Behind the scenes, the force-guided relay is the real star of modern safety applications. Before force-guided relays were developed in the late 1970s, safety-critical applications were implemented with standard components (e.g., an auxiliary contactor). The number of fatal work-related accidents in Germany has fallen over the years. The successful implementation of the force-guided relay has certainly played an important role in this.
Occupational safety Number of fatal work-related accidents in Germany between 1974 and 2018
Force-guided relays are also mistakenly called safety relays. A safety relay is a functional switching device. The force-guided relay is a variant of an electromechanical elementary relay and can be used as a basic module for a safety relay.
What is the difference between a standard relay and a force-guided relay?
In simple terms, a force-guided relay is a set of contacts that are connected together mechanically. In the most basic configuration, a contact set consists of a N/C contact and a N/O contact. The two are connected together and the contact clearance of the N/C contact must be 0.5 mm, even in a disturbance state, over the entire service life. It is therefore possible to determine the state of the N/O contact based on the state of the N/C contact. This makes force-guided relays the ideal basic module of a safety relay module.
As mentioned, a force-guided relay is an important component in a safety relay module. In principle, a distinction is made between safety relay modules, which act as the central logic element in the safety chain, and coupling relays, which are part of a logic element.
Safety relays for high-demand applications
Safety relay modules with force-guided elementary relays are typically used in high-demand applications in machine building. They are subject to the Machinery Directive. High demand means that the safety function is demanded more than once a year. From safety level PL c onwards, extensive diagnostics are advantageous, which is why the properties of force-guided elementary relays are so outstanding. With the aid of forced guidance, the required diagnostics, i.e., the diagnostic coverage (DC), can be achieved with little effort. This is why the use of force-guided elementary relays has become established in machine building.
Safe coupling relays for low-demand applications
The second category of safety relay modules are safe coupling relays, which are primarily used in the process industry. In contrast to machine building, the central logic of the safety chain is, in most cases, a safety instrumented system (SIS). Parallel to the distributed control system (DCS), an SIS is part of the production system that controls the process. The SIS monitors the process and only intervenes if there is a valid safety reason.
In the safety chain, the safe coupling relay is part of the logic unit. In some circumstances, the safe coupling relay is not required. That is, provided the digital output is able to drive the load and electrical isolation is not required. In contrast to machine building, the demand rate for the safety function is less than once a year (low demand) for most applications in the process industry. This means that different demands are placed on the safe coupling relay, e.g., it is not necessary to implement continuous diagnostics. However, the current trend is to further increase the safety and availability of the application by incorporating a sophisticated diagnostic function.
In the environment of the process industry, with regard to safe coupling relays, two different variants of relays currently prevail: A distinction is made between safe coupling relays with standard elementary relays and a 1oo3 structure and safe coupling relays with positively driven elementary relays with a 1oo2 structure. With regard to emergency shutdown applications (ESD), a 1oo2 or 1oo3 structure means that two or three elementary relays are installed in series. All system operators must perform a mandatory proof test at regular intervals. This tests the proper function of the installed components within the safety chain. The force-guided relays have a great advantage here. With the aid of forced guidance, the state of the load contact is determined by monitoring the state of the N/C contact. This can also be done during operation by means of integrated diagnostics. In short, to perform the proof test of the safe coupling relay, the digital output is switched just once. The proof test is not as easy to implement with a standard elementary relay. In most cases, the safe coupling relay is removed and each elementary relay that is installed is tested individually for continuity or is replaced with a new one.
In contrast to the 1oo3 structure, higher system availability can be ensured with a 1oo2 structure. There is a lower probability of failure with a 1oo2 structure, because there are fewer components installed that could potentially fail. The figure showing the balance of availability and safety in safety structures provides an overview of this.