Electronic Design

A Brief Safety Lexicon: Selected Definitions From EN 60950

• Basic insulation: Insulation to provide basic protection against electric shock.
• Bounding surface: The outer surface of the electrical enclosure.
• Class I: Equipment where protection against electric shock is achieved by using basic insulation, also providing a means of connecting to the protective earthing conductor in the building wiring if the basic insulation fails.
• Class II: Equipment in which protection against electric shock does not rely solely on basic insulation, but in which additional safety precautions, such as double insulation or reinforced insulation, are provided.
• Clearance: The shortest distance between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured through air.
• Clearance distance: Shortest distance in air between two conductive elements.
• Creepage distance: The shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured along the surface of the insulation.
• Double insulation: Insulation comprising both basic insulation and supplementary insulation.
• Functional insulation: Insulation needed for the correct operation of the equipment.
• Hazardous energy level: A stored energy level of 20 J or more, or an available continuous power level of 240 V A or more, at a potential of 2 V or more.
• Hazardous voltage: A voltage exceeding 42.4 V peak or 60 V dc, existing in a circuit that does not meet the requirements for either a limited current circuit or a telephone network voltage (TNV) circuit.
• SELV circuit (safety extra-low voltage): A secondary circuit that is so designed and protected whereby, under normal and single fault conditions, its voltages do not exceed a safe value (definitely less than
42.4 V peak or 60 V dc).
• Touch current: Electric current through a human body when it touches one or more accessible parts.
• Tracking resistance: Evaluation of insulating materials by determining their creepage distance formation (accomplished by dripping a watery solution onto a horizontal surface so that it leads to electrolytic
• Rated and surge voltages: The “rated” voltage is the value above which the creepage distance is measured. The “surge” voltage is a test impulse of short duration with a specified impulse form and polarity that is applied to test insulation paths.

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