Hi George:
Here is what I sent to this group a week or so ago: >5. What are the IEC 950 enclosure flammability class requirements? > > In Sub-clause 4.4.4, enclosure flammability class is specified > depending on the function of the enclosure: > > Mechanical enclosure: HB or better. > Electrical enclosure: HB or better. > Fire enclosure (<18 kg) V-1 or better. > Fire enclosure (>18 kg) 5V or better. > Jasmine Wong and Jim Eichner are correct: IEC 950 DOES (today) have a "sweeping generalization" that electrical enclosures need only be rated HB. See Sub-clause 4.4.4, third paragraph. Your associate quotes Sub-clause 4.4.5 which identifies COMPONENTS, not circuits, which require a fire enclosure, (It also identifies those COMPONENTS which DO NOT require a fire enclosure). Jasmine is wrong in identifying SELV circuits as the criterion for not requiring a fire enclosure. Rather, as your associate stated, it is the COMPONENTS in "limited power circuits" which do not require a fire enclosure. (Often, but not necessarily, SELV circuits also qualify as limited power circuits.) These are fine points, distinguishing between circuits and components. IEC 950 permits primary and secondary wiring comprised of PVC, et al, wire insulation, enclosed switches and relays, and certain motors to be within an HB enclosure. Indeed, if one reads further (Sub-clause 4.4.1), an HB enclosure can be used for circuits which are NOT limited power by performing extensive fault testing! The issue of fire from electrical energy is that of electrical heating resulting from power dissipation in a resistance or impedance. The trick is to identify the resistances that can dissipate power for an extended time -- long enough to heat the plastic material to ignition temperature. The resistance can't be too low or too high. For each power source, there is a range of resistance that can dissipate sufficient power. Usually, this is in the 0.5 to 5 ohm range. So, we have candidate components such as contact resistances in switches and connectors, forward resistances of semiconductors, and the dc winding resistances of transformers and chokes. So, it is really not out of the question to do fault testing that would qualify a circuit and its components for an HB enclosure. Best regards, Rich ------------------------------------------------------------- Richard Nute Quality Department Hewlett-Packard Company Product Regulations Group San Diego Division (SDD) Tel : 619 655 3329 16399 West Bernardo Drive FAX : 619 655 4979 San Diego, California 92127 e-mail: [email protected] -------------------------------------------------------------
