See also the buzz word “electrical protection”. In working with electrical equipment it must be ensured that live parts cannot be touched directly (categories one or two) or indirectly (category three). This can be achieved by insulating live parts or by applying coatings (sheaths). It must be assured that casings can and may only be opened once the circuit has been interrupted. If this should not be possible for some reason (i.e. during trial operation), casings may be removed only by skilled electricians who have to use special wrenches or tools.