Class I
The tool is equipped with grounding device, and all or most parts of the insulation structure are basically insulated. If the insulation is damaged, because the accessible metal parts are connected with the protective grounding (see grounding) or protective zero wire installed in the fixed line through the grounding device, they will not become electrified body, which can prevent the operator from electric shock.
Class II
The insulation structure of such tools consists of double insulation or reinforced insulation composed of basic insulation and additional insulation. When the basic insulation is damaged, the operator is separated from the electrified body by the additional insulation without electric shock. Class II tools must be non re connected to the power plug and shall not be grounded.
Class III
These tools are powered by a safe voltage source. The effective value of no-load voltage between the safety voltage guide body or between any conductor and the ground shall not exceed 50V; for three-phase power supply, the effective value of no-load voltage between the conductor and the middle line shall not exceed 29V. Safety voltage is usually supplied by safety isolation transformer or converter with independent winding. It is not allowed to set protective grounding device on class III tools.
radio interference
The single-phase series motor and DC motor with commutator will produce serious electromagnetic interference to TV and radio, so the interference of pitch system to radio should be considered in the design of electric tools. The main measures are shielding, symmetrical connection of excitation winding, setting of electric filter and connection of filter into △ shape. If necessary, small inductive coils can also be connected in series at both ends of the motor armature.
