An earth leak is when the insulation of the live conductors fails and current flows to earth (hopefully direct to the protective earth conductor). If you had a big earth leak (for example if you bang a nail into a cable or drop a toaster into the sink) then it will, unless very slight, blow the fuse.
Immersion heaters also sometimes leak slightly when they get old. This is one of the reasons why all the copper pipes are securely earthed, so that any current will flow straight to earth without giving risk of electric shock.
Slight leakage is also caused, for example, when the insulation in an oven heating element breaks down with age. In this case it is usually very slight and because the oven is securely earthed, not movable, and is not used outside the house or in the bathroom, it is not considered dangerous.
The RCD is a very sensitive device that cuts off the power from a very small earth leakage, before it gets big enough to cause a dangerous shock. It is important to use these with appliances that are likely yo be used outdoors (for example, sockets that might be used when mowing the lawn or hoovering the car) because you are outside the area protected by your house's earthing installation; or with electric shower (because of the danger of electricity and water).
I'm not sure if my description is clear, accurate and complete, perhaps someone will correct or add?
Edited: BTW - the reason why I suggested the 10Amp oven should be on a 13Amp fuse (not connected to the 32A Cooker Circuit) is so that the fuse is quite close to the normal current, so a fault will not need to be big to blow the fuse). Not that I think a fault is likely.
Immersion heaters also sometimes leak slightly when they get old. This is one of the reasons why all the copper pipes are securely earthed, so that any current will flow straight to earth without giving risk of electric shock.
Slight leakage is also caused, for example, when the insulation in an oven heating element breaks down with age. In this case it is usually very slight and because the oven is securely earthed, not movable, and is not used outside the house or in the bathroom, it is not considered dangerous.
The RCD is a very sensitive device that cuts off the power from a very small earth leakage, before it gets big enough to cause a dangerous shock. It is important to use these with appliances that are likely yo be used outdoors (for example, sockets that might be used when mowing the lawn or hoovering the car) because you are outside the area protected by your house's earthing installation; or with electric shower (because of the danger of electricity and water).
I'm not sure if my description is clear, accurate and complete, perhaps someone will correct or add?
Edited: BTW - the reason why I suggested the 10Amp oven should be on a 13Amp fuse (not connected to the 32A Cooker Circuit) is so that the fuse is quite close to the normal current, so a fault will not need to be big to blow the fuse). Not that I think a fault is likely.