[email protected] wrote: > > I have some questions that are probably old hat to some of you but have me > running in circles! I would take these to my TUV contact, but its a > holiday south of the 49th! Any help would be appreciated. > > The discussion relates to a high-frequency switching transformer with a > primary (mains) winding and an SELV winding. The core is ferrite and is > floating (ie not conductively connected to anything). While ferrite is only > electrically conductive to the level of a few thousand ohms (according to my > DMM), I have treated it as a conductive part. The core is not accessible. > > 1. We need to meet Reinforced insulation requirements from primary to SELV. > Does it matter what insulation level we have from primary to the core and > from secondary to the core? In other words can the core be simply treated > as an interspersed conductive part which counts for zero in creepage and > clearance measurements from primary to secondary? If so, does it matter how > the total is divided - can most of it be on one side or the other, or do you > have to meet at least Basic from each winding to the core?Based on IEC 950, > 2nd edition, and amendment 3, clause 2.9.1 we read: "It is permitted for clearances and creepage distances to be divided by intervening, unconnected (floating) conductive parts, such as unused contacts of a connector, provided that the sum of the individual distances meets the specified minimum requirements. See figure F.15." This gives the following answers: It does not matter what insulation level is between primary and the core or between secondary and core, the sum must be reinforced insulation. The core can be treated as an intervening coductive and floating part. Most of the spacings can be on one side. An alternativeto reinforced insulation would be you treat one side as basic insulation, then the other side would have to be supplementary insulation (both sides as basic is not sufficient), resulting in double insulation, see definitions in 1.2.9.2 - 1.2.9.5. > > 2. Does our varnish impregnation help us in any way? It covers the complete > core and windings, but obviously not the coil terminations.If your > impregnation is according to 2.9.7 then there are no clearances and creepage distances in the transformer. The construction can then be treated as solid insulation according to 2.9.4, except the coil terminations, as you correctly state. > > 3. Is hipot testing required from either winding to the core or just between > the windings? Table C.2 only gives hipot requirements to the core if you > are trying to achieve Basic or Reinforced insulation from a winding to the > core.Annex C.2 says that no electric strength test is applied if the core is > totally enclosed or encapsulated and there is no electrical connection to the core. > > Thanks in advance for your help, > > Jim Eichner > Statpower Technologies Corporation > [email protected]
-- Kind regards, Horst Dierich, Convenor IEC TC74/WG6 Germany EMAIL: [email protected]
