An RCD is a safety device in your fuse board. It constantly monitors the electric current travelling through the live and neutral cable. As long as both currents measure the same, it will let the electric continue. The moment it changes, it will isolate the power. Why? well if the electric current changes between the two then it must have gone somewhere right? well that could be through a dodgy appliance casing or it could be through you. So its a device we all need in our fuse boards and many older installations don’t have them. That’s why a fuse board upgrade is always advised should they not.