Class I
There is a grounding device in the tool, and all or most parts of the insulating structure have basic insulation. If the insulation is damaged, because the accessible metal parts are connected with the protective grounding (see grounding) or protective neutral conductor installed in the fixed line through the grounding device, they will not become live objects, which can prevent the operator from getting electric shock.
Class II
The insulating structure of such tools consists of double or reinforced insulation consisting of basic insulation and supplementary insulation. When the basic insulation is damaged, the operator is separated from the charged body by the additional insulation to prevent electric shock. Class II tools must use non-reconnectable power plugs, and grounding is not allowed.
Class III
Such tools are powered by a safe voltage supply. The rms value of the no-load voltage between the safety voltage conductors or between any conductor and the ground shall not exceed 50V; for three-phase power supplies, the rms value of the no-load voltage between the conductors and the neutral line shall not exceed 29V. The safety voltage is usually supplied by a safety isolation transformer or a converter with separate windings. Protective grounding devices are not allowed on Class III tools.
Radio interference
Single-phase series motors and DC motors with commutators will cause serious electromagnetic interference to TV sets and radios, so the interference to radios should be considered in the design of power tools. Mainly adopt measures such as shielding, symmetrical connection of excitation windings, setting up electrical filters, and connecting filters in a delta shape. If necessary, small inductance coils can also be connected in series at both ends of the motor armature.
