Double insulation — protection against contact with live parts

It is used to protect against touching parts that are normally or accidentally under voltage double insulation — electrical insulation consisting of working and additional insulation. Operational isolation — isolation of live parts of the electrical installation, ensuring its normal operation and protection against electric shock.

Supplemental Insulation — Insulation provided in addition to the working insulation to protect against electric shock in the event of failure of the working insulation.

The simplest double insulation is done by covering metal boxes and handles of electrical equipment with a layer of electrical insulating material and using insulating handles.

The surface layer of the insulation is susceptible to mechanical loads and damage. When this layer is destroyed, access is opened to metal parts that may be under voltage. Damage and even complete destruction of the second layer of insulation does not prevent the continuation of work and thus does not signal a loss of protection. Therefore, this method of implementing double insulation does not provide reliable protection and can only be recommended in rare cases — for equipment that is not subjected to shock loading.

A better way is to make a case of insulating material. Such a body carries all live parts, non-live metal parts and mechanical part. When the case is destroyed, access to metal current-carrying and non-carrying parts is released, but the electrical equipment cannot work, because the relative position of its parts is disturbed.

An example of this is an electric drill with a plastic body. The stator magnetic core, brush holders and bearings are fixed in the insulating housing. In case of minor damage to the housing, access to the metal parts remains closed. Touching these parts is only possible if the box is destroyed. Obviously, it is impossible to work with such a tool, because the displacement and misalignment of the bearings lead to jamming of the rotor.

The presence of protective double insulation, of course, does not exclude the possibility of electric shock when touching live parts in the event of a breakdown of the main phase insulation.

The protective double insulation can ensure the safety of operation of any electrical equipment.However, due to the presence of some disadvantages in plastics, such as insufficient mechanical strength, the possibility of significant permanent deformation, unreliability of joints with metal, a change in the direction of deterioration of mechanical properties with aging, the field of application of double insulation is limited to electrical equipment low-wattage-electrified hand tools, some portable devices, household appliances, and hand-held electric lamps.

Double insulation cannot be used when exposed to heat due to the low thermal resistance of plastics.

Double-insulated hand-held electric lamps, hand-held power tools and some household appliances are commonly produced.