I too am re-posting an e-mail. I'm pretty sure this one did make it onto the forum, but I've only received one reply, so I am begging for more, particularly to my questions 2 and 3 below.
Thanks, Jim > -----Original Message----- > From: Jim Eichner [SMTP:[email protected]] > Sent: Tuesday, May 18, 1999 11:21 AM > To: 'EMC-PSTC - forum' > Cc: Tom Unger > Subject: RE: Surge Suppressors to ground > > > Sorry to re-open a thread that is almost 2 years old, but here goes... > > We are designing a product for 120V markets only, so there are no > immunity requirements from a regulatory point of view. We want to do > what we can to ensure trouble-free operation though, so we are looking > at up to 3 MOV's on the AC input: one line-to-line and one from each > line to ground. The line-to-ground MOV's have been the subject of > much > discussion regarding various European agencies disallowing them, and > we > don't really want to give up the real estate that they would take up, > anyway. On the other hand, common-mode transients are a real world > event, so we're having trouble making up our minds. > > My questions: > > 1. Am I correct that the North American agencies (CSA, UL, ETL, etc.) > allow line-to-ground MOV's? The standards we work with do not > disallow > them, but this is the sort of issue for which agencies often invoke > "desk drawer requirements". > > 2. Many of you have lots of experience with surge testing for European > compliance. How common is it (relative to differential mode surges) > for > common-mode surges to cause upset or damage? Just as common but it > takes higher voltages / currents? Less common even at higher levels? > > 3. Does anyone have any real experiences with the supposed shock > hazard > that has caused several European agencies to ban line-to-ground MOV's? > > Thanks for your help, > > Regards, > > Jim Eichner > > Senior Regulatory Compliance Engineer > Statpower Technologies Corporation > [email protected] > http://www.statpower.com > Any opinions expressed are those of my invisible friend, who really > exists. Honest. > > > -----Original Message----- > > From: richn@anetMHS (Rich Nute){MHS:[email protected]} > > Sent: Wednesday, May 14, 1997 3:36 PM > > To: WOODS@anetMHS (WOODS RICHARD){MHS:[email protected]}; > > JEichner; bceresne > > Cc: emc-pstc@anetMHS (Product Safety Technical > > Committee){MHS:[email protected] > > Subject: Re: Surge Suppressors > > > > Hi Richard: > > > > You commented that the major reason a surge suppressor > > is not allowed from line to earth is because the surge > > suppressor is prone to shorting. > > > > Let us assume that the surge suppressor only fails when > > a voltage is impressed across it. > > > > For a voltage to appear across the surge suppressor, the > > chassis of the product must be connected to ground. If > > it is not connected to ground, then there is no current, > > and there is no voltage across the suppressor. > > > > So, a surge suppressor can only fail when the product is > > truly grounded. If it is grounded, then the grounding > > SHOULD provide the protection for which it is intended, > > namely protection against electric shock in the event of > > a failure of some sort from mains to ground! > > > > When it fails, hopefully its impedance is sufficiently > > low so as to cause the operation of an overcurrent device > > (e.g, fuse or circuit-breaker). Hopefully, the operator > > will notice that the circuit-breaker has tripped and the > > unit is taken out of service for repair. > > > > If the unit is not taken out of service, but moved to a > > site where there is no ground, then the unit will indeed > > be hazardous, with mains applied to the chassis through > > the shorted suppressor. This is the sequence of events > > that would lead to a shock hazard. > > > > Evaluation of products for safety does not include moving > > a unit (with a single fault) from a grounded state to an > > ungrounded state. > > > > Note that, in every product, we PRESUME failure of basic > > insulation. Grounding is one scheme for mitigating the > > failure of basic insulation. What is the logic -- or > > engineering basis -- for permitting grounding as a mitigator > > for failure of basic insulation but not for failure of a > > surge suppressor? > > > > I submit that not allowing a surge suppressor from line > > to chassis because it is subject to shorting and causing > > a shock is nonsense. > > > > Nevertheless, the various certification houses indeed > > have rules prohibiting surge suppressors from line to > > chassis. Their justification is that the failure of the > > surge suppressor WILL cause a shock hazard. The construction > > is DEFINED as hazardous. No amount of engineering evaluation > > or testing is going to change the certification house rules! > > > > > > Best regards, > > Rich > > > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
