Machinery Protection Devices - British Automation & Robot ...

5.2.5 MutingThe muting of protective devices raises the problem of aninstallation’s safety. For example, EN 415-4 (Palletizers anddepalletizers) relates to packaging machinery on which all operationson the palletised load are carried out entirely and automatically bymachine. Under normal operating conditions, there is a risk at boththe entrance and exit of the interior zone. The AOPD must be mutedat the moment the pallet passes through, but it must also be possibleto detect the presence of an operator. The muting system musttherefore be able to discriminate between the pallet and the operator.26122Machinery ProtectionDevicesThe muting conditions defined in standard EN 415-4 state that:Muting may only occur during the operating cycle when theloaded pallet obstructs access to the hazardous zoneMuting shall be automaticMuting shall not depend on a single electrical signalMuting shall not depend entirely on software signalsIf muting signals occur as part of an invalid combination,they shall not allow a state of muting, or they shall ensurethat the machine is locked outThe state of muting must be deactivated as soon as thepallet has passed through the detection zone.The diagrams below show how a light curtain can be used to meet allthese requirements. The device incorporates a system of temporarymuting by automatic discrimination. The AOPD is muted by thesensor pairs A1/A2 and B1/2. In this case the distance between A1and B2 must be less than the length of the pallet. The light curtaincan also be used to define the maximum duration of the mutingperiod, in stages of 1 second.