For the earth-current breaker to work when there is a fault on the enlarger, you will need to have an earth on the metal chassis of the enlarger (assuming that the lamp-socket is in a metal head, which it is in the Valoy I think). If the earth wire only starts in the timer, then a short inside there could pop the switch, but the device doesn't give protection automagically with just a two-core cable to the enlarger. And you do have the correct size fuse in the plug of course, not 13A?
[Bearing in mind that I'm not a professional electrical engineer, your safety is your responsibility etc. etc.]
EDIT: Maybe I misunderstood about the timer - if there is no earth wire out of the timer then how can one expect to trip the earth-current breaker at all???
Last edited by MartinP; 01-15-2014 at 07:13 PM. Click to view previous post history.