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: https://support.industry.siemens.com/cs/de/en/view/76676971

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.