliability
  • Plug-in relays to ensure quick replacement of the relays with permanent wiring
  • Semiconductor output for an extremely long electrical service life with an unlimited number of switching cycles
  • Universal bridging option using connecting combs for all terminals

LZS coupling relays

Powerful with plug-in relays

SIRIUS LZS coupling relays with plug-in relays

SIRIUS LZS coupling relays are available both as complete units, as individual modules for customer assembly, or for spare parts requirements. These are characterized by increased power for the switching of greater loads.


The plug-in relays allow quick replacement of the relays.


  • Tested AC-15 and DC-13 switching capacity
  • Available coil voltages: 24 V DC, 24 V AC, 115 V AC, 230 V AC
  • Hard gold-plated contacts for optimum interaction with electronic controls

3RS70 signal converters

For analog standard and universal signals

SIRIUS 3RS70 signal converters in narrow compact design

3RS70 signal converters are mainly used to electrically isolate and convert analog signals. The frequency outputs implemented offer a further application. Only minimal space required on the standard mounting rail due to the narrow design of just 6.2 mm and reduced depth and height of the enclosure, ideal for flat switchboxes and control cabinets with small gaps between tiers.


Reduced device variance resulting from exclusive use of 3-way separation. For the 3-way separation, each circuit is galvanically separated from the other circuits, i.e. input, output, and supply voltage do not have equipotential bonding.

3TG10 power relays

For performance range below S00

Compact SIRIUS 3TG10 power relays for switching of high loads

3TG10 power relays are the ideal option wherever small, low-noise relays or contactors are required. They are suitable for simple controllers and for applications that do not require an overload relay and need only one auxiliary switch – but which therefore need more switching power, higher switching voltage, and a longer service life.


  • Can be mounted in any position, hum-free
  • Narrow width of just 36 mm
  • Integrated auxiliary switch
  • Protective separation
  • Integrated overvoltage damping

Things to know about coupling relays

A coupling relay is mainly used for electrical separation and voltage transformation, for example, from 24 V DC to 230 V AC. It is also used to amplify weak signals and can be used for general relay controls and for overvoltage and EMC protection of controls.

The utilization categories for coupling relays are defined in IEC 60974-5-1 and stipulate the characteristic operating conditions. The most common are AC-15 and DC-13.

AC stands for the utilization categories for alternating voltage. The typical application for AC-15 is control of electromagnetic load (for alternating voltage).

DC stands for the utilization categories for direct voltage. The typical application for DC-13 is control of electromagnetic load (for direct voltage).

The maximum number of switching cycles depends on the load to be switched, on the technical design of the elementary relay and on the contact material used, e.g. AgNi + Au, AgNi, AgCdO, AgSnO2.

To switch very small currents, e.g. at control inputs, hard gold-plated contacts are used.

For silent, fast and frequent switching, semiconductor contacts can also be used.

According to the standard IEC 60947, it is defined as the number of switching cycles without load that is achieved or exceeded by 90% of all devices under test, without repair or replacement of any component.

This is tested at a switching frequency of 8 Hz and without a contact load and also provides an indication of the electrical endurance at very small contact loads.

Simple selection and worldwide standardization, e.g. a 3RQ1/2 coupling relay with a voltage of 24 V AC/DC to 240 V AC/DC can be used both for the US market (110 V AC) and for the CE market (240 V AC) or also with a 24 V power supply. 

Functional safety defines a state in which the risk of damage is reduced to a tolerable level, or that can be regarded as risk-free.  This definition consequently applies to the functional safety of persons, machines and the environment.

Functional safety is intended to ensure equal protection of people and the environment in all countries with internationally harmonized standards. Apart from that, many countries have laws that mandate compliance with safety aspects. This obligation can often be met by applying the standards. In particular, the following standards must be applied to factory automation: IEC 61508. IEC 62061 and EN ISO 13849.

Safe coupling relays are used for the same purpose as standard coupling relays (see "What is a coupling relay used for?") and usually have positively-driven contacts and safety certification.

In mechanically linked operation according to IEC 60947-5-1 (IEC 61810-3), the contacts of the installed relays are connected to each other so that NO contacts and NC contacts are never closed at the same time. In this way, opening failures are reliably detected and maximum safety is ensured.

They are used as a coupling link for signals, e.g.: for fail-safe control and output expansion (SIL 1-3) for safety relays with direct control of actuators and signaling elements.


They are also used in applications without requirements for functional safety. In this case, the mechanically linked operation of the contacts is used for reliable feedback, for anti-parallel switching or for diagnostics of the NC contact.

The SIRIUS 3SK1 safety relays and the force-guided SIRIUS 3RQ1 coupling relays offer such a possibility. This means both the number of contacts and the switching capacity of the fail-safe control can be increased and amplified up to 10 A.

You will find more information in this FAQ:

The IEC 62061 standard "Safety of machines – Functional safety of electrical, electronic and programmable electronic control systems" defines comprehensive requirements. It includes recommendations for the development, integration and validation of safety-related electrical, electronic and programmable electronic control systems (SRECS) for machines. For the first time, the standard covers the entire safety chain, from the sensor to the actuator. The "Safety Integrity Level", or "SIL" for short, is defined as the application parameter for this standard. The standard does not specify performance capability requirements for non-electric – e.g. hydraulic and pneumatic – safety-related control elements for machines.

The standard EN ISO 13849-1 "Safety of machines – Safety-related components of control systems, Part 1: General principles for design" superseded EN 954-1 at the end of 2011. It takes into consideration the complete safety chain with all the devices involved in its performance. EN ISO 13849-1 also makes a quantitative analysis of the safety functions. The standard describes how to determine the "Performance Level", "PL" for short, for safety-related parts of control systems on the basis of architectures specified for the intended service life. When several safety-related parts are combined to form a complete system, the standard explains how to determine the resulting PL. It can be applied to safety-related parts of control systems (SRP/CS) and all types of machines, regardless of the technology and energy used, e.g. electrical, hydraulic, pneumatic or mechanical.


It provides both a very high electrical endurance and an almost unlimited number of switching cycles. It also offers the greatest contact reliability, silent and short switching and a high DC switching capacity. The SIRIUS coupling relays with semiconductor output feature electrical separation between the input and output circuit.

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