Integrity of interlock guards
These can fail as a result of age or rough treatment. They should be designed to work in the positive mode so that under normal circumstances a failure of the interlock switch prevents machine operation. Opening of an interlock guard should stop the movement of hazardous parts before they can be reached.
Where switches are installed in the positive mode, the contacts are opened by the positive mechanical action of the cam. Where a switch is installed in the negative mode, the contacts are opened by spring pressure when the cam is rotated. When switches are used singly, the positive mode should be used. Its failure should prevent operation of the machine. Positive switches are also more difficult to defeat. The use of two cam-operated switches positioned next to each other but operating in opposite modes provides an even better standard.