Hints & Tips
Identifying the various parts of a consumer unit #2
This is an RCD or Residual Current Device.
In this case, the RCD is protecting the five circuits on the right hand side of the board. This device is designed to switch off all five circuits if it detects any leakage of current to earth in excess of 30mA and to do it quickly enough to save a life. Usually less than 1/5 second.