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).
