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Moving parts in or on machines are a significant hazard. Movable guards are a basic measure used to keep operators and other personnel at a safe distance from these parts and to prevent people coming into contact with them.
According to the Machinery Directive, a guard is "a part of the machinery used specifically to provide protection by means of a physical barrier"; simply put, a person cannot reach into the hazardous area past the guard. This can be achieved by using a protective grid on machines or an enclosure, for example.
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In order to attach machine tools or maintain systems, for example, you must be able to access the workspace.
This is made possible by using movable guards, such as protective doors for machines or flaps. The position of these guards must be monitored using sensors or safety switches and be connected to the machine's control system. The relevant requirements are defined in EN ISO 12100.
A distinction is made between safety switches with interlocking and safety switches with guard locking: An interlock detects when a protective door is opened so that the machine can be brought to a safe state using the connected controller. A guard lock, on the other hand, only allows the protective door of a machine to be opened when the system is not running. It keeps the guard closed using spring force or an electromagnet until it receives a signal from the controller that a safe state has been reached.
Guard locks are used for situations where a person would be able to reach into the hazardous area before the dangerous machine function has stopped (e.g. in the case of long deceleration times). DIN EN ISO 14119 provides information on the design and selection of interlocking devices associated with guards. Additionally, general requirements for the design and construction of fixed and movable guards are defined in EN ISO 14120.
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Assumption: Normal operation of the system requires cyclic intervention in the hazardous area. The stop time is less than the access time, i.e. the operator cannot reach into the hazardous area until the machine has come to a safe standstill.
For this application, the protective grid or protective door of a machine can be monitored via two channels with a safety relay and a no-contact safety switch. A prerequisite for this setup is the use of a no-contact safety switch that complies with IEC 60947-5-3 and type 3 or 4 function according to ISO 14119.
The RS2 no-contact safety switch from Eaton's RS-Titan series fulfills this requirement: Its potential-free reed contacts are controlled by multiple magnets in the actuator to make the switch even more tamper proof.
They have a rugged construction and stand up particularly well in terms of safety: According to EN ISO 13849, the switches attain PLe, while according to EN IEC 62061 they are certified up to SILCL3.
There are, of course, many other ways to monitor movable guards.
For anyone wanting to study "machine and system safety" in more depth, the Eaton Safety Manual provides detailed information about this subject. Not only does it explain all relevant standards, it also offers you the opportunity to get to grips with the extensive subject of functional safety technology, one step at a time. A separate, comprehensive chapter lists numerous circuit examples showing how movable guards can be monitored depending on the application and the safety integrity level required.
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