Thanks, I wonder then, by increasing the voltage rating of Y capacitors can you increase mains transient immunity?…..…I say this because some years ago I was working for a multinational lighting company, and we managed to reverse engineer a competitor product and got the schematic for it…..it was a 15W , 230VAC , Offline Flyback LED driver. There was the usual Y capacitor across the isolation barrier from primary to secondary…but in series with the Y capacitor was a high voltage diode. This surely must have been put there to increase mains transient immunity?…..but it makes you think, hey, hang on, having no Y capacitor at all would surely make it have even better mains transient immunity.