THR. Also Tolerable risk level. Tolerable level of risk. In security systems it indicates the maximum level of risk, in the operation of a system or during a specific operation, which the railway authority is willing to accept. For example, in the Hazard and Risk Analysis phase of a certain system used for a specific “vital” application, the SIL4 requirement corresponding to a THR value (tolerable hazard rate), as established in the European standard ENV 50129, lower than the value of 1×10-8h-1. This risk level is usually defined as “the lowest reasonably practicable”. See ALARP
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