David, You have to ask yourself, what would secondary protection be protecting?
As part of your Risk Assessment or circuit Fault Analysis, you must consider what would happen if the VDR/MOV faulted in a high current or short circuit condition. If the high current can cause overheating in wiring, traces, connectors, components, etc. which could cause a fire, then yes, some kind of "additional" protection device must be added. If not, the answer is no. I hate it when I'm told by "safety people" that you must do this or do that without consideration of the specific design. For example, most power supplies (PSU) have a Line to Line VDR/MOV, but it is after the onboard fuse provided to protect the entire PSU. If the VDR/MOV faulted, the fuse would open. So an additional protection device for the VDR/MOV would not be needed. If you do need to add protection for your VDRs, make it robust enough so it does not open after your first thunderstorm. The Other Brian -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Spencer, David H Sent: Wednesday, December 15, 2010 7:12 PM To: Bill Owsley; [email protected]; [email protected] Subject: RE: OT A BIT........ [PSES] IEC 60950-1 Ed. 2 Class A Pluggable equipment and VDR bridging basic insulation. My safety people are telling me that part of Ed. 2 also requires that VDR's line to line ( or line to neutral) have secondary protection. Either a fuse, gas tube, or double insulated cap. They have been with great gusto been pulling out the VDR's or replacing them with fused versions. >From an EMC aspect, we never know when a VDR (from now on MOV) goes bad. SO.... with respect to line to line (neutral) MOV's only and Ed.2 Do MOV's need a secondary protection? thanks for any comments Regards David Spencer EH&S EMC Engineer Xerox Corp -----Original Message----- From: [email protected] on behalf of Bill Owsley Sent: Wed 12/15/2010 3:57 PM To: [email protected]; [email protected] Subject: RE: [PSES] IEC 60950-1 Ed. 2 Class A Pluggable equipment and VDR bridging basic insulation. In EMC are testing to 2 kV surge. But Brazil deemed our product "telecom" and applied 4 kV. A couple of our products spark and arc impressively and continue running. A couple of others quit, died, castors up, and the smoke got out... (It fun when they do that!) EMC says it has to continue to work. Safety says it only has to fail safely. Engineering says fix it but don't change anything. VDR between line and earth, and between line to line fixes it to 4 kV. But I run right into the safety guy and all these notes, about pluggable, single/double fused, screwed down earths, permanent connect, etc. Bill In the event of a national emergency, click on the following links to provide directions to your duly elected mis-representatives.http://www.usa.gov/Contact/Elected.shtml or... https://writerep.house.gov/writerep/welcome.shtml http://www.senate.gov/general/contact_information/senators_cfm.cfmif really desperate... http://www.usa.gov/Contact/Elected.shtml --- On Wed, 12/15/10, Brian O'Connell <[email protected]> wrote: From: Brian O'Connell <[email protected]> Subject: RE: [PSES] IEC 60950-1 Ed. 2 Class A Pluggable equipment and VDR bridging basic insulation. To: [email protected] List-Post: [email protected] Date: Wednesday, December 15, 2010, 3:33 PM The problem with the VDR being rated for a level 3 surge, which is significantly greater than the typical test V for BI, is that the VDR will typically start conducting long before required test voltage is reached. Di-electric withstand and surge immunity are very different animals. I saw a VDR from line to chassis in a competitor's component P/S - never understood the reason for this construction. Is the VDR rating for 62368 conformity a working voltage or surge rating ? Brian > -----Original Message----- > From: [email protected] [mailto:[email protected]]On Behalf > Of Richard > Nute > Sent: Wednesday, December 15, 2010 12:01 PM > To: JIM WIESE > Cc: [email protected] > Subject: Re: [PSES] IEC 60950-1 Ed. 2 Class A Pluggable equipment and > VDR bridging basic insulation. > > > Hi Jim: > > Thanks for your remarks. > > You CAN remove the VDR during hi-pot testing. > This is specified in 5.2.2: > > "To avoid damage to components or insulation > that are not involved in the test, > disconnection of integrated circuits or the > like and the use of equipotential bonding are > permitted." > > This is for the "type" test, not the "routine" > test. Nevertheless, your point is well taken. > > Also note that 5.2.1 specifies: > > "The electric strength of the SOLID INSULATION > used in the equipment shall be adequate." > > So, the hi-pot test only applies to solid > insulation. > > The other option is to specify the VDR at a higher > voltage than the hi-pot test voltage. > > Regarding VDRs, I don't know why any equipment > would need a VDR between mains and earth. The > requirements for clearance, creepage, and solid > insulations require an electric strength at least > as great as the expected transient overvoltage, > regardless whether a VDR is between mains and > earth or not. So, the VDR does not protect > anything against any voltage up to the required > electric strength of the equipment. > > The VDR *may* be useful to protect against > transient voltages exceeding the required electric > strength.n which case it will pass the hi-pot > test. > > In the new IEC 62368, the requirement is that any > VDR between mains and earth shall be rated greater > than the required electric strength. > > > Best wishes for the Christmas season, > 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://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. 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