Treggers, Does anyone know by practical experience/experimentation what is the minimum OPERATIONAL creepage and clearance distance required for the following conditions:
Circuit type - Secondary Circuit Electric strength - 1000V 50Hz for 1 minute (i.e BS EN 60950/A4 : 1997 clause 6.4.2.2) Pollution degree - 2 Material Group - IIIa (CTI 200) Maximum transient - 1.5kV Transient repetition rate - very low (lightning, TNV-3) I realise that in BS EN 60950/A4:1997 table 6 for a TNV working voltage of 125V d.c the creepage distance stated is 1.5mm and from table 5 for a TNV working voltage of 140V d.c (transient overvoltage of 1.5kV) the clearance stated is 0.7mm, but after reading BS 7822:1995 (IEC 664-1:1992) it would appear that the creepage distance is based on long term stresses whereas the flash test requirement of EN60950 clause 6.4.2.2 is for one minute only. BS 6221:1991 (IEC 326-3:1991) section 6.4.1 gives a graph of "voltage against spacing between conductors" that differs greatly from BS EN 60950 even after derating by a factor of 11 and doesn't appear to take the material group into account. Does anyone know the source of this graph? BS EN 41003 : 1991 in Annex F (informative) gives a clearance distance of 0.5mm (inhomogeneous field) for the 1.5kV impulse test of BS EN 60950 clause 6.4.2.1 which ties up with BS 7822:1995 (IEC 326-3:1991) table 2 and a creepage distance of 1.5mm which ties up with BS 6221:1991 (IEC 326-3:1991) table 4. I have always used 1mm per 1kV for clearance through air but does anyone know the formula for creepage? Any help or pointers to other specifications is welcomed. Regards Richard Steele
