Patent Publication Number: US-6337786-B1

Title: Electric switching device with closed-circuit protection

Description:
The invention relates to an electric switching device. The electric switching device has a signaling element, e.g. a switching strip, which leads to a switching function in the event of a closed circuit being interrupted. The switching device also includes a relay safety assembly connected into the closed circuit. 
     Electric switching devices of this type are described e.g. in EP 234 523 B1, to be precise in connection with a closing-edge fuse. Closing-edge fuse systems of this type are provided for use on motor-driven doors and gates in buildings and public transport means, movable shelves and on protection devices for machines. A closed-circuit current flows in the actual switching strip and is monitored by an evaluating unit. Contact is made as a result of the closed-circuit current being interrupted. Load reduction is followed by automatic resetting for the purpose of closing the closed circuit. 
     The so-called evaluating unit—or also called relay safety assembly—is likewise known in many different forms in the prior art and monitors itself continually by the closed-circuit principle. Faults such as short circuit-cable breakage-profile interruptions and incorrect connection lead to the disconnection of the load circuits. In the prior art, evaluating units of this type are referred to by the technical designation “relay safety assembly”. 
     These known arrangements have the disadvantage that a single-ended earth fault is not identified since the device continues to remain functional. 
     The invention is based on the object of providing a line monitoring arrangement which both demonstrates a single earth fault and reacts appropriately in the event of a short circuit. 
     This object on which the invention is based is achieved by means of the fact that the incoming electrical line of the closed circuit to the signaling element is surrounded by a conductive safety screen which is connected to the protective conductor terminal of the relay safety assembly, the secondary circuit of the transformer in the relay safety assembly being connected to the safety screen on one side. 
     If such an arrangement is used in conjunction with a closing-edge fuse, a control signal which effects disconnection of the load circuits occurs even in the event of an earth fault of the incoming line of the closed circuit. 
     In order to illustrate the invention, a block diagram of a relay safety assembly known in the prior art, type: “BN 5983”, is enclosed in the Appendix, the screening according to the invention being additionally illustrated in this block diagram. 
    
    
     In the drawing, the switching strip is represented by  1  and the fixing strip for the switching strip  1  is represented by  6 , the fixing strip  6  usually being composed of metal. In the switching strip  1 , the actual conductor  3  lies above the fixing strip  6 , as is known in the prior art. 
    
    
     According to the invention, this conductor  3  is surrounded by a safety screening  5 , which, as shown in the drawing, is connected to the protective-conductor terminal PE(−) of the relay safety assembly  2 , with the result that disconnection of the load circuits is thereby effected even in the event of an earth fault of the incoming line of the closed circuit.