This International Standard specifies the safety requirements of a THCD and the dependency of the output signal from the input signals. This International Standard describes the main characteristics of THCDs for the achievement of safety and sets out combinations of functional characteristics for three types. It does not apply to devices intended to be used as enabling devices, as hold-to-run devices or as special control devices. This International Standard does not specify with which machines THCDs shall be used. It also does not specify which types of two-hand-control device shall be used. Moreover, it does not specify the distance between the THCD and the danger zone (see 8.8). This International Standard provides requirements and guidance on the design and selection (based on a risk assessment) of THCDs including their assessment, the prevention of defeat and the avoidance of faults. This International Standard applies to all THCDs, independent of the energy used, including: THCDs which are or are not integral parts of a machine; THCDs which consist of one or more than one separate element.