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There are additional key terms that are important to understand relating to the IEC 60950-1 standard. Class I equipment must have a protective earth conductor. If insulated it must have a green/yellow or transparent covering. No switch or fuse is allowed. Resistance between earthed parts and the earth termination must not exceed 0.1 Ω, which is tested by a current 1.5 times the current capacity of any hazardous voltage circuit at the point where failure of basic insulation would make the earthed part live. Test voltage maximum is 12V. Test current may be AC or DC but must not exceed 25A. Clearance is the shortest unimpeded distance between two conductors or circuit components measured though the air. Power products are intended for general application and should be designed for worst case conditions (pollution degree 3 and mains voltages up to 264VAC) for minimum clearances of 4.00mm for reinforced or double insulation and 2.00mm for basic and supplementary insulation. Where formal quality control processes are in place relaxation to 3.4mm and 1.7mm is allowed, but reinforced insulation is then subjected to 100% electric strength testing. If an air gap serves as the insulator between a hazardous voltage and the enclosure, the required clearance is 10mm. Creepage is the shortest distance between two conductors measured along the surface of the board. Tables of creepage distances for basic insulation are given for various pollution conditions and materials, the distances depending on working voltages. These distances are doubled for reinforced insulation.
