This document specifies principles for the design and selection independent of the nature of the energy source of interlocking devices associated with guards and provides guidance of measures to minimize the possibility of defeat of interlocking devices in a reasonably foreseeable manner. This document covers principles for the design, selection and application " of parts of the guards which actuate interlocking devices; " of trapped key interlocking devices and systems for machinery applications. NOTE ISO 14120:2015 specifies general requirements for the design and construction of guards provided primarily to protect persons from mechanical hazards. The processing of the signal from the interlocking device to stop the machine and prevent unexpected start up is covered in ISO 13849-1:2015 or IEC 62061:2005+AMD1: 2012+ AMD2: 2015.