Hi Joe: You said:
" Unfortunately, Annex Q does not help to define the term "adequate breaking capacity" as called for in clause 1.5.9.2 for fuses required in series with the MOV." The term "adequate breaking capacity" means the authors (committee) did not know how to determine the parameters. Or, such determination would take many pages in the standard. Or, most of the committee did not understand the parameters necessary to determine "adequate breaking capacity." The use of the term is a cop-out on the part of the committee. Instead, they left it up to the person or organization using the standard to determine the parameters. IEC 60950-1, 1.5.9.2, says the VDR must be provided with an interrupting means for: * Temporary overvoltages * Thermal overload due to increased leakage current * Burning and bursting of the MOV in the event of a short-circuit fault 1) A VDR should not operate in the event of a temporary overvoltage. Temporary over voltages are root 3 times the normal voltage, and at most 2 times the normal voltage (for 120 or 230 volt systems). A VDR that would operate at a temporary overvoltage would mean the equipment would not pass the electric strength test. And, of course, it would overheat and fail. A fuse that would provide protection of the VDR in the event of a temporary overvoltage would need to be rated for the temporary overvoltage and fault condition current. In the worst case, the fuse would need to be rated more than 460 volts! 2) Because of the small continuous leakage current of a VDR, to protect against thermal overload is most effectively done with a thermal protector rather than a fuse. Due to the small thermal mass of a VDR, and the small power dissipation due to increased leakage current, coupling a thermal protector to the VDR may be difficult. 3) Short circuit of what? A true short-circuit does not dissipate power. If the MOV is short-circuited, then a fuse can do the job. Or, the MOV can be in a fire enclosure that contains the burning MOV and contains the burst fragments (but this is not an "interrupting means" and might not be accepted by a certification house). Note that if the fuse or thermal protector operates, you can't tell the difference in the equipment operation until the next transient of sufficient voltage to cause a fault in the equipment. For 120 or 230 volt equipment, this may take months or even years! The easy solution is not use a VDR in equipment. The installation should have transient voltage protection, not equipment. And, the operation of a VDR creates transient overvoltages in other equipment as you concluded in your paper at the PSES Symposium several years ago. My comments don't answer your questions, but widen the perspective. 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]>
