The present invention relates to a safety arrangement in a device for indicating a pressure or force applied to, for example, push-buttons, door edge sensors, pressure sensitive doormats or position detecting equipment in general, said device being of the type which comprises one or more channels in an elastic electrially insulating material, at least one of said channels containing an electrically conducting fluid through which, in normal operation, an electric current passes and which thereby forms a current path comprised in an electric primary circuit adapted to activate pressure and force responsive means, said current path being interrupted when the said fluid-containing channel is compressed under the action of an external force displacing the fluid. SE-B-398,684 discloses a safety strip designed according to this principle and used on, for example, electrically operated doors to prevent squeezing of a person entering through the doorway. The current path is comprised in the electric circuit for operating the door. When the door hits an obstacle, the electric circuit thus is interrupted and the door is made inoperable. A broken conductor wire in the safety strip and power failure give the same result. Thus, the device is self- regulating and may seem to provide "failsafe" reliability. There is, however, one type of fault to which the device does not respond correctly, i.e. by interrupting the door operating circuit, and which involves short-cir¬ cuiting of the safety strip, as may occur if the lead-in of the safety strip makes contact with an electrically conducting part which is separate from the safety strip and which may be, for example, a portion of the motor operated door which usually is made of metal. In such a case, an electric circuit is closed which operates

the door while the safety strip is put out of service, i.e. the strip cannot make the door inoperable when compressed by an obstacle.

It is the object of the present invention to eli- minate this source of fault in the function of a device of the type described above. To this end, the invention proposes a safety arrangement of the type as defined in the claims.

An embodiment of the invention in the form of a safety strip for motor operated doors will be described in detail below, reference being had to the accompanying drawing. The safety means comprises a rubber strip A which is filled with an electrically conducting fluid. The fluid is supplied with current from an a.c. source 3 via lead-ins 1 and 2. A current transformer core 4 em¬ braces the strip 1, and the core winding is connected to the door operating circuit 5. The transformer thus senses whether or not current passes through the fluid in the rubber strip A. If not, the reason is either that the force F has compressed the strip such that the current path has been interrupted, or that a power failure has occurred, caused by, for example, an external short-circuiting 6. In both cases, the door opening circuit 5 is interrupted. In an alternative embodiment, the current transformer may be replaced by an optocoupler mounted in the strip and connected to the door operating circuit.