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23 September, 2021

Switching into safety engineering - Designing a Safety Related Control System with Locking Controls

01 September, 2021

Our fourth topic in the Euchner series drills down into the necessities of good machinery safety control system design. Here, Andy Pye introduces issues which will be explored in greater detail in our fourth seminar on 8 September.

The EN ISO 14119 standard describes the selection and the usage of interlocking devices/interlocks with or without guard locking on safety doors, safety covers and other movable safety guards. Like all safety-related standards of this type, the application of the standard gives the machine design engineer robust technical information that can, in many cases, ensure a level of conformity to the machinery regulations.

The task of safety switches
Safety switches have the task of preventing the operation of a machine in the case of a potential hazard. This task is defined in EN ISO 14119 (Safety of machinery. Interlocking devices associated with guards. Principles for design and selection). For this purpose the safety circuit must be opened by the safety switch. Safety switches are therefore key elements of an interlocking device. In this context an interlocking device is, for example, the interruption of machine operation if the safety door is open – the stop state of the machine is “interlocked” so to speak and unintentional starting is therefore prevented. In relation to movable guards this means that if safety doors or safety flaps are open, the machine or system cannot be operated if the machine or system can produce a hazard. For this reason the safety switch for a guard must be attached such that a malfunction is excluded. Safety switches must also not be tampered with or bypassed.

Guard Locking & personnel protection vs process protection
Guard locking is a feature that prevents the unintentional opening of a door as long as there is a hazard. The door is locked by preventing the removal of the actuator from the safety switch.

Guard locking is required if a hazard cannot be removed immediately by shutting down a machine (e.g. due to machine movements with overtravel). In this case fail-safe control of the guard locking solenoid is required. This requirement can, for instance, be achieved by a safe standstill monitor or a safe delay. The safety switch must also provide a facility for monitoring the position of the locking system – a task which is very hard to achieve with magnetic-type locking systems.

Often a guard is only to be locked to prevent interruption to the process due to unintentional opening of the guard. In this case the position of the guard locking solenoid does not need to be integrated in the safety circuit. Often magnetic-type locking mechanisms are used for this type of application.

Principles of guard locking
EN ISO 14119 contains four different principles of operation for guard locking devices. Two of these are so-called closed-circuit current principles, whereby the guard locking device is closed (locked) in the event of a power failure. A guard locking device for personnel protection must use one of these two principles.

Fig 1 Schematic depiction of an interlocking guard
  • Spring applied – Power-ON released: aka mechanical guard locking, this is a closed-circuit current principle in relation to the function of guard locking. It means that the guard locking device is moved to the “locked” position by a spring on the removal of the power. On switching on the power the guard locking device opens.
  • Power-ON applied – Spring released: this operates in the opposite manner (aka electrical guard locking). It is an open-circuit current principle.
  • Power-ON applied – Power-ON released: also called the bistable principle, the position does not change on the removal of power. Power must be applied to change it to the other state. As the removal of the power does not change the position of the guard locking device, this is a closed-circuit current principle.
  • Power-ON applied – Power-OFF released: corresponds to an open-circuit current principle, as the guard locking device opens on the removal of the power.

References




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