Standard s

Electromechanical systems that require redundancy and checking ofrelay contacts should use relays that are designed with mechanical linkages to provide a positive relation between normally open and normally closed contacts to check the contact operation. Solid-state devices do not have a mutually exclusive normally open - normally closed contact arrangement. Other methods must be used to monitor the performance of these devices.” Category 4: All conditions of B apply, but now single fault detection isimperative and calls for not only redundancy in the input and output devices, but also for self-checking and cross monitoring. Again dual channel controls are called for. Category 1: Category 3: All conditions of B apply, but the safety related systemmust use “well tried” principles and components, see 7.2.2 EN (TC114/JWG 6). All conditions of B apply, but the complete safety controlsystem shall be designed so that any single fault shall not lead to the loss of the safety function and where practical, the single fault shall be detected. This now calls for not only redundancy in the interface relay but also in the input devices, pointing to dual channel systems.

Risk Categories: European Standard EN-954-1

Category B: Category 2: Safety devices and control systems at a minimum must bedesigned, selected and assembled to meet the operational requirements of design limits and influence of the processed materials and other external influences. Most domestic appliances fall into this category, and providing the components are correctly specified (load, switching frequency, etc.), then no other special features are required. All conditions of B apply, but in addition the machine shallbe prevented from starting if a fault is detected on power up. This suggests the use of an interface relay with redundancy and self checking on energization. Single channel operation is permitted providing that the input devices (E Stop buttons, gate switches) are tested for operation on a regular basis. Requirement of the safety related control circuit to meet the various categories are listed in section 7 of EN 954-1, but in general their requirements are as follows:
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