Class I

The tool is equipped with a grounding device, and all or most parts of the insulation structure have basic insulation. If the insulation is damaged, the accessible metal parts are connected to the protective grounding (see Grounding) or protective neutral conductor installed in the fixed line through the grounding device, which prevents the operator from being electrocuted as a charged body.

Class II

The insulation structure of this type of tool consists of dual insulation or reinforced insulation consisting of basic insulation and additional insulation. When the basic insulation is damaged, the operator is separated from the live body by the additional insulation, preventing electric shock. Class II tools must use non-rewirable power plugs and are not allowed to be grounded.

Class III

This type of tool is powered by a safe voltage power supply. The effective value of the no-load voltage between safe voltage conductors or between any one conductor and the ground does not exceed 50V; for three-phase power supplies, the effective value of the no-load voltage between the conductor and the neutral line does not exceed 29V. Safety voltage is usually supplied by a safety isolation transformer or a converter with independent windings. Protective grounding devices are not allowed on Class III tools.

radio interference

Single-phase series motors with commutators and DC motors can cause serious electromagnetic interference to televisions and radios.  Therefore, measures to suppress radio interference should be considered during the design of power tools. These include shielding, symmetrical connection of excitation windings, installation of electrical filters, and connecting the filters in a delta shape. When necessary, small inductance coils can be connected in series across the armature terminals of the motor.