As post # 17 mentions a 25 A MCB at the supply end of the 50 meter cable run I think it is VERY unlikely that two MCBs are faulty.
When you say in post #1 that a cable smokes is that the cable that you THINK is 4 sq mm or some other cable ? Can you post a diagram showing everything between the incoming supply to your house to where you short live to neutral. Also could you do this test. In your garage monitor the mains voltage as close as possible to a socket where you can plug in a reasonably heavy load such as a 3KW electric kettle. (This will take about 12 amps.) Note the voltage reading before you switch the kettle on and again when the kettle is switched on. This should allow us to calculate the resistance between your garage and the incoming supply. (I will assume that the impedance of the incoming supply is low enough to be ignored.)
Les.
When you say in post #1 that a cable smokes is that the cable that you THINK is 4 sq mm or some other cable ? Can you post a diagram showing everything between the incoming supply to your house to where you short live to neutral. Also could you do this test. In your garage monitor the mains voltage as close as possible to a socket where you can plug in a reasonably heavy load such as a 3KW electric kettle. (This will take about 12 amps.) Note the voltage reading before you switch the kettle on and again when the kettle is switched on. This should allow us to calculate the resistance between your garage and the incoming supply. (I will assume that the impedance of the incoming supply is low enough to be ignored.)
Les.