I need to replace some plastic light switches and would like to use metal replacements. The old switches had no provision for an earth wire and most of them had a sleeved unconnected earth wire which I was able to utilise. However, in three of the back-boxes the earth wire was cut short, is it ok to strip back the outer sheath a bit and join a new earth wire on (and sleeve it)? And can I safely use a crimp to do this? Thanks.