May 2021

SWITCHING INTO SAFETY ENGINEERING n together, the related contact is opened safely because the external forces are transferred via the rigid mechanical joint. If the spring is broken, the contact is not closed again. The contact bridge remains in the safe, open position. Tongue-type actuators Mechanical switches using a tongue-type actuator are universally common. They use the same principle as discussed above, but with one major difference – the switch has to convert the negative action of ‘pulling-out’ the actuating tongue from the switch into a positive action of physically breaking the safety contacts. It does this via a well-engineered assembly within the head of the switch. When the tongue type actuator is removed from the switch, such as upon opening a guard door, it forces the normally closed contacts to open. Slow-action contact elements A primary characteristic of mechanical safety switches is their use of slow-action contact elements. Here, the speed and travel of the contact bridge movement is the same as the speed of the actuation, because they are directly linked. The actuating travel is therefore transferred evenly to the contact bridge. Due to the simple design of slow-action contact elements, various safety and signalling contact combinations can be accommodated in very small housings. This is one reason why they are often installed in safety switches. Design variations There are many variants on the basic concept of the safety switch, suitable for many different circumstances. These are covered in depth in the Euchner Safety Book (pages 23- 79) to which readers are referred. In reality, to comply with the strict requirements on machinery safety, modern safety circuits are mostly of dual- channel design. This design requires two positively driven contacts, either within one body or as two separate switches. Switching elements with multiple contacts do not have just one normally closed contact or normally open contact, but instead they have two or more contacts (Fig 3). Additional contacts may be included for further machine control functions. These requirements lead to an array of options featuring switching elements with different contact assemblies. Furthermore, modern electronic controls can feature dynamic signal transmission and signal processing with fault detection. This makes it possible to implement a purely electronically controlled shutdown that is just as safe as positively driven contacts. These new technologies will be covered in greater depth later in the series. n This is the second part of our ‘Switching into safety engineering’ series which will include comprehensive articles and follow-up ZoomQ&A sessions – to register for the series or to request a copy of the free machinery safety guidebook, please visit www.drivesncontrols.com Fig 1: A normally open position switch which remains closed (fail) if the spring is broken or the contacts weld. * This article (and a longer version of the first article) - is also available on our website: https://drivesncontrols.com/news/fullstory.php/aid/6671/Safety_Engineering.html Fig 2: A safety limit switch with positive opening operation - the contact is open if the spring breaks or the contacts weld. Fig 3: A switching element with four contacts. noitaut ac hetyb ol csitcanto cheT eds ngirp shetyb edenp ositcanto cheT desolc s y t sta ctancoeth kor bsignirp sehtfI edsolc ctancoeth anto cehtfI ,en s y t sta ,eddlewsitc g niprsybedsolc sitcantocheT rotaut ac hetyb edenp ositcanto che ecrof T eop lli wtcatnochet reaks b pring shetfI cerfo pobe lliwti anto cehtfI n , ybedne ,eddlewsitc our Behavi right ef Behaviour l t 