I have not performed any actual testing, but I believe that the basic problem is that MOV leakage current increases slightly each time the MOV experiences a big surge. So, in the early stages of this mechanism, the leakage current will increase step-wise each time the MOV experiences a big surge.
In practice, these successive "big surges" could be spaced months or years apart. Each successive surge will increase the leakage current, but if the current remains well below the level that causes significant self-heating of the MOV, the situation will remain stable. After some number of big surges (which could take years to accumulate), the leakage current will have increased enough that the self-heating process itself leads to higher leakage current. This sets up a positive feedback loop that causes the MOV to go into thermal runaway and self-destruct. I'm not sure the thermal runaway could be described as an "avalanche," since the process might take considerable time to destroy the MOV, but the general direction of the failure gets establishes as soon as the positive feedback mechanism gets started. If it could be shown that any dissipation level less than 240 W cannot cause ignition of nearby materials, then a 1A fuse would appear to provide the "adequate breaking capacity" called for in 60950-1. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com -----Original Message----- From: Ralph McDiarmid [mailto:ralph.mcdiar...@schneider-electric.com] Sent: Friday, November 17, 2017 1:05 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on AC mains ports? I wonder if a 1A fuse would protect against MOV flameout just as well as a 100mA, if these MOV fail as an avalanche. (get hotter = more leakage) I've seen some standards use 240VA (assume 240W) as a power limit for protection against fire. However, I wonder if a MOV could burn nicely at say 0.5A without further increase in current. Ralph McDiarmid Product Compliance Engineering Solar Business Schneider Electric -----Original Message----- From: Joe Randolph [mailto:j...@randolph-telecom.com] Sent: Monday, November 13, 2017 7:56 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on AC mains ports? Hi Mick: Annex Q in 60950-1 shows that the authors were concerned about increased leakage current in MOVs that are subjected to multiple surges. Perhaps the authors assumed that if the equipment passed the earth leakage test with new MOVs, allowing the MOVs to deteriorate by up to 10% after the Annex Q surges would be acceptable. 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. As Rich Nute has pointed out, a fuse that will open when the leakage current exceeds 0.5 mA cannot be expected to survive a 3 kA surge. So, the purpose of the fuse does not appear to be directed at the 0.5 mA leakage current requirement. Rather, I think the authors were trying to use the fuse to protect against the three conditions itemized in clause 1.5.9.2: * Temporary overvoltages * Thermal overload due to increased leakage current * Burning and bursting of the MOV in the event of a short-circuit fault The problem with this is that it takes very little leakage current to cause thermal overload of the MOV. At 230 VRMS, and RMS leakage current of 100 mA would dissipate 23 Watts in the MOV. A fuse that reliably opens at 100 mA will not survive a 3 kA surge. In general, MOVs subjected to thermal overload allow more leakage current as they heat up. With a fixed voltage supply (such as 230 VRMS), this creates a positive-feedback condition of ever-increasing power dissipation that eventually drives the MOV to destruction. So, perhaps the requirement for "adequate breaking capacity" was directed at trying to prevent burning and busting of the MOV, rather than excessive earth leakage current. So far, it appears that the term "adequate breaking capacity" is not actually defined in 60950-1. I suppose that the authors intended "adequate breaking capacity" to mean "adequate to prevent burning and bursting" of the MOV. I'm not sure that this is even possible if that same fuse has to survive a 3 kA combination wave surge. However, it is possible that 60950-1 does not actually require the circuit to survive such surges. Perhaps all that 60950-1 requires is that the MOV itself tolerates such surges with no more than 10% degradation. It would then be left to the product designer to select a suitable fuse. Between the conflicting goals of preventing nuisance tripping of the fuse and preventing overheating of the MOV, I'm not sure that an ordinary fuse can be found that meets both goals. This may be why 62368-1 seems to be steering us to using a thermal fuse that senses overheating of the MOV, rather than a fuse that responds only to current. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com -----Original Message----- From: Mick Maytum [mailto:mjmay...@gmail.com] Sent: Sunday, November 12, 2017 3:55 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on AC mains ports? For my sins I am currently editing a 40 page MOV application guide. As the guide was written by a Chinese expert I have the additional task of translating the Chinlish to English. However this forces understanding and I found the document contains many gems I did not appreciate. On degradation three areas are mentioned: 1. Varistor voltage at 1mA has decreased by more than 10 % of the initial value. 2. Surge limiting voltage at a specified impulse current has increased by more than 10 % of the initial value. 3. Leakage current or watt-loss shows a steady increase. Then the comment is made that generally if the Varistor voltage has decreased by 10 %, the limiting voltage will have only increased by 3 %. Thus the Annex Q limiting voltage change limit of 10 % would be better replaced by a Varistor voltage change. On fuses I did some calculations and found an anti-surge fuse in the 4 A region was required to withstand the Annex Q MOV requirement of a 3 kA 8/20 surge. It should be remembered that most fuses will not interrupt an 8/20 surge as the fuse link plasma carries on conducing the 8/20 impulse current. Ted makes the excellent point that in the equipment additional series impedances will exist that reduce the peak the peak surge current from an 1.2/50-8/20 generator. Tests are now appearing for thermally protected MOVs, which disconnect the MOV in the event of exceeding a pre-set body temperature limit. Thermal disconnect, rather than current disconnect, gets to the heart of the matter. But, as far as I'm aware, none of these tests apply a surge voltage to check for open thermal switch arc over. Regards, Mick Maytum Safety and Telecom Standards mjmay...@gmail.com https://ictsp-essays.info - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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