Cooker Hood Shorts When the Outer Cover Touches a Wall Bracket Installed to Stabilise the Cover
I have a feeling you are in UK or Europe and your MCB includes RCD/RCBO functionality.** 

The problem is that your device has residual current leakage, or what we in the states call a "ground fault".   It is leaking current in a way it should not, and which could stun/harm/kill a human. Being stunned unconscious onto a hot cooking surface would not be fun.  

When you install the cover, you are causing the problem to manifest in a way that doesn't involve bloody mayhem.  This is a good thing. 

The temptation would be to defeat the symptom, but the real problem is the hood is defective.  Sorry. 



**if not, tripping the main circuit breaker and not the branch circuit breaker would require an arc flash brighter than the sun, and I'm pretty sure you would not be typing!