It is advisable to use a residual current device (RCD) whenever possible but particularly in wet or damp locations such as outdoors. An RCD rated at no more than 30mA limits the energy in a particular type of electric shock and can save your life. However, an RCD cannot protect you from every type of electric shock, so you should still make sure that circuits are securely isolated before you work on them.
It is best to use an RCD that is incorporated into the switchboard of your installation. This means that all circuits fed from that RCD are protected by the RCD. An RCD that is incorporated into an ordinary mains socket, or plugged into it, will protect anything that is attached to that socket, but it is possible that equipment may be plugged into another, unprotected, socket.
RCDs should be regularly tested by pressing the ‘test’ button, and by making sure that the RCD trips. Faulty or inoperative RCDs should be removed from use.