Juan Pedro Pena wrote: > I have some questions about IEC/EN 60950: > > a) Last version of the standard doesn't show explicitly the > required insulation between conductive parts of different polarity in > primary circuit. It looks like obvious that operational insulation is > the minimum, but other standards requires basic insulation, at least > until the other side of the fuse where operational insulation may be > enough. Moreover, X capacitors between phase and neutral conductor has > higher requirements (# 1.5.6) than only operational insulation. What's > the right interpretation? I am working with IEC 950 (newly IEC 60950) Second edition 1991-09, amended with A1 (1992), 2 (1993), 3 (1995) and 4 (1995). The types of insulation are defined in 1.2.9.1 (OPERATIONAL) through 1.2.9.5 (REINFORCED). Between conductive parts of different polarity in a primary cicuit you need operational insulation, as you may learn also from figure 5A on page 79c. We are not talking here about protection against electric shock. In 1.5.6 Mains capacitors, I can not see any connection to OPERATIONAL and/or BASIC or any other INSULATION. Did I miss something?
> b) Insulation between both sides of a transformer shall be > calculate according to the maximum voltage that can be found between > them. If we have a primary circuit with 230 V AC and a secondary circuit > with 100 V AC, what voltage should we consider? 230 + 100?, 325-141 V > peak if both circuits have the same phase? 325+141 V peak if both > circuits have a phase of 180º? Does it depend on the real phase? The general answer is given in 2.2.7.1. The applicable working voltage depends on the construction of the transformer, and how the primary and secondary windings can be/are wired. You are on the safe side if you add up the two voltages. Do not forget to add the tolerances on the distribution system. To determine the clearances you follow 2.2.7.2 and use peak voltages to go into table 3. For creepage distances you apply r.m.s. voltages according to 2.2.7.4 and table 6. > c) It looks like if the old 4th method of protecting a SELV circuit > (never admitted in Europe), has been substituted by # 2.3.5 Isn't it? The old method 4 was never accepted in EN 60950 as you correctly state. It was eliminated without replacement. 2.3.5 deals with connection of SELV to other circuits. > d) If somebody uses a SELV circuit not earthed, there is no > insulation requirement of that circuit from earth. The earth conductor > has a reinforce insulation from primary circuit, but no insulation > requirement from the SELV circuit. Primary to SELV insulation may be > reinforce, but it isn't because it has a way through earth with only > basic insulation. What's the right interpretation? Earth is safe to touch. SELV is safe to touch. Why shall there be any need for an insulation to protect against electric shock? -- Kind regards/mit freundlichen Gruessen, Horst Dierich, Germany EMAIL: [email protected]
