The most cost efficient and safest type of maintenance is maintenance that is planned and carried out before a defect occurs. Trouble shooting under time pressure based on improvisation is not only dangerous but, in most cases, also expensive. In principle, maintenance work may only be performed when machines are shut off, and unauthorized, inadvertent or unintended switching on can be ruled out. If several employees are involved in maintenance work, the main switch should be secured by means of several padlocks (one for each maintenance engineer). If maintenance work has to be done on running machines, special protective devices or additional equipment must be used or other special measures taken. For this type of operation, staff must be specially instructed and trained.