Ned, Will this method also cover UL 916?
Joe Josiah P. Burch Compliance Engineer II Andover Controls Corporation 300 Brickstone Square Andover,Ma 01810 (978)-470-0555 x335 (978)-470-3615 Fax > -----Original Message----- > From: Ned Devine [SMTP:[email protected]] > Sent: Thursday, November 30, 2000 8:55 AM > To: [email protected] > Subject: RE: component spacing question > > > 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] >
