Shorting neutral and earthing trips MCB
You have probably got a residual current device built into your MCB. Normal operation is such that current in "live" precisely equals current in "neutral" i.e. no leakage currents through the earth wire.

When you bypass the device, it detects an imbalance in both live and neutral wires and trips: -

 

Alternatively the predecessor of the RTD was called an earth leakage detection unit and it actually looked for earth currents above a certain value. These have been superceded by the RCD in a lot of homes.