Safety protection of electric tools

Class I

There is a grounding device in the electric tool, and all or most parts of the insulating structure have basic insulation. If the insulation is damaged, since the accessible metal parts are connected with the protective grounding (see grounding) or the protective zero wire installed in the fixed line through the grounding device, they will not become charged objects, which can prevent the operator from getting an electric shock.

Class II

The insulation structure of electric tools consists of double insulation 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 additional insulation, so as not to get an electric shock. Class II tools must use non-reconnectable power plugs, and grounding is not allowed.

Class III 

Power tools are powered by a safe voltage source. The effective value of the no-load voltage between the safety voltage conductors or between any conductor and the ground does not exceed 50V; for a three-phase power supply, the effective value of the no-load voltage between the conductor and the neutral line does not exceed 29V. The safety voltage is usually supplied by a safety isolating transformer or a current transformer with separate windings. Protective earthing 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 power tools should be designed to control radio interference. Mainly adopt measures such as shielding, symmetrical connection of excitation windings, setting of electrical filters, and forming of filters. If necessary, small inductance coils can also be connected in series at both ends of the motor armature.