The electrician wiring this dimmer switch was certainly on the "dim" side

The earth wire is being used as a live wire!

This is a deliberate and dangerous situation where the culprit must have known how bad it was when they did it! The black and red cable could be correct, the black should have a red or brown sleeve to show it is being used as a live wire, however the green and yellow earth wire will become live when the dimmer switch is turned off! If these are connected to the main earthing terminal of the house the fuse or circuit breaker would blow whenever the lights are off. This was not happening and after further investigation we found it was being used as a 3rd live wire allowing a second "2 way" switch to be added. This is very dangerous and could lead to a significant electric shock for anyone working on the electrics.