Hi, I agree with Rich. This never made any sense to me even when I worked at UL. Why are spacing not critical where the leads enter the case but are critical on the PCB? Oh well, as Rich mentioned, there is an out. In many cases the insulation if OPERATIONAL and can be short-circuited. For example, see UL 1950, Third Edition, Clause 5.4.4 c).
Ned Devine Entela, Inc. Program Manager III Phone 616 248 9671 Fax 616 574 9752 e-mail [email protected] -----Original Message----- From: Rich Nute [mailto:[email protected]] Sent: Wednesday, November 29, 2000 7:38 PM To: [email protected] Cc: [email protected] Subject: Re: component spacing question Hi Ken: > I wanted to pose the group a question in regards to spacing requirements for > the U.S. Many UL standards allow for the exception of components (such as > semi conductors, switches, etc) to not meet spacing requirements (as they > usually have different standards they meet anyways) The question is what > about PCB trace spacing for those components, such as a capacitor in a > switching power supply to ground. High voltage, the components is an > exception, but what about the pads under the component, are they required to > meet the spacing requirements, or would that fall under the component > exception? You've brought our attention to the anomaly that a component must meet its spacing requirements, and that the end-product (i.e. PWB) must meet its spacing requirements, even when the component requirements are less than that of the end-product. >From an engineering point of view, this is nonsense. Nevertheless, our standards have such requirements, and our certification houses must enforce such requirements. There are several options: 1) Design the PWB to meet the spacing requirements by trimming edges off circular pads, or by bending the component leads. 2) Test by short-circuiting the spacing and observing the results (i.e., no shock or fire or damage to basic insulation). This is especially appropriate for semiconductors since, by definition, they alternate or vary between open and nearly short. 3) Remember that "spacings" is a special case of insulation, either air insulation (clearance) or a surface insulation (creepage). Only those insulations that are required by the safety standard (i.e., basic, supplementary, or reinforced) are subject to the spacings requirements. Typically, these insulations only exist between primary and ground, and between primary and secondary. Typical products do not have components between primary and ground and between primary and secondary except those specifically rated for such use such as Y-caps, transformers, and opto-isolators (and which therefore meet the spacing requirements of the end-product). Some standards may require pole-to-pole spacings in primary circuits. In this case you must measure the voltage and then determine the spacing from a table. A typical SMPS has lots of low-voltage control circuits with respect to the negative rail. So, you can lump all those circuits together as not requiring insulation from each other. Then, they can be taken as a whole and spaced from the positive rail. That will generally only leave the bulk capacitor and the switching transistor(s) as requiring spacings. And, the snubber circuit, which can be considered a voltage divider so that the spacings across any individual snubber component need not be the full voltage across the snubber. Etc. It really makes no sense to require a PWB to have greater spacings than the component itself. It further makes no sense to enforce spacings across capacitors (that are not Y capacitors) and semiconductors and similar components. Internally, these components do not have insulations that are equivalent to their terminal spacings or to the PWB spacings. So, why require a higher level of insulation than the device itself can provide? Short-circuiting of the component will tell the story of whether the circuit is safe; if safe, then the spacing is inconsequential to the safety of the product. Best regards, Rich ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
