Richard Recently I asked some of these same questions about MOVs to the group. Most of the products I represent need to meet IEC950 or IEC1010-1. Some of my questions were relative to use of MOVs from line to ground. I heard that there are some local country requirements that do not allow MOVs line to ground due to the lack of a valid ground for many cord powered products that connect to wall outlets and the fact that MOVs often fail short. After much talk and investigation I have found that the standard EMI filter with its large common mode choke (3 to 20 mH) and surge rated Y caps (such as found in a typical OEM AC/DC power supply) is more than adequate for the common mode exposure of at least level 3 of IEC 1000-4-5 surge withstand and in some products even level 4. As a result I plan not to use MOVs to ground.
This leaves what to do with the normal mode (line to line) exposure. I assume this is the basis of your questions. I see many products with a MOV across the lines but it is then fused to insure that a fault on the MOV will not be a fire hazard. If the fuse is in series with the source then the product will loose power . If the fuse is in series with the MOV but that group is in parallel with the source then the product can keep running even if the MOV shorts. The problem here is how to indicate to the user that the MOV has failed. Relative to the overvoltage exposure data I believe the following standard may help: IEEE C62.41 (1991) "IEEE Recommended Practice on Surge Voltages in Low Voltage AC Power Circuits" This standard has lots of empirical/statistical data (111 pages) and clearly addresses your questions. As to how big of a MOV. I believe you must rely on the standards like IEC 664 and IEC 1000-4-5 to give you guidance and a reference to design for. They will dictate what the exposure energy levels will be. Then you have to decide what to do about any limiting impedance you may want to use in front of the MOV to help control what size (joule rating) you need. >This is an area I also would like to hear some advice on. - What do other designers use, if anything, in front of the MOV to control the joule rating? Power resistors? Inductors? If inductors, do they need to be VDE 565-2 qualified? etc. Or more simply said - What do others do to control surge? Even if you study the data and over design your MOV for your anticipated exposure beyond these standards , one of your customers will exceed it and you will have to deal with inevitable failure mode anyway. That is why I was talking about fuses. Hope this was useful Chris Wells Sr Des. Eng. Cutler-Hammer [email protected] ------------- Original Text From: C=US/A=INTERNET/DDA=ID/R.E.Hughes(a)nortel.co.uk, on 7/24/97 9:18 AM: Dear Safety & EMC folks, I am interested in obtaining data on the number, duration and amplitude of mains transients at different parts of the mains distribution network. It would be great if someone could point me to information published by the IEE, IEEE or whoever. IEC 664-1 classifies different parts of the mains distribution network as being Installation Category II, III, IV etc. IEC 950 re-states IEC 664-1 in saying that equipment plugged into a normal socket outlet in an office or home is Installation Category II: hence the clearances in Clause 2.9.2 of IEC 950 are based on 2.5 kV transients for a 230 V supply. I am also aware that various EMC (well, immunity) standards require equipment to be hit with transients, but not being an EMC engineer I cannot quote standard numbers or values at you. It is one thing knowing the assumed maximum transient voltage, but that is not sufficient to work out the amount of energy to be absorbed by an MOV or other such device during its working life. I have seen MOVs of various sizes used in power supplies, but just how big is big enough I dont know (but Id like to find out). For those of you that dont keep up with standards let me say that the draft 3rd edition of IEC 950 has an Annex that provides an alternative method for determining minimum clearance distances that includes a test for injecting the assumed transient at the mains input and then measuring what the resultant transient at the insulation under consideration. Unfortunately there is also a requirement that "surge suppressors in PRIMARY CIRCUITS are disconnected". This restriction may be due to the nasty habits that MOVs can get as they get older (i.e hot and leaky) - but then maybe devices that exhibit this behavior were not adequately rated in the first place?! Information and constructive comments please. Thanks (I hope!) Richard Hughes Nortel plc (UK) (By the way, this is all the signature I think is needed, but no doubt someone will correct me).
