Tony Polson wrote:

I have a 1950s German slide projector (anyone remember slides?) which
I have been using with its German mains plug and an adaptor.

I decided to replace the plug with a UK 13A plug. Having removed the
German plug, I cannot remember which wire went were, and the colour
coding is unfamiliar to me.

There are three wires, coloured red, black and light grey. Can anyone
tell me which is which? I guessed at

red = live
black = neutral

but is the grey wire really the earth?

Definitive advice will be very much appreciated!

a multimeter will tell you which is which in no time.

NT