Rich, Thanks for the quick lesson. After reading the standard I can see the perspective (a new perspective for me - I've always required fire enclosures.)
Going back to Mike's original question "can we anticipate and design tests to simulate all the abnormal and fault conditions that could start and spread a fire beyond the equipment?" My answer would be that it would be very difficult, especially if you have a switching power supply. If I am not going to provide a fire enclosure for my power supply, then I better be darn sure that every combination of failure will not start a fire. Trying to get something like that approved by UL would be extremely difficult in my opinion. If I'm the UL engineer, I would want to do abnormals on virtually every component (up to limited current circuitry.) If there are 20 components in the primary and another 10 in the secondary that must be tested, that's a lot of UL's time (and your money) Best Regards, Kendall ______________________________ Forward Header __________________________________ Subject: Re: Do UL1950 Recognized PS's Need Fire Enclosures Author: Rich Nute <[email protected]> at SMTP-MAIL List-Post: [email protected] Date: 10/15/96 4:10 PM Hi Kendall: You ask if 4.4.5.1 still applies even if the power supply passes the tests of 5.4.6? I admit that this is an ambiguous situation. Note that all of the sub-clauses providing the "confusion" are in Sub-clause 4.4. (We are concerned with 4.4.1 and 4.4.5.) Sub-clause 4.4.1 says that there are two methods for providing protection against ignition and spread of fire: Method 1: Requirements in 4.4.2 and 4.4.3. Method 2: Fault testing in 5.4.6. The ambiguousness is that 4.4.1 Method 1 doesn't list 4.4.5 as being within Method 1. So, it appears that 4.4.5 is a stand- alone requirement, unrelated ot 4.4.1. Sub-clause 4.4.5.1 is not related to primary circuits, but to components. In essence, it says that all components must be within a fire enclosure. (Except those components identified in 4.4.5.2.) This is Method 1. Sub-clause 4.4.1 also identifies components, although the list is not equal to that of 4.4.5.1. So, it would appear that the parts identified in 4.4.5.1 that are not also identified in 4.4.1, namely unenclosed arcing parts, transformers, solenoids, and relays require a fire enclosure. But, 4.4.1 uses the phrase "such as." So, this is an admission that the list is not complete, and that other components can be tested according to 5.4.6. I am not at all sure that the statement implies that some components cannot be included in 5.4.6 testing. If you think of the standard as a flow-chart, then if you invoke Method 2, you skip all of the fire requirements and go directly to 5.4.6. That's how I arrive at the conclusion that primary circuits need not be required to be within a fire 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] -------------------------------------------------------------
