Hi Rich: Thanks for your feedback on this topic.
My impression is that the intent of the requirement for a fuse in series with an MOV is not related to concerns about protecting users from surge transients. I believe the intent of the requirement is to protect users from the effects of an MOV that has become leaky and is allowing excess current to flow into the ground conductor. While the cause of the MOV becoming leaky is typically considered to be degradation caused by surges, the surges themselves are not the focus of the requirement for a fuse. Rather, the focus is on hazards created by leaky MOVs. Regarding the interpretation of 60950-1, I have two questions: 1) For equipment that has two MOVs, each connected line-to-ground, does 60950-1 require fusing both of the line leads? It appears to me that the answer is yes, but I'm not an expert on this topic. 2) Clause 1.5.9.2 in 60950-1 requires that the fuse must have "adequate breaking capacity." Where in 60950-1 do we find a definition and/or test for "adequate breaking capacity"? I agree with your observation that if the purpose of the fuse is to keep the leakage current under 0.5 mA, there is no practical way to achieve this. The same fuse must also be able to survive the anticipated surge currents for which the MOV was installed. So, the authors of clause 1.5.9.2 must have had some other definition in mind for the term, "adequate surge protection." Thanks, Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com -----Original Message----- From: Richard Nute [mailto:[email protected]] Sent: Thursday, November 09, 2017 6:34 PM To: 'Joe Randolph' <[email protected]>; [email protected] Subject: RE: [PSES] Regulatory requirements for MOVs placed line-to-ground on AC mains ports? Hi Joe: "I still have a question about whether clause 1.5.9.2 in 60950-1 means that there must be fuses in each of the Line conductors when two MOVs are installed line-to-ground on the AC mains input. It sure looks that way when I read the requirement." I have some doubts as to whether a fuse will open the circuit when an MOV conducts. Or, when the MOV leakage is excessive. Continuous current in the earth conductor must not exceed 0.5 mA. I couldn't find any fuses rated less than 32 mA. So the fuse cannot protect against MOV deterioration. Supposedly, the MOV operates when the transient voltage exceeds, say, 2100 volts peak. When the MOV operates, the line conductor is almost shorted to ground. So, 2100 volts peak across the fuse exceeds its voltage rating by 500% (assuming the fuse is rated for the mains voltage). If you exceed the fuse voltage rating, the fuse is not likely reliable, and may become a conductor. Another interesting point. While the standard severely limits the use of an SPD for "Type A" plug-connected equipment, the SPD cannot operate if the ground is open. For an injury to occur, three conditions must be met: 1) Transient voltage exceeding 2100 volts peak (maybe only a few times per year). 2) Person touching an otherwise grounded part of the equipment. 3) The transient voltage must have enough energy to cause an injury. The probability of an injury is the multiplication of the individual probabilities. Very small. If the transient waveform is the standard transient waveform, 1.2 usec x 50 usec, then the duration is much shorter than the curves shown in IEC 60479-5. So, there is insufficient energy in the transient waveform to cause an injury. Best regards, Rich - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
