Joe Finlayson wrote: > > To any and all safety gurus, > > I am in the midst of trying to minimize the creepage and clearance > requirements between TNV and SELV called out in IEC 950. My > interpretation of tables 5 and 6 in sections 2.9.2. and 2.9.3 are that > minimum clearance is 2.0mm and minimum creepage is 2.5mm. This is > based on a global design using up to 250V mains, pollution degree 1 & > 2 and Material Group IIIa + IIIb. The application is for Class I as > well as Class II systems. > > I could not find a reference explaining the definition of the > Material Groups and would appreciate any insight into a clause, etc. > which I could reference to reduce these spacing requirements. Please > also advise if you are in need of further details. > > Thx, > > > Joe >
Joe, You have to firstly establish the maximum working voltage across the barrier, if its a transformer then as a rule of thumb you should add the primary voltage to the secondary voltage and use the total as the working voltage - see clauses 2.2.7.3 and 2.2.7.4. Peak values are used for clearance and r.m.s or d.c for creepage. IEC 950/A3 clause 6.2.1.2 states Basic insulation between TNV and unearthed SELV. Clearance (Secondary circuits Table 5) - I assume that your TNV will be subject to transients up to 1500V so using the line for your peak working voltage and column 2 "Nominal mains supply voltage >150V < 300V with the appropriate Pollution degree (I - Sealed unit, II - office environment, III - dirty environment) you will get the required clearance. Example - TNV 120V peak, Secondary side of Tx 40V peak = Working V of 160V Pollution Degree 2 and Basic gives a clearance of 1mm (I used the 210V working voltage line as it is the nearest). Creepage - You have to know the CTI of the material in question and this can be found from the material manufacturer and it is also detailed in the UL component directory. Using the clearance working voltage and multiplying by 0.707 to get the r.m.s value = 160 x 0.707 = 113. If the CTI is 120 = Material group of IIIb (see note 3 of table 6) you will get a creepage distance of 1.5mm (125V used as this is the nearest). As the creepage distance is the greater distance then this is the figure to be used for clearance as well. I hope this is of some help - please remember this is my interpretation of the standard it may not be quoted as authority. Richard
