Hi
I am replacing plastic light switches for chrome switches. The back boxes are metal and are earthed with the single earth from the main supply. Do I just need to run another single length of earth from the earth terminal on the box to the earth terminal on the switch?

If so, what wire... 1.5mm OK? Does it matter?

Thanks

Knowing your location would help as it is obvious that you are not in the US or Canada, where most of our posters are from.