May 2021

n SWITCHING INTO SAFETY ENGINEERING Switching into safety engineering - Distinguishing normal switches and safety switches T he purpose of all switches used in safety systems is to interrupt the safety circuit when necessary. Selection of the correct safety device for any given application is a major engineering decision, one which quite literally can result in life or death. Guarding safety switches are used to monitor gates and doors, as well as any other movable physical guards. They operate by sending a signal to a machine’s control system if a guard has been opened, removed or is out of alignment. In addition, process control elements can also have a safety implication such as pressure or flow. The most well-known and probably the simplest example of a safety-switch is the emergency stop button, which ensures that relevant parts of the installation are safely shut down. Safety switches come in many differing forms to suit the demands of the designer and it is important the right technology is used for any particular application. This includes non-contact types, based on coding and transponder, or mechanical switches with or without door- locking. Although the popularity of the electronic-based ‘coded’ type switches is growing, mechanical varieties using physical contacts instead of failsafe- semiconductors are still extremely popular in machinery design. In many cases, the safety variants look very similar to the standard devices. At first glance, the standard units look suitable, having the much needed, normally closed contacts required by the evaluation devices. But the technology inside the two differs markedly. The main differences are shown in Table 1 below. Electromechanical switching elements In a simple electromechanical switch, the key element comprises a plunger with at least one contact bridge in a housing. The contact bridge forms the connection between the contacts. It is used as either a normally open contact (Fig 1), where contacts are closed on the actuation of the switch); or a normally closed contact, where contacts are opened on the actuation of the switch. The switching elements have a spring for resetting after actuation. The spring force acts against the actuating force applied externally to the plunger and determines how much pressure is needed to operate the switch. If the plunger is pressed down by an external actuating force (such as when a pallet arrives at a station), the normally open contact closes. A signal can then be transferred to the machine control system to tell it to do something. As soon as the external force is no longer acting on the plunger, the contact bridge is moved away again by the spring force and the contact is opened again. For safety applications it is vital that the switching element must function effectively, even when there is a fault. Typical faults include welding of the contacts due to overcurrent or fracture of the spring over time. If one of these faults occurs, the switch's contact bridge position cannot be determined and therefore the switch could continue to give a healthy signal back to the control system. This switching principle therefore does not guarantee safe interruption of the actual circuit in all situations. For this reason, switching elements with at least one positively driven contact are used for safety switches (Fig 2). In principle, position switches with positively driven, normally closed contacts - the result of a rigid connection between the plunger and contact bridge - have the same design as switches with standard design, normally open contacts. However, opening of the switching contacts is positively driven by the plunger. Springs and clips are no longer relied upon, so even if the spring breaks or the contacts weld together, the physical force of depressing the plunger guarantees that the safety contacts will open. The principle of operation is shown in Fig 2: on the actuation of the plunger, the normally closed contact is positively driven over the entire actuating travel. If the external force is no longer acting on the plunger, the contacts are closed again by the spring. Even if the contacts weld The first article in this series covered the philosophy behind safe machinery and the role that the various types of safety switches and devices can provide. This second article in a series of six covers the key design principles and differences between normal switches and safety switches and provides an overview of the technologies which can be adopted, both conventional andmodern. Sponsored by Part Two of a Six-Part Series Feature Non-Safety Standard Versions Safety Versions Redundancy of switch signals No Yes Positive actuation (mechanical) No Yes Special contact material to prevent welding No Yes ‘Coded’ actuator to operate No Yes Table 1: Main differences between standard switches and safety switches with physical contacts