The contacts of relays can fuse together due to the heat of the electric arcs that occur during switching operations - with the dangerous consequence that a machine would continue to run even if the emergency stop switch were actuated.

To prevent this, safety controls with at least two relays with forcibly guided contacts are used. The output contacts of the relays are connected in series. If one of the relays fails, for example with a welded NO contact, the NO contacts of the other relays can switch off the machine in any case and thus bring it to a safe standstill. Due to the forced guidance feature, the NC contact in the defective relay remains open in the event of a fault - with a minimum opening of 0.5 mm according to the standard. This open NC contact detects the fault that has occurred and prevents the machine from being switched on again. This prevents the system from continuing to run uncontrollably in the event of a fault in the relay.

Solid state versus safety relays

Solid state relays are not really relays at all. They do not consist of moving parts like a relay, but of semiconductor components, transistors, MOSFETs, thyristors, diacs and triacs, with which a relay function is realized. The switching element in a solid state relay is a switching transistor, triac or MOSFET, which is controlled by a photo resistor or transistor (optocoupler). Solid state relays are particularly suitable for applications in which high loads have to be switched on and off frequently.

Safety relay from Hengstler.

© Stallion

Relays with forcibly guided contacts (safety relays) generate a significantly lower power loss (self-heating) than solid state relays and therefore - unlike the latter - do not need to be equipped with heat sinks. It is irrelevant whether the relays carry direct or alternating current. Relays with forcibly guided contacts are also very resistant to voltage and overcurrent peaks, which can hardly cause any damage to the contacts or the coil. Solid-state relays, on the other hand, are relatively sensitive to surge currents, as the integrated C/R damping circuit is not sufficient to suppress the peaks. As a result, the short-term maximum peak voltage of the solid-state relay is exceeded and overvoltage damage occurs.

In contrast to electromechanical relays, solid-state relays do not have moving parts and are therefore not subject to wear. Electronic relays are also more compact than electromechanical relays and have a longer average electrical service life. In addition, solid-state relays offer significantly shorter switching times. However, this property only really plays a role with photoelectric switches that are actuated every second. Overall, solid-state relays score highly in direct comparison with safety relays, primarily due to their very long service life.

Both and

Leslie Wenzler is Marketing and Communications Manager at Hengstler in Aldingen.

© Stallion

Due to their different switching technologies, safety and solid-state relays complement each other perfectly. The sum of their properties results in very effective safety control - especially for applications in which high loads are switched at short intervals. Solid-state relays withstand these high loads much better than safety relays. These in turn protect the semiconductor in the event that it does become alloyed and unusable. The safety relays run 'idle' most of the time and reliably shut down the system in the event of a fault.

The main areas of application for solid state relays are motion controls and heating controls, for example in the food industry, the plastics industry (extruders) or in air conditioning systems and soldering systems. The wear-free relays are also often installed in lighting and pump control systems.

Railroad and elevator technology

In addition to applications with both relay types, there are numerous applications in which only safety relays are installed. As a rule, these are applications with very high safety requirements, such as in railroad and elevator technology. Many electrical circuits there have to meet the requirements of Safety Integrity Levels (SIL) 3 and 4. The Safety Integrity Level refers to the reliability of safety functions - the requirements for the individual SILs are defined in the EN 61508 standard. The greater the danger posed by a system, the higher the requirements for its functional safety and the higher the Safety Integrity Level.

Functionally safe circuits

Based on relays with forcibly guided contacts, the circuits for SIL3 and SIL4 applications can be implemented with significantly less effort than with solid-state relays - provided the circuit is designed correctly. The cost savings result, among other things, from the lower unit costs of the relays and the reduced installation effort, as no heat sinks are required. At the same time, the safety relays offer a high level of protection against unwanted movements of system components in the event of a fault.

In the field of railroad technology, for example, safety relays from Hengstler can be found in many applications - they are used in door controls and braking systems of rail vehicles as well as for monitoring level crossings or in the so-called 'dead man's switch', which the train or locomotive driver must actuate regularly. If he fails to do so, the train is automatically brought to a safe stop.

Hengstler safety relays are also frequently used with main signals. These signals indicate to the train driver whether the section of track in front of him may be used or whether it is still blocked by a train ahead. The main signal is controlled and monitored by the responsible signal box. This is done by means of several safety relays. If a train passes a red signal, a solenoid coil on the signal automatically transmits this information to the corresponding solenoid coil on the locomotive. The solenoid coil on the locomotive then immediately activates the train's brakes and brings it to a standstill.

Safe exit guaranteed

In elevators, safety relays are mainly used to control the doors. They ensure that people can only get out when the car has reached its safe end position. In elevators with comfort control, it is also possible to get out before the elevator comes to a standstill: The relays with positively driven contacts ensure that the doors can only be opened in a certain zone and only at a certain speed.